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[mirror_ubuntu-bionic-kernel.git] / fs / xfs / xfs_iomap.c

/*
 * Copyright (c) 2000-2006 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#include <linux/iomap.h>
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_shared.h"
#include "xfs_format.h"
#include "xfs_log_format.h"
#include "xfs_trans_resv.h"
#include "xfs_mount.h"
#include "xfs_defer.h"
#include "xfs_inode.h"
#include "xfs_btree.h"
#include "xfs_bmap_btree.h"
#include "xfs_bmap.h"
#include "xfs_bmap_util.h"
#include "xfs_error.h"
#include "xfs_trans.h"
#include "xfs_trans_space.h"
#include "xfs_iomap.h"
#include "xfs_trace.h"
#include "xfs_icache.h"
#include "xfs_quota.h"
#include "xfs_dquot_item.h"
#include "xfs_dquot.h"


#define XFS_WRITEIO_ALIGN(mp,off)       (((off) >> mp->m_writeio_log) \
                                                << mp->m_writeio_log)
#define XFS_WRITE_IMAPS         XFS_BMAP_MAX_NMAP

STATIC int
xfs_iomap_eof_align_last_fsb(
        xfs_mount_t     *mp,
        xfs_inode_t     *ip,
        xfs_extlen_t    extsize,
        xfs_fileoff_t   *last_fsb)
{
        xfs_extlen_t    align = 0;
        int             eof, error;

        if (!XFS_IS_REALTIME_INODE(ip)) {
                /*
                 * Round up the allocation request to a stripe unit
                 * (m_dalign) boundary if the file size is >= stripe unit
                 * size, and we are allocating past the allocation eof.
                 *
                 * If mounted with the "-o swalloc" option the alignment is
                 * increased from the strip unit size to the stripe width.
                 */
                if (mp->m_swidth && (mp->m_flags & XFS_MOUNT_SWALLOC))
                        align = mp->m_swidth;
                else if (mp->m_dalign)
                        align = mp->m_dalign;

                if (align && XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, align))
                        align = 0;
        }

        /*
         * Always round up the allocation request to an extent boundary
         * (when file on a real-time subvolume or has di_extsize hint).
         */
        if (extsize) {
                if (align)
                        align = roundup_64(align, extsize);
                else
                        align = extsize;
        }

        if (align) {
                xfs_fileoff_t   new_last_fsb = roundup_64(*last_fsb, align);
                error = xfs_bmap_eof(ip, new_last_fsb, XFS_DATA_FORK, &eof);
                if (error)
                        return error;
                if (eof)
                        *last_fsb = new_last_fsb;
        }
        return 0;
}

STATIC int
xfs_alert_fsblock_zero(
        xfs_inode_t     *ip,
        xfs_bmbt_irec_t *imap)
{
        xfs_alert_tag(ip->i_mount, XFS_PTAG_FSBLOCK_ZERO,
                        "Access to block zero in inode %llu "
                        "start_block: %llx start_off: %llx "
                        "blkcnt: %llx extent-state: %x",
                (unsigned long long)ip->i_ino,
                (unsigned long long)imap->br_startblock,
                (unsigned long long)imap->br_startoff,
                (unsigned long long)imap->br_blockcount,
                imap->br_state);
        return -EFSCORRUPTED;
}

int
xfs_iomap_write_direct(
        xfs_inode_t     *ip,
        xfs_off_t       offset,
        size_t          count,
        xfs_bmbt_irec_t *imap,
        int             nmaps)
{
        xfs_mount_t     *mp = ip->i_mount;
        xfs_fileoff_t   offset_fsb;
        xfs_fileoff_t   last_fsb;
        xfs_filblks_t   count_fsb, resaligned;
        xfs_fsblock_t   firstfsb;
        xfs_extlen_t    extsz, temp;
        int             nimaps;
        int             quota_flag;
        int             rt;
        xfs_trans_t     *tp;
        struct xfs_defer_ops dfops;
        uint            qblocks, resblks, resrtextents;
        int             error;
        int             lockmode;
        int             bmapi_flags = XFS_BMAPI_PREALLOC;
        uint            tflags = 0;

        rt = XFS_IS_REALTIME_INODE(ip);
        extsz = xfs_get_extsz_hint(ip);
        lockmode = XFS_ILOCK_SHARED;    /* locked by caller */

