dax: Define DAX lock bit for radix tree exceptional entry

[mirror_ubuntu-bionic-kernel.git] / fs / dax.c

/*
 * fs/dax.c - Direct Access filesystem code
 * Copyright (c) 2013-2014 Intel Corporation
 * Author: Matthew Wilcox <matthew.r.wilcox@intel.com>
 * Author: Ross Zwisler <ross.zwisler@linux.intel.com>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms and conditions of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope it will 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.
 */

#include <linux/atomic.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>
#include <linux/dax.h>
#include <linux/fs.h>
#include <linux/genhd.h>
#include <linux/highmem.h>
#include <linux/memcontrol.h>
#include <linux/mm.h>
#include <linux/mutex.h>
#include <linux/pagevec.h>
#include <linux/pmem.h>
#include <linux/sched.h>
#include <linux/uio.h>
#include <linux/vmstat.h>
#include <linux/pfn_t.h>
#include <linux/sizes.h>

/*
 * We use lowest available bit in exceptional entry for locking, other two
 * bits to determine entry type. In total 3 special bits.
 */
#define RADIX_DAX_SHIFT (RADIX_TREE_EXCEPTIONAL_SHIFT + 3)
#define RADIX_DAX_PTE (1 << (RADIX_TREE_EXCEPTIONAL_SHIFT + 1))
#define RADIX_DAX_PMD (1 << (RADIX_TREE_EXCEPTIONAL_SHIFT + 2))
#define RADIX_DAX_TYPE_MASK (RADIX_DAX_PTE | RADIX_DAX_PMD)
#define RADIX_DAX_TYPE(entry) ((unsigned long)entry & RADIX_DAX_TYPE_MASK)
#define RADIX_DAX_SECTOR(entry) (((unsigned long)entry >> RADIX_DAX_SHIFT))
#define RADIX_DAX_ENTRY(sector, pmd) ((void *)((unsigned long)sector << \
                RADIX_DAX_SHIFT | (pmd ? RADIX_DAX_PMD : RADIX_DAX_PTE) | \
                RADIX_TREE_EXCEPTIONAL_ENTRY))

static long dax_map_atomic(struct block_device *bdev, struct blk_dax_ctl *dax)
{
        struct request_queue *q = bdev->bd_queue;
        long rc = -EIO;

        dax->addr = (void __pmem *) ERR_PTR(-EIO);
        if (blk_queue_enter(q, true) != 0)
                return rc;

        rc = bdev_direct_access(bdev, dax);
        if (rc < 0) {
                dax->addr = (void __pmem *) ERR_PTR(rc);
                blk_queue_exit(q);
                return rc;
        }
        return rc;
}

static void dax_unmap_atomic(struct block_device *bdev,
                const struct blk_dax_ctl *dax)
{
        if (IS_ERR(dax->addr))
                return;
        blk_queue_exit(bdev->bd_queue);
}

struct page *read_dax_sector(struct block_device *bdev, sector_t n)
{
        struct page *page = alloc_pages(GFP_KERNEL, 0);
        struct blk_dax_ctl dax = {
                .size = PAGE_SIZE,
                .sector = n & ~((((int) PAGE_SIZE) / 512) - 1),
        };
        long rc;

        if (!page)
                return ERR_PTR(-ENOMEM);

        rc = dax_map_atomic(bdev, &dax);
        if (rc < 0)
                return ERR_PTR(rc);
        memcpy_from_pmem(page_address(page), dax.addr, PAGE_SIZE);
        dax_unmap_atomic(bdev, &dax);
        return page;
}

static bool buffer_written(struct buffer_head *bh)
{
        return buffer_mapped(bh) && !buffer_unwritten(bh);
}