        ASSERT(xfs_isilocked(ip, lockmode));

        offset_fsb = XFS_B_TO_FSBT(mp, offset);
        last_fsb = XFS_B_TO_FSB(mp, ((xfs_ufsize_t)(offset + count)));
        if ((offset + count) > XFS_ISIZE(ip)) {
                /*
                 * Assert that the in-core extent list is present since this can
                 * call xfs_iread_extents() and we only have the ilock shared.
                 * This should be safe because the lock was held around a bmapi
                 * call in the caller and we only need it to access the in-core
                 * list.
                 */
                ASSERT(XFS_IFORK_PTR(ip, XFS_DATA_FORK)->if_flags &
                                                                XFS_IFEXTENTS);
                error = xfs_iomap_eof_align_last_fsb(mp, ip, extsz, &last_fsb);
                if (error)
                        goto out_unlock;
        } else {
                if (nmaps && (imap->br_startblock == HOLESTARTBLOCK))
                        last_fsb = MIN(last_fsb, (xfs_fileoff_t)
                                        imap->br_blockcount +
                                        imap->br_startoff);
        }
        count_fsb = last_fsb - offset_fsb;
        ASSERT(count_fsb > 0);

        resaligned = count_fsb;
        if (unlikely(extsz)) {
                if ((temp = do_mod(offset_fsb, extsz)))
                        resaligned += temp;
                if ((temp = do_mod(resaligned, extsz)))
                        resaligned += extsz - temp;
        }

        if (unlikely(rt)) {
                resrtextents = qblocks = resaligned;
                resrtextents /= mp->m_sb.sb_rextsize;
                resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0);
                quota_flag = XFS_QMOPT_RES_RTBLKS;
        } else {
                resrtextents = 0;
                resblks = qblocks = XFS_DIOSTRAT_SPACE_RES(mp, resaligned);
                quota_flag = XFS_QMOPT_RES_REGBLKS;
        }

        /*
         * Drop the shared lock acquired by the caller, attach the dquot if
         * necessary and move on to transaction setup.
         */
        xfs_iunlock(ip, lockmode);
        error = xfs_qm_dqattach(ip, 0);
        if (error)
                return error;

        /*
         * For DAX, we do not allocate unwritten extents, but instead we zero
         * the block before we commit the transaction.  Ideally we'd like to do
         * this outside the transaction context, but if we commit and then crash
         * we may not have zeroed the blocks and this will be exposed on
         * recovery of the allocation. Hence we must zero before commit.
         *
         * Further, if we are mapping unwritten extents here, we need to zero
         * and convert them to written so that we don't need an unwritten extent
         * callback for DAX. This also means that we need to be able to dip into
         * the reserve block pool for bmbt block allocation if there is no space
         * left but we need to do unwritten extent conversion.
         */
        if (IS_DAX(VFS_I(ip))) {
                bmapi_flags = XFS_BMAPI_CONVERT | XFS_BMAPI_ZERO;
                if (ISUNWRITTEN(imap)) {
                        tflags |= XFS_TRANS_RESERVE;
                        resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;
                }
        }
        error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, resrtextents,
                        tflags, &tp);
        if (error)
                return error;

        lockmode = XFS_ILOCK_EXCL;
        xfs_ilock(ip, lockmode);

        error = xfs_trans_reserve_quota_nblks(tp, ip, qblocks, 0, quota_flag);
        if (error)
                goto out_trans_cancel;

        xfs_trans_ijoin(tp, ip, 0);

        /*
         * From this point onwards we overwrite the imap pointer that the
         * caller gave to us.
         */
        xfs_defer_init(&dfops, &firstfsb);
        nimaps = 1;
        error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
                                bmapi_flags, &firstfsb, resblks, imap,
                                &nimaps, &dfops);
        if (error)
                goto out_bmap_cancel;

        /*
         * Complete the transaction
         */
        error = xfs_defer_finish(&tp, &dfops, NULL);
        if (error)
                goto out_bmap_cancel;

        error = xfs_trans_commit(tp);
        if (error)
                goto out_unlock;

        /*
         * Copy any maps to caller's array and return any error.
         */
        if (nimaps == 0) {
                error = -ENOSPC;
                goto out_unlock;
        }

        if (!(imap->br_startblock || XFS_IS_REALTIME_INODE(ip)))
                error = xfs_alert_fsblock_zero(ip, imap);

out_unlock:
        xfs_iunlock(ip, lockmode);
        return error;

out_bmap_cancel:
        xfs_defer_cancel(&dfops);
        xfs_trans_unreserve_quota_nblks(tp, ip, (long)qblocks, 0, quota_flag);
out_trans_cancel:
        xfs_trans_cancel(tp);
        goto out_unlock;
}

/*
 * If the caller is doing a write at the end of the file, then extend the
 * allocation out to the file system's write iosize.  We clean up any extra
 * space left over when the file is closed in xfs_inactive().
 *
 * If we find we already have delalloc preallocation beyond EOF, don't do more
 * preallocation as it it not needed.
 */
STATIC int
xfs_iomap_eof_want_preallocate(
        xfs_mount_t     *mp,
        xfs_inode_t     *ip,
        xfs_off_t       offset,
        size_t          count,
        xfs_bmbt_irec_t *imap,
        int             nimaps,
        int             *prealloc)
{
        xfs_fileoff_t   start_fsb;
        xfs_filblks_t   count_fsb;
        int             n, error, imaps;
        int             found_delalloc = 0;