/*
 * When ext4 encounters a hole, it returns without modifying the buffer_head
 * which means that we can't trust b_size.  To cope with this, we set b_state
 * to 0 before calling get_block and, if any bit is set, we know we can trust
 * b_size.  Unfortunate, really, since ext4 knows precisely how long a hole is
 * and would save us time calling get_block repeatedly.
 */
static bool buffer_size_valid(struct buffer_head *bh)
{
        return bh->b_state != 0;
}


static sector_t to_sector(const struct buffer_head *bh,
                const struct inode *inode)
{
        sector_t sector = bh->b_blocknr << (inode->i_blkbits - 9);

        return sector;
}

static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
                      loff_t start, loff_t end, get_block_t get_block,
                      struct buffer_head *bh)
{
        loff_t pos = start, max = start, bh_max = start;
        bool hole = false, need_wmb = false;
        struct block_device *bdev = NULL;
        int rw = iov_iter_rw(iter), rc;
        long map_len = 0;
        struct blk_dax_ctl dax = {
                .addr = (void __pmem *) ERR_PTR(-EIO),
        };
        unsigned blkbits = inode->i_blkbits;
        sector_t file_blks = (i_size_read(inode) + (1 << blkbits) - 1)
                                                                >> blkbits;

        if (rw == READ)
                end = min(end, i_size_read(inode));

        while (pos < end) {
                size_t len;
                if (pos == max) {
                        long page = pos >> PAGE_SHIFT;
                        sector_t block = page << (PAGE_SHIFT - blkbits);
                        unsigned first = pos - (block << blkbits);
                        long size;

                        if (pos == bh_max) {
                                bh->b_size = PAGE_ALIGN(end - pos);
                                bh->b_state = 0;
                                rc = get_block(inode, block, bh, rw == WRITE);
                                if (rc)
                                        break;
                                if (!buffer_size_valid(bh))
                                        bh->b_size = 1 << blkbits;
                                bh_max = pos - first + bh->b_size;
                                bdev = bh->b_bdev;
                                /*
                                 * We allow uninitialized buffers for writes
                                 * beyond EOF as those cannot race with faults
                                 */
                                WARN_ON_ONCE(
                                        (buffer_new(bh) && block < file_blks) ||
                                        (rw == WRITE && buffer_unwritten(bh)));
                        } else {
                                unsigned done = bh->b_size -
                                                (bh_max - (pos - first));
                                bh->b_blocknr += done >> blkbits;
                                bh->b_size -= done;
                        }

                        hole = rw == READ && !buffer_written(bh);
                        if (hole) {
                                size = bh->b_size - first;
                        } else {
                                dax_unmap_atomic(bdev, &dax);
                                dax.sector = to_sector(bh, inode);
                                dax.size = bh->b_size;
                                map_len = dax_map_atomic(bdev, &dax);
                                if (map_len < 0) {
                                        rc = map_len;
                                        break;
                                }
                                dax.addr += first;
                                size = map_len - first;
                        }
                        max = min(pos + size, end);
                }

                if (iov_iter_rw(iter) == WRITE) {
                        len = copy_from_iter_pmem(dax.addr, max - pos, iter);
                        need_wmb = true;
                } else if (!hole)
                        len = copy_to_iter((void __force *) dax.addr, max - pos,
                                        iter);
                else
                        len = iov_iter_zero(max - pos, iter);

                if (!len) {
                        rc = -EFAULT;
                        break;
                }

                pos += len;
                if (!IS_ERR(dax.addr))
                        dax.addr += len;
        }

        if (need_wmb)
                wmb_pmem();
        dax_unmap_atomic(bdev, &dax);

        return (pos == start) ? rc : pos - start;
}

/**
 * dax_do_io - Perform I/O to a DAX file
 * @iocb: The control block for this I/O
 * @inode: The file which the I/O is directed at
 * @iter: The addresses to do I/O from or to
 * @pos: The file offset where the I/O starts
 * @get_block: The filesystem method used to translate file offsets to blocks
 * @end_io: A filesystem callback for I/O completion
 * @flags: See below
 *
 * This function uses the same locking scheme as do_blockdev_direct_IO:
 * If @flags has DIO_LOCKING set, we assume that the i_mutex is held by the
 * caller for writes.  For reads, we take and release the i_mutex ourselves.
 * If DIO_LOCKING is not set, the filesystem takes care of its own locking.
 * As with do_blockdev_direct_IO(), we increment i_dio_count while the I/O
 * is in progress.
 */
ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode,
                  struct iov_iter *iter, loff_t pos, get_block_t get_block,
                  dio_iodone_t end_io, int flags)
{
        struct buffer_head bh;
        ssize_t retval = -EINVAL;
        loff_t end = pos + iov_iter_count(iter);

        memset(&bh, 0, sizeof(bh));
        bh.b_bdev = inode->i_sb->s_bdev;

        if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ)
                inode_lock(inode);