        *prealloc = 0;
        if (offset + count <= XFS_ISIZE(ip))
                return 0;

        /*
         * If the file is smaller than the minimum prealloc and we are using
         * dynamic preallocation, don't do any preallocation at all as it is
         * likely this is the only write to the file that is going to be done.
         */
        if (!(mp->m_flags & XFS_MOUNT_DFLT_IOSIZE) &&
            XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_writeio_blocks))
                return 0;

        /*
         * If there are any real blocks past eof, then don't
         * do any speculative allocation.
         */
        start_fsb = XFS_B_TO_FSBT(mp, ((xfs_ufsize_t)(offset + count - 1)));
        count_fsb = XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes);
        while (count_fsb > 0) {
                imaps = nimaps;
                error = xfs_bmapi_read(ip, start_fsb, count_fsb, imap, &imaps,
                                       0);
                if (error)
                        return error;
                for (n = 0; n < imaps; n++) {
                        if ((imap[n].br_startblock != HOLESTARTBLOCK) &&
                            (imap[n].br_startblock != DELAYSTARTBLOCK))
                                return 0;
                        start_fsb += imap[n].br_blockcount;
                        count_fsb -= imap[n].br_blockcount;

                        if (imap[n].br_startblock == DELAYSTARTBLOCK)
                                found_delalloc = 1;
                }
        }
        if (!found_delalloc)
                *prealloc = 1;
        return 0;
}

/*
 * Determine the initial size of the preallocation. We are beyond the current
 * EOF here, but we need to take into account whether this is a sparse write or
 * an extending write when determining the preallocation size.  Hence we need to
 * look up the extent that ends at the current write offset and use the result
 * to determine the preallocation size.
 *
 * If the extent is a hole, then preallocation is essentially disabled.
 * Otherwise we take the size of the preceeding data extent as the basis for the
 * preallocation size. If the size of the extent is greater than half the
 * maximum extent length, then use the current offset as the basis. This ensures
 * that for large files the preallocation size always extends to MAXEXTLEN
 * rather than falling short due to things like stripe unit/width alignment of
 * real extents.
 */
STATIC xfs_fsblock_t
xfs_iomap_eof_prealloc_initial_size(
        struct xfs_mount        *mp,
        struct xfs_inode        *ip,
        xfs_off_t               offset,
        xfs_bmbt_irec_t         *imap,
        int                     nimaps)
{
        xfs_fileoff_t   start_fsb;
        int             imaps = 1;
        int             error;

        ASSERT(nimaps >= imaps);

        /* if we are using a specific prealloc size, return now */
        if (mp->m_flags & XFS_MOUNT_DFLT_IOSIZE)
                return 0;

        /* If the file is small, then use the minimum prealloc */
        if (XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_dalign))
                return 0;

        /*
         * As we write multiple pages, the offset will always align to the
         * start of a page and hence point to a hole at EOF. i.e. if the size is
         * 4096 bytes, we only have one block at FSB 0, but XFS_B_TO_FSB(4096)
         * will return FSB 1. Hence if there are blocks in the file, we want to
         * point to the block prior to the EOF block and not the hole that maps
         * directly at @offset.
         */
        start_fsb = XFS_B_TO_FSB(mp, offset);
        if (start_fsb)
                start_fsb--;
        error = xfs_bmapi_read(ip, start_fsb, 1, imap, &imaps, XFS_BMAPI_ENTIRE);
        if (error)
                return 0;

        ASSERT(imaps == 1);
        if (imap[0].br_startblock == HOLESTARTBLOCK)
                return 0;
        if (imap[0].br_blockcount <= (MAXEXTLEN >> 1))
                return imap[0].br_blockcount << 1;
        return XFS_B_TO_FSB(mp, offset);
}

STATIC bool
xfs_quota_need_throttle(
        struct xfs_inode *ip,
        int type,
        xfs_fsblock_t alloc_blocks)
{
        struct xfs_dquot *dq = xfs_inode_dquot(ip, type);

        if (!dq || !xfs_this_quota_on(ip->i_mount, type))
                return false;

        /* no hi watermark, no throttle */
        if (!dq->q_prealloc_hi_wmark)
                return false;

        /* under the lo watermark, no throttle */
        if (dq->q_res_bcount + alloc_blocks < dq->q_prealloc_lo_wmark)
                return false;

        return true;
}

STATIC void
xfs_quota_calc_throttle(
        struct xfs_inode *ip,
        int type,
        xfs_fsblock_t *qblocks,
        int *qshift,
        int64_t *qfreesp)
{
        int64_t freesp;
        int shift = 0;
        struct xfs_dquot *dq = xfs_inode_dquot(ip, type);

        /* no dq, or over hi wmark, squash the prealloc completely */
        if (!dq || dq->q_res_bcount >= dq->q_prealloc_hi_wmark) {
                *qblocks = 0;
                *qfreesp = 0;
                return;
        }

        freesp = dq->q_prealloc_hi_wmark - dq->q_res_bcount;
        if (freesp < dq->q_low_space[XFS_QLOWSP_5_PCNT]) {
                shift = 2;
                if (freesp < dq->q_low_space[XFS_QLOWSP_3_PCNT])
                        shift += 2;
                if (freesp < dq->q_low_space[XFS_QLOWSP_1_PCNT])
                        shift += 2;
        }

        if (freesp < *qfreesp)
                *qfreesp = freesp;

        /* only overwrite the throttle values if we are more aggressive */
        if ((freesp >> shift) < (*qblocks >> *qshift)) {
                *qblocks = freesp;
                *qshift = shift;
        }
}

/*
 * If we don't have a user specified preallocation size, dynamically increase
 * the preallocation size as the size of the file grows. Cap the maximum size
 * at a single extent or less if the filesystem is near full. The closer the
 * filesystem is to full, the smaller the maximum prealocation.
 */
STATIC xfs_fsblock_t
xfs_iomap_prealloc_size(
        struct xfs_mount        *mp,
        struct xfs_inode        *ip,
        xfs_off_t               offset,
        struct xfs_bmbt_irec    *imap,
        int                     nimaps)
{
        xfs_fsblock_t           alloc_blocks = 0;
        int                     shift = 0;
        int64_t                 freesp;
        xfs_fsblock_t           qblocks;
        int                     qshift = 0;

        alloc_blocks = xfs_iomap_eof_prealloc_initial_size(mp, ip, offset,
                                                           imap, nimaps);
        if (!alloc_blocks)
                goto check_writeio;
        qblocks = alloc_blocks;

        /*
         * MAXEXTLEN is not a power of two value but we round the prealloc down
         * to the nearest power of two value after throttling. To prevent the
         * round down from unconditionally reducing the maximum supported prealloc
         * size, we round up first, apply appropriate throttling, round down and
         * cap the value to MAXEXTLEN.
         */
        alloc_blocks = XFS_FILEOFF_MIN(roundup_pow_of_two(MAXEXTLEN),
                                       alloc_blocks);

        freesp = percpu_counter_read_positive(&mp->m_fdblocks);
        if (freesp < mp->m_low_space[XFS_LOWSP_5_PCNT]) {
                shift = 2;
                if (freesp < mp->m_low_space[XFS_LOWSP_4_PCNT])
                        shift++;
                if (freesp < mp->m_low_space[XFS_LOWSP_3_PCNT])
                        shift++;
                if (freesp < mp->m_low_space[XFS_LOWSP_2_PCNT])
                        shift++;
                if (freesp < mp->m_low_space[XFS_LOWSP_1_PCNT])
                        shift++;
        }

        /*
         * Check each quota to cap the prealloc size, provide a shift value to
         * throttle with and adjust amount of available space.
         */
        if (xfs_quota_need_throttle(ip, XFS_DQ_USER, alloc_blocks))
                xfs_quota_calc_throttle(ip, XFS_DQ_USER, &qblocks, &qshift,
                                        &freesp);
        if (xfs_quota_need_throttle(ip, XFS_DQ_GROUP, alloc_blocks))
                xfs_quota_calc_throttle(ip, XFS_DQ_GROUP, &qblocks, &qshift,
                                        &freesp);
        if (xfs_quota_need_throttle(ip, XFS_DQ_PROJ, alloc_blocks))
                xfs_quota_calc_throttle(ip, XFS_DQ_PROJ, &qblocks, &qshift,
                                        &freesp);

        /*
         * The final prealloc size is set to the minimum of free space available
         * in each of the quotas and the overall filesystem.
         *
         * The shift throttle value is set to the maximum value as determined by
         * the global low free space values and per-quota low free space values.
         */
        alloc_blocks = MIN(alloc_blocks, qblocks);
        shift = MAX(shift, qshift);

        if (shift)
                alloc_blocks >>= shift;
        /*
         * rounddown_pow_of_two() returns an undefined result if we pass in
         * alloc_blocks = 0.
         */
        if (alloc_blocks)
                alloc_blocks = rounddown_pow_of_two(alloc_blocks);
        if (alloc_blocks > MAXEXTLEN)
                alloc_blocks = MAXEXTLEN;

        /*
         * If we are still trying to allocate more space than is
         * available, squash the prealloc hard. This can happen if we
         * have a large file on a small filesystem and the above
         * lowspace thresholds are smaller than MAXEXTLEN.
         */
        while (alloc_blocks && alloc_blocks >= freesp)
                alloc_blocks >>= 4;

check_writeio:
        if (alloc_blocks < mp->m_writeio_blocks)
                alloc_blocks = mp->m_writeio_blocks;

        trace_xfs_iomap_prealloc_size(ip, alloc_blocks, shift,
                                      mp->m_writeio_blocks);

        return alloc_blocks;
}

int
xfs_iomap_write_delay(
        xfs_inode_t     *ip,
        xfs_off_t       offset,
        size_t          count,
        xfs_bmbt_irec_t *ret_imap)
{
        xfs_mount_t     *mp = ip->i_mount;
        xfs_fileoff_t   offset_fsb;
        xfs_fileoff_t   last_fsb;
        xfs_off_t       aligned_offset;
        xfs_fileoff_t   ioalign;
        xfs_extlen_t    extsz;
        int             nimaps;
        xfs_bmbt_irec_t imap[XFS_WRITE_IMAPS];
        int             prealloc;
        int             error;

        ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));

        /*
         * Make sure that the dquots are there. This doesn't hold
         * the ilock across a disk read.
         */
        error = xfs_qm_dqattach_locked(ip, 0);
        if (error)
                return error;

        extsz = xfs_get_extsz_hint(ip);
        offset_fsb = XFS_B_TO_FSBT(mp, offset);

        error = xfs_iomap_eof_want_preallocate(mp, ip, offset, count,
                                imap, XFS_WRITE_IMAPS, &prealloc);
        if (error)
                return error;

retry:
        if (prealloc) {
                xfs_fsblock_t   alloc_blocks;

                alloc_blocks = xfs_iomap_prealloc_size(mp, ip, offset, imap,
                                                       XFS_WRITE_IMAPS);

                aligned_offset = XFS_WRITEIO_ALIGN(mp, (offset + count - 1));
                ioalign = XFS_B_TO_FSBT(mp, aligned_offset);
                last_fsb = ioalign + alloc_blocks;
        } else {
                last_fsb = XFS_B_TO_FSB(mp, ((xfs_ufsize_t)(offset + count)));
        }

        if (prealloc || extsz) {
                error = xfs_iomap_eof_align_last_fsb(mp, ip, extsz, &last_fsb);
                if (error)
                        return error;
        }

        /*
         * Make sure preallocation does not create extents beyond the range we
         * actually support in this filesystem.
         */
        if (last_fsb > XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes))
                last_fsb = XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes);

        ASSERT(last_fsb > offset_fsb);

        nimaps = XFS_WRITE_IMAPS;
        error = xfs_bmapi_delay(ip, offset_fsb, last_fsb - offset_fsb,
                                imap, &nimaps, XFS_BMAPI_ENTIRE);
        switch (error) {
        case 0:
        case -ENOSPC:
        case -EDQUOT:
                break;
        default:
                return error;
        }

        /*
         * If bmapi returned us nothing, we got either ENOSPC or EDQUOT. Retry
         * without EOF preallocation.
         */
        if (nimaps == 0) {
                trace_xfs_delalloc_enospc(ip, offset, count);
                if (prealloc) {
                        prealloc = 0;
                        error = 0;
                        goto retry;
                }
                return error ? error : -ENOSPC;
        }

        if (!(imap[0].br_startblock || XFS_IS_REALTIME_INODE(ip)))
                return xfs_alert_fsblock_zero(ip, &imap[0]);

        /*
         * Tag the inode as speculatively preallocated so we can reclaim this
         * space on demand, if necessary.
         */
        if (prealloc)
                xfs_inode_set_eofblocks_tag(ip);

        *ret_imap = imap[0];
        return 0;
}

/*
 * Pass in a delayed allocate extent, convert it to real extents;
 * return to the caller the extent we create which maps on top of
 * the originating callers request.
 *
 * Called without a lock on the inode.
 *
 * We no longer bother to look at the incoming map - all we have to
 * guarantee is that whatever we allocate fills the required range.
 */
int
xfs_iomap_write_allocate(
        xfs_inode_t     *ip,
        xfs_off_t       offset,
        xfs_bmbt_irec_t *imap)
{
        xfs_mount_t     *mp = ip->i_mount;
        xfs_fileoff_t   offset_fsb, last_block;
        xfs_fileoff_t   end_fsb, map_start_fsb;
        xfs_fsblock_t   first_block;
        struct xfs_defer_ops    dfops;
        xfs_filblks_t   count_fsb;
        xfs_trans_t     *tp;
        int             nimaps;
        int             error = 0;
        int             nres;

        /*
         * Make sure that the dquots are there.
         */
        error = xfs_qm_dqattach(ip, 0);
        if (error)
                return error;

        offset_fsb = XFS_B_TO_FSBT(mp, offset);
        count_fsb = imap->br_blockcount;
        map_start_fsb = imap->br_startoff;

        XFS_STATS_ADD(mp, xs_xstrat_bytes, XFS_FSB_TO_B(mp, count_fsb));

        while (count_fsb != 0) {
                /*
                 * Set up a transaction with which to allocate the
                 * backing store for the file.  Do allocations in a
                 * loop until we get some space in the range we are
                 * interested in.  The other space that might be allocated
                 * is in the delayed allocation extent on which we sit
                 * but before our buffer starts.
                 */
                nimaps = 0;
                while (nimaps == 0) {
                        nres = XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
                        /*
                         * We have already reserved space for the extent and any
                         * indirect blocks when creating the delalloc extent,
                         * there is no need to reserve space in this transaction
                         * again.
                         */
                        error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, 0,
                                        0, XFS_TRANS_RESERVE, &tp);
                        if (error)
                                return error;