        /* Protects against truncate */
        if (!(flags & DIO_SKIP_DIO_COUNT))
                inode_dio_begin(inode);

        retval = dax_io(inode, iter, pos, end, get_block, &bh);

        if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ)
                inode_unlock(inode);

        if (end_io) {
                int err;

                err = end_io(iocb, pos, retval, bh.b_private);
                if (err)
                        retval = err;
        }

        if (!(flags & DIO_SKIP_DIO_COUNT))
                inode_dio_end(inode);
        return retval;
}
EXPORT_SYMBOL_GPL(dax_do_io);

/*
 * The user has performed a load from a hole in the file.  Allocating
 * a new page in the file would cause excessive storage usage for
 * workloads with sparse files.  We allocate a page cache page instead.
 * We'll kick it out of the page cache if it's ever written to,
 * otherwise it will simply fall out of the page cache under memory
 * pressure without ever having been dirtied.
 */
static int dax_load_hole(struct address_space *mapping, struct page *page,
                                                        struct vm_fault *vmf)
{
        if (!page)
                page = find_or_create_page(mapping, vmf->pgoff,
                                                GFP_KERNEL | __GFP_ZERO);
        if (!page)
                return VM_FAULT_OOM;

        vmf->page = page;
        return VM_FAULT_LOCKED;
}

static int copy_user_bh(struct page *to, struct inode *inode,
                struct buffer_head *bh, unsigned long vaddr)
{
        struct blk_dax_ctl dax = {
                .sector = to_sector(bh, inode),
                .size = bh->b_size,
        };
        struct block_device *bdev = bh->b_bdev;
        void *vto;

        if (dax_map_atomic(bdev, &dax) < 0)
                return PTR_ERR(dax.addr);
        vto = kmap_atomic(to);
        copy_user_page(vto, (void __force *)dax.addr, vaddr, to);
        kunmap_atomic(vto);
        dax_unmap_atomic(bdev, &dax);
        return 0;
}

#define NO_SECTOR -1
#define DAX_PMD_INDEX(page_index) (page_index & (PMD_MASK >> PAGE_SHIFT))

static int dax_radix_entry(struct address_space *mapping, pgoff_t index,
                sector_t sector, bool pmd_entry, bool dirty)
{
        struct radix_tree_root *page_tree = &mapping->page_tree;
        pgoff_t pmd_index = DAX_PMD_INDEX(index);
        int type, error = 0;
        void *entry;

        WARN_ON_ONCE(pmd_entry && !dirty);
        if (dirty)
                __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);

        spin_lock_irq(&mapping->tree_lock);

        entry = radix_tree_lookup(page_tree, pmd_index);
        if (entry && RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD) {
                index = pmd_index;
                goto dirty;
        }

        entry = radix_tree_lookup(page_tree, index);
        if (entry) {
                type = RADIX_DAX_TYPE(entry);
                if (WARN_ON_ONCE(type != RADIX_DAX_PTE &&
                                        type != RADIX_DAX_PMD)) {
                        error = -EIO;
                        goto unlock;
                }

                if (!pmd_entry || type == RADIX_DAX_PMD)
                        goto dirty;

                /*
                 * We only insert dirty PMD entries into the radix tree.  This
                 * means we don't need to worry about removing a dirty PTE
                 * entry and inserting a clean PMD entry, thus reducing the
                 * range we would flush with a follow-up fsync/msync call.
                 */
                radix_tree_delete(&mapping->page_tree, index);
                mapping->nrexceptional--;
        }

        if (sector == NO_SECTOR) {
                /*
                 * This can happen during correct operation if our pfn_mkwrite
                 * fault raced against a hole punch operation.  If this
                 * happens the pte that was hole punched will have been
                 * unmapped and the radix tree entry will have been removed by
                 * the time we are called, but the call will still happen.  We
                 * will return all the way up to wp_pfn_shared(), where the
                 * pte_same() check will fail, eventually causing page fault
                 * to be retried by the CPU.
                 */
                goto unlock;
        }

        error = radix_tree_insert(page_tree, index,
                        RADIX_DAX_ENTRY(sector, pmd_entry));
        if (error)
                goto unlock;

        mapping->nrexceptional++;
 dirty:
        if (dirty)
                radix_tree_tag_set(page_tree, index, PAGECACHE_TAG_DIRTY);
 unlock:
        spin_unlock_irq(&mapping->tree_lock);
        return error;
}

static int dax_writeback_one(struct block_device *bdev,
                struct address_space *mapping, pgoff_t index, void *entry)
{
        struct radix_tree_root *page_tree = &mapping->page_tree;
        int type = RADIX_DAX_TYPE(entry);
        struct radix_tree_node *node;
        struct blk_dax_ctl dax;
        void **slot;
        int ret = 0;

        spin_lock_irq(&mapping->tree_lock);
        /*
         * Regular page slots are stabilized by the page lock even
         * without the tree itself locked.  These unlocked entries
         * need verification under the tree lock.
         */
        if (!__radix_tree_lookup(page_tree, index, &node, &slot))
                goto unlock;
        if (*slot != entry)
                goto unlock;

        /* another fsync thread may have already written back this entry */
        if (!radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE))
                goto unlock;

        if (WARN_ON_ONCE(type != RADIX_DAX_PTE && type != RADIX_DAX_PMD)) {
                ret = -EIO;
                goto unlock;
        }

        dax.sector = RADIX_DAX_SECTOR(entry);
        dax.size = (type == RADIX_DAX_PMD ? PMD_SIZE : PAGE_SIZE);
        spin_unlock_irq(&mapping->tree_lock);

        /*
         * We cannot hold tree_lock while calling dax_map_atomic() because it
         * eventually calls cond_resched().
         */
        ret = dax_map_atomic(bdev, &dax);
        if (ret < 0)
                return ret;

        if (WARN_ON_ONCE(ret < dax.size)) {
                ret = -EIO;
                goto unmap;
        }

        wb_cache_pmem(dax.addr, dax.size);

        spin_lock_irq(&mapping->tree_lock);
        radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
        spin_unlock_irq(&mapping->tree_lock);
 unmap:
        dax_unmap_atomic(bdev, &dax);
        return ret;

 unlock:
        spin_unlock_irq(&mapping->tree_lock);
        return ret;
}

/*
 * Flush the mapping to the persistent domain within the byte range of [start,
 * end]. This is required by data integrity operations to ensure file data is
 * on persistent storage prior to completion of the operation.
 */
int dax_writeback_mapping_range(struct address_space *mapping,
                struct block_device *bdev, struct writeback_control *wbc)
{
        struct inode *inode = mapping->host;
        pgoff_t start_index, end_index, pmd_index;
        pgoff_t indices[PAGEVEC_SIZE];
        struct pagevec pvec;
        bool done = false;
        int i, ret = 0;
        void *entry;

        if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT))
                return -EIO;

        if (!mapping->nrexceptional || wbc->sync_mode != WB_SYNC_ALL)
                return 0;

        start_index = wbc->range_start >> PAGE_SHIFT;
        end_index = wbc->range_end >> PAGE_SHIFT;
        pmd_index = DAX_PMD_INDEX(start_index);

        rcu_read_lock();
        entry = radix_tree_lookup(&mapping->page_tree, pmd_index);
        rcu_read_unlock();

        /* see if the start of our range is covered by a PMD entry */
        if (entry && RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
                start_index = pmd_index;

        tag_pages_for_writeback(mapping, start_index, end_index);

        pagevec_init(&pvec, 0);
        while (!done) {
                pvec.nr = find_get_entries_tag(mapping, start_index,
                                PAGECACHE_TAG_TOWRITE, PAGEVEC_SIZE,
                                pvec.pages, indices);

                if (pvec.nr == 0)
                        break;

                for (i = 0; i < pvec.nr; i++) {
                        if (indices[i] > end_index) {
                                done = true;
                                break;
                        }