                        xfs_ilock(ip, XFS_ILOCK_EXCL);
                        xfs_trans_ijoin(tp, ip, 0);

                        xfs_defer_init(&dfops, &first_block);

                        /*
                         * it is possible that the extents have changed since
                         * we did the read call as we dropped the ilock for a
                         * while. We have to be careful about truncates or hole
                         * punchs here - we are not allowed to allocate
                         * non-delalloc blocks here.
                         *
                         * The only protection against truncation is the pages
                         * for the range we are being asked to convert are
                         * locked and hence a truncate will block on them
                         * first.
                         *
                         * As a result, if we go beyond the range we really
                         * need and hit an delalloc extent boundary followed by
                         * a hole while we have excess blocks in the map, we
                         * will fill the hole incorrectly and overrun the
                         * transaction reservation.
                         *
                         * Using a single map prevents this as we are forced to
                         * check each map we look for overlap with the desired
                         * range and abort as soon as we find it. Also, given
                         * that we only return a single map, having one beyond
                         * what we can return is probably a bit silly.
                         *
                         * We also need to check that we don't go beyond EOF;
                         * this is a truncate optimisation as a truncate sets
                         * the new file size before block on the pages we
                         * currently have locked under writeback. Because they
                         * are about to be tossed, we don't need to write them
                         * back....
                         */
                        nimaps = 1;
                        end_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
                        error = xfs_bmap_last_offset(ip, &last_block,
                                                        XFS_DATA_FORK);
                        if (error)
                                goto trans_cancel;

                        last_block = XFS_FILEOFF_MAX(last_block, end_fsb);
                        if ((map_start_fsb + count_fsb) > last_block) {
                                count_fsb = last_block - map_start_fsb;
                                if (count_fsb == 0) {
                                        error = -EAGAIN;
                                        goto trans_cancel;
                                }
                        }

                        /*
                         * From this point onwards we overwrite the imap
                         * pointer that the caller gave to us.
                         */
                        error = xfs_bmapi_write(tp, ip, map_start_fsb,
                                                count_fsb, 0, &first_block,
                                                nres, imap, &nimaps,
                                                &dfops);
                        if (error)
                                goto trans_cancel;

                        error = xfs_defer_finish(&tp, &dfops, NULL);
                        if (error)
                                goto trans_cancel;

                        error = xfs_trans_commit(tp);
                        if (error)
                                goto error0;

                        xfs_iunlock(ip, XFS_ILOCK_EXCL);
                }

                /*
                 * See if we were able to allocate an extent that
                 * covers at least part of the callers request
                 */
                if (!(imap->br_startblock || XFS_IS_REALTIME_INODE(ip)))
                        return xfs_alert_fsblock_zero(ip, imap);

                if ((offset_fsb >= imap->br_startoff) &&
                    (offset_fsb < (imap->br_startoff +
                                   imap->br_blockcount))) {
                        XFS_STATS_INC(mp, xs_xstrat_quick);
                        return 0;
                }

                /*
                 * So far we have not mapped the requested part of the
                 * file, just surrounding data, try again.
                 */
                count_fsb -= imap->br_blockcount;
                map_start_fsb = imap->br_startoff + imap->br_blockcount;
        }

trans_cancel:
        xfs_defer_cancel(&dfops);
        xfs_trans_cancel(tp);
error0:
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        return error;
}

int
xfs_iomap_write_unwritten(
        xfs_inode_t     *ip,
        xfs_off_t       offset,
        xfs_off_t       count)
{
        xfs_mount_t     *mp = ip->i_mount;
        xfs_fileoff_t   offset_fsb;
        xfs_filblks_t   count_fsb;
        xfs_filblks_t   numblks_fsb;
        xfs_fsblock_t   firstfsb;
        int             nimaps;
        xfs_trans_t     *tp;
        xfs_bmbt_irec_t imap;
        struct xfs_defer_ops dfops;
        xfs_fsize_t     i_size;
        uint            resblks;
        int             error;

        trace_xfs_unwritten_convert(ip, offset, count);

        offset_fsb = XFS_B_TO_FSBT(mp, offset);
        count_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + count);
        count_fsb = (xfs_filblks_t)(count_fsb - offset_fsb);