                        ret = dax_writeback_one(bdev, mapping, indices[i],
                                        pvec.pages[i]);
                        if (ret < 0)
                                return ret;
                }
        }
        wmb_pmem();
        return 0;
}
EXPORT_SYMBOL_GPL(dax_writeback_mapping_range);

static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh,
                        struct vm_area_struct *vma, struct vm_fault *vmf)
{
        unsigned long vaddr = (unsigned long)vmf->virtual_address;
        struct address_space *mapping = inode->i_mapping;
        struct block_device *bdev = bh->b_bdev;
        struct blk_dax_ctl dax = {
                .sector = to_sector(bh, inode),
                .size = bh->b_size,
        };
        int error;

        i_mmap_lock_read(mapping);

        if (dax_map_atomic(bdev, &dax) < 0) {
                error = PTR_ERR(dax.addr);
                goto out;
        }
        dax_unmap_atomic(bdev, &dax);

        error = dax_radix_entry(mapping, vmf->pgoff, dax.sector, false,
                        vmf->flags & FAULT_FLAG_WRITE);
        if (error)
                goto out;

        error = vm_insert_mixed(vma, vaddr, dax.pfn);

 out:
        i_mmap_unlock_read(mapping);

        return error;
}

/**
 * __dax_fault - handle a page fault on a DAX file
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * When a page fault occurs, filesystems may call this helper in their
 * fault handler for DAX files. __dax_fault() assumes the caller has done all
 * the necessary locking for the page fault to proceed successfully.
 */
int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
                        get_block_t get_block)
{
        struct file *file = vma->vm_file;
        struct address_space *mapping = file->f_mapping;
        struct inode *inode = mapping->host;
        struct page *page;
        struct buffer_head bh;
        unsigned long vaddr = (unsigned long)vmf->virtual_address;
        unsigned blkbits = inode->i_blkbits;
        sector_t block;
        pgoff_t size;
        int error;
        int major = 0;

        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (vmf->pgoff >= size)
                return VM_FAULT_SIGBUS;

        memset(&bh, 0, sizeof(bh));
        block = (sector_t)vmf->pgoff << (PAGE_SHIFT - blkbits);
        bh.b_bdev = inode->i_sb->s_bdev;
        bh.b_size = PAGE_SIZE;

 repeat:
        page = find_get_page(mapping, vmf->pgoff);
        if (page) {
                if (!lock_page_or_retry(page, vma->vm_mm, vmf->flags)) {
                        put_page(page);
                        return VM_FAULT_RETRY;
                }
                if (unlikely(page->mapping != mapping)) {
                        unlock_page(page);
                        put_page(page);
                        goto repeat;
                }
        }

        error = get_block(inode, block, &bh, 0);
        if (!error && (bh.b_size < PAGE_SIZE))
                error = -EIO;           /* fs corruption? */
        if (error)
                goto unlock_page;

        if (!buffer_mapped(&bh) && !vmf->cow_page) {
                if (vmf->flags & FAULT_FLAG_WRITE) {
                        error = get_block(inode, block, &bh, 1);
                        count_vm_event(PGMAJFAULT);
                        mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
                        major = VM_FAULT_MAJOR;
                        if (!error && (bh.b_size < PAGE_SIZE))
                                error = -EIO;
                        if (error)
                                goto unlock_page;
                } else {
                        return dax_load_hole(mapping, page, vmf);
                }
        }

        if (vmf->cow_page) {
                struct page *new_page = vmf->cow_page;
                if (buffer_written(&bh))
                        error = copy_user_bh(new_page, inode, &bh, vaddr);
                else
                        clear_user_highpage(new_page, vaddr);
                if (error)
                        goto unlock_page;
                vmf->page = page;
                if (!page)
                        i_mmap_lock_read(mapping);
                return VM_FAULT_LOCKED;
        }

        /* Check we didn't race with a read fault installing a new page */
        if (!page && major)
                page = find_lock_page(mapping, vmf->pgoff);

        if (page) {
                unmap_mapping_range(mapping, vmf->pgoff << PAGE_SHIFT,
                                                        PAGE_SIZE, 0);
                delete_from_page_cache(page);
                unlock_page(page);
                put_page(page);
                page = NULL;
        }

        /* Filesystem should not return unwritten buffers to us! */
        WARN_ON_ONCE(buffer_unwritten(&bh) || buffer_new(&bh));
        error = dax_insert_mapping(inode, &bh, vma, vmf);

 out:
        if (error == -ENOMEM)
                return VM_FAULT_OOM | major;
        /* -EBUSY is fine, somebody else faulted on the same PTE */
        if ((error < 0) && (error != -EBUSY))
                return VM_FAULT_SIGBUS | major;
        return VM_FAULT_NOPAGE | major;

 unlock_page:
        if (page) {
                unlock_page(page);
                put_page(page);
        }
        goto out;
}
EXPORT_SYMBOL(__dax_fault);