        /*
         * Reserve enough blocks in this transaction for two complete extent
         * btree splits.  We may be converting the middle part of an unwritten
         * extent and in this case we will insert two new extents in the btree
         * each of which could cause a full split.
         *
         * This reservation amount will be used in the first call to
         * xfs_bmbt_split() to select an AG with enough space to satisfy the
         * rest of the operation.
         */
        resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;

        do {
                /*
                 * Set up a transaction to convert the range of extents
                 * from unwritten to real. Do allocations in a loop until
                 * we have covered the range passed in.
                 *
                 * Note that we can't risk to recursing back into the filesystem
                 * here as we might be asked to write out the same inode that we
                 * complete here and might deadlock on the iolock.
                 */
                error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,
                                XFS_TRANS_RESERVE | XFS_TRANS_NOFS, &tp);
                if (error)
                        return error;

                xfs_ilock(ip, XFS_ILOCK_EXCL);
                xfs_trans_ijoin(tp, ip, 0);

                /*
                 * Modify the unwritten extent state of the buffer.
                 */
                xfs_defer_init(&dfops, &firstfsb);
                nimaps = 1;
                error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
                                        XFS_BMAPI_CONVERT, &firstfsb, resblks,
                                        &imap, &nimaps, &dfops);
                if (error)
                        goto error_on_bmapi_transaction;

                /*
                 * Log the updated inode size as we go.  We have to be careful
                 * to only log it up to the actual write offset if it is
                 * halfway into a block.
                 */
                i_size = XFS_FSB_TO_B(mp, offset_fsb + count_fsb);
                if (i_size > offset + count)
                        i_size = offset + count;

                i_size = xfs_new_eof(ip, i_size);
                if (i_size) {
                        ip->i_d.di_size = i_size;
                        xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
                }

                error = xfs_defer_finish(&tp, &dfops, NULL);
                if (error)
                        goto error_on_bmapi_transaction;

                error = xfs_trans_commit(tp);
                xfs_iunlock(ip, XFS_ILOCK_EXCL);
                if (error)
                        return error;

                if (!(imap.br_startblock || XFS_IS_REALTIME_INODE(ip)))
                        return xfs_alert_fsblock_zero(ip, &imap);

                if ((numblks_fsb = imap.br_blockcount) == 0) {
                        /*
                         * The numblks_fsb value should always get
                         * smaller, otherwise the loop is stuck.
                         */
                        ASSERT(imap.br_blockcount);
                        break;
                }
                offset_fsb += numblks_fsb;
                count_fsb -= numblks_fsb;
        } while (count_fsb > 0);

        return 0;

error_on_bmapi_transaction:
        xfs_defer_cancel(&dfops);
        xfs_trans_cancel(tp);
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        return error;
}

void
xfs_bmbt_to_iomap(
        struct xfs_inode        *ip,
        struct iomap            *iomap,
        struct xfs_bmbt_irec    *imap)
{
        struct xfs_mount        *mp = ip->i_mount;

        if (imap->br_startblock == HOLESTARTBLOCK) {
                iomap->blkno = IOMAP_NULL_BLOCK;
                iomap->type = IOMAP_HOLE;
        } else if (imap->br_startblock == DELAYSTARTBLOCK) {
                iomap->blkno = IOMAP_NULL_BLOCK;
                iomap->type = IOMAP_DELALLOC;
        } else {
                iomap->blkno = xfs_fsb_to_db(ip, imap->br_startblock);
                if (imap->br_state == XFS_EXT_UNWRITTEN)
                        iomap->type = IOMAP_UNWRITTEN;
                else
                        iomap->type = IOMAP_MAPPED;
        }
        iomap->offset = XFS_FSB_TO_B(mp, imap->br_startoff);
        iomap->length = XFS_FSB_TO_B(mp, imap->br_blockcount);
        iomap->bdev = xfs_find_bdev_for_inode(VFS_I(ip));
}

static inline bool imap_needs_alloc(struct xfs_bmbt_irec *imap, int nimaps)
{
        return !nimaps ||
                imap->br_startblock == HOLESTARTBLOCK ||
                imap->br_startblock == DELAYSTARTBLOCK;
}

static int
xfs_file_iomap_begin(
        struct inode            *inode,
        loff_t                  offset,
        loff_t                  length,
        unsigned                flags,
        struct iomap            *iomap)
{
        struct xfs_inode        *ip = XFS_I(inode);
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_bmbt_irec    imap;
        xfs_fileoff_t           offset_fsb, end_fsb;
        int                     nimaps = 1, error = 0;

        if (XFS_FORCED_SHUTDOWN(mp))
                return -EIO;

        xfs_ilock(ip, XFS_ILOCK_EXCL);

        ASSERT(offset <= mp->m_super->s_maxbytes);
        if ((xfs_fsize_t)offset + length > mp->m_super->s_maxbytes)
                length = mp->m_super->s_maxbytes - offset;
        offset_fsb = XFS_B_TO_FSBT(mp, offset);
        end_fsb = XFS_B_TO_FSB(mp, offset + length);

        error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
                               &nimaps, XFS_BMAPI_ENTIRE);
        if (error) {
                xfs_iunlock(ip, XFS_ILOCK_EXCL);
                return error;
        }

        if ((flags & IOMAP_WRITE) && imap_needs_alloc(&imap, nimaps)) {
                /*
                 * We cap the maximum length we map here to MAX_WRITEBACK_PAGES
                 * pages to keep the chunks of work done where somewhat symmetric
                 * with the work writeback does. This is a completely arbitrary
                 * number pulled out of thin air as a best guess for initial
                 * testing.
                 *
                 * Note that the values needs to be less than 32-bits wide until
                 * the lower level functions are updated.
                 */
                length = min_t(loff_t, length, 1024 * PAGE_SIZE);
                if (xfs_get_extsz_hint(ip)) {
                        /*
                         * xfs_iomap_write_direct() expects the shared lock. It
                         * is unlocked on return.
                         */
                        xfs_ilock_demote(ip, XFS_ILOCK_EXCL);
                        error = xfs_iomap_write_direct(ip, offset, length, &imap,
                                        nimaps);
                } else {
                        error = xfs_iomap_write_delay(ip, offset, length, &imap);
                        xfs_iunlock(ip, XFS_ILOCK_EXCL);
                }

                if (error)
                        return error;

                trace_xfs_iomap_alloc(ip, offset, length, 0, &imap);
        } else {
                ASSERT(nimaps);

                xfs_iunlock(ip, XFS_ILOCK_EXCL);
                trace_xfs_iomap_found(ip, offset, length, 0, &imap);
        }

        xfs_bmbt_to_iomap(ip, iomap, &imap);
        return 0;
}

static int
xfs_file_iomap_end_delalloc(
        struct xfs_inode        *ip,
        loff_t                  offset,
        loff_t                  length,
        ssize_t                 written)
{
        struct xfs_mount        *mp = ip->i_mount;
        xfs_fileoff_t           start_fsb;
        xfs_fileoff_t           end_fsb;
        int                     error = 0;

        start_fsb = XFS_B_TO_FSB(mp, offset + written);
        end_fsb = XFS_B_TO_FSB(mp, offset + length);

        /*
         * Trim back delalloc blocks if we didn't manage to write the whole
         * range reserved.
         *
         * We don't need to care about racing delalloc as we hold i_mutex
         * across the reserve/allocate/unreserve calls. If there are delalloc
         * blocks in the range, they are ours.
         */
        if (start_fsb < end_fsb) {
                xfs_ilock(ip, XFS_ILOCK_EXCL);
                error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
                                               end_fsb - start_fsb);
                xfs_iunlock(ip, XFS_ILOCK_EXCL);

                if (error && !XFS_FORCED_SHUTDOWN(mp)) {
                        xfs_alert(mp, "%s: unable to clean up ino %lld",
                                __func__, ip->i_ino);
                        return error;
                }
        }

        return 0;
}

static int
xfs_file_iomap_end(
        struct inode            *inode,
        loff_t                  offset,
        loff_t                  length,
        ssize_t                 written,
        unsigned                flags,
        struct iomap            *iomap)
{
        if ((flags & IOMAP_WRITE) && iomap->type == IOMAP_DELALLOC)
                return xfs_file_iomap_end_delalloc(XFS_I(inode), offset,
                                length, written);
        return 0;
}

struct iomap_ops xfs_iomap_ops = {
        .iomap_begin            = xfs_file_iomap_begin,
        .iomap_end              = xfs_file_iomap_end,
};

static int
xfs_xattr_iomap_begin(
        struct inode            *inode,
        loff_t                  offset,
        loff_t                  length,
        unsigned                flags,
        struct iomap            *iomap)
{
        struct xfs_inode        *ip = XFS_I(inode);
        struct xfs_mount        *mp = ip->i_mount;
        xfs_fileoff_t           offset_fsb = XFS_B_TO_FSBT(mp, offset);
        xfs_fileoff_t           end_fsb = XFS_B_TO_FSB(mp, offset + length);
        struct xfs_bmbt_irec    imap;
        int                     nimaps = 1, error = 0;
        unsigned                lockmode;

        if (XFS_FORCED_SHUTDOWN(mp))
                return -EIO;

        lockmode = xfs_ilock_data_map_shared(ip);

        /* if there are no attribute fork or extents, return ENOENT */
        if (XFS_IFORK_Q(ip) || !ip->i_d.di_anextents) {
                error = -ENOENT;
                goto out_unlock;
        }

        ASSERT(ip->i_d.di_aformat != XFS_DINODE_FMT_LOCAL);
        error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
                               &nimaps, XFS_BMAPI_ENTIRE | XFS_BMAPI_ATTRFORK);
out_unlock:
        xfs_iunlock(ip, lockmode);

        if (!error) {
                ASSERT(nimaps);
                xfs_bmbt_to_iomap(ip, iomap, &imap);
        }

        return error;
}

struct iomap_ops xfs_xattr_iomap_ops = {
        .iomap_begin            = xfs_xattr_iomap_begin,
};