/**
 * dax_fault - handle a page fault on a DAX file
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * When a page fault occurs, filesystems may call this helper in their
 * fault handler for DAX files.
 */
int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
              get_block_t get_block)
{
        int result;
        struct super_block *sb = file_inode(vma->vm_file)->i_sb;

        if (vmf->flags & FAULT_FLAG_WRITE) {
                sb_start_pagefault(sb);
                file_update_time(vma->vm_file);
        }
        result = __dax_fault(vma, vmf, get_block);
        if (vmf->flags & FAULT_FLAG_WRITE)
                sb_end_pagefault(sb);

        return result;
}
EXPORT_SYMBOL_GPL(dax_fault);

#if defined(CONFIG_TRANSPARENT_HUGEPAGE)
/*
 * The 'colour' (ie low bits) within a PMD of a page offset.  This comes up
 * more often than one might expect in the below function.
 */
#define PG_PMD_COLOUR   ((PMD_SIZE >> PAGE_SHIFT) - 1)

static void __dax_dbg(struct buffer_head *bh, unsigned long address,
                const char *reason, const char *fn)
{
        if (bh) {
                char bname[BDEVNAME_SIZE];
                bdevname(bh->b_bdev, bname);
                pr_debug("%s: %s addr: %lx dev %s state %lx start %lld "
                        "length %zd fallback: %s\n", fn, current->comm,
                        address, bname, bh->b_state, (u64)bh->b_blocknr,
                        bh->b_size, reason);
        } else {
                pr_debug("%s: %s addr: %lx fallback: %s\n", fn,
                        current->comm, address, reason);
        }
}

#define dax_pmd_dbg(bh, address, reason)        __dax_dbg(bh, address, reason, "dax_pmd")

int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                pmd_t *pmd, unsigned int flags, get_block_t get_block)
{
        struct file *file = vma->vm_file;
        struct address_space *mapping = file->f_mapping;
        struct inode *inode = mapping->host;
        struct buffer_head bh;
        unsigned blkbits = inode->i_blkbits;
        unsigned long pmd_addr = address & PMD_MASK;
        bool write = flags & FAULT_FLAG_WRITE;
        struct block_device *bdev;
        pgoff_t size, pgoff;
        sector_t block;
        int error, result = 0;
        bool alloc = false;

        /* dax pmd mappings require pfn_t_devmap() */
        if (!IS_ENABLED(CONFIG_FS_DAX_PMD))
                return VM_FAULT_FALLBACK;

        /* Fall back to PTEs if we're going to COW */
        if (write && !(vma->vm_flags & VM_SHARED)) {
                split_huge_pmd(vma, pmd, address);
                dax_pmd_dbg(NULL, address, "cow write");
                return VM_FAULT_FALLBACK;
        }
        /* If the PMD would extend outside the VMA */
        if (pmd_addr < vma->vm_start) {
                dax_pmd_dbg(NULL, address, "vma start unaligned");
                return VM_FAULT_FALLBACK;
        }
        if ((pmd_addr + PMD_SIZE) > vma->vm_end) {
                dax_pmd_dbg(NULL, address, "vma end unaligned");
                return VM_FAULT_FALLBACK;
        }

        pgoff = linear_page_index(vma, pmd_addr);
        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (pgoff >= size)
                return VM_FAULT_SIGBUS;
        /* If the PMD would cover blocks out of the file */
        if ((pgoff | PG_PMD_COLOUR) >= size) {
                dax_pmd_dbg(NULL, address,
                                "offset + huge page size > file size");
                return VM_FAULT_FALLBACK;
        }

        memset(&bh, 0, sizeof(bh));
        bh.b_bdev = inode->i_sb->s_bdev;
        block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);

        bh.b_size = PMD_SIZE;

        if (get_block(inode, block, &bh, 0) != 0)
                return VM_FAULT_SIGBUS;

        if (!buffer_mapped(&bh) && write) {
                if (get_block(inode, block, &bh, 1) != 0)
                        return VM_FAULT_SIGBUS;
                alloc = true;
                WARN_ON_ONCE(buffer_unwritten(&bh) || buffer_new(&bh));
        }

        bdev = bh.b_bdev;

        /*
         * If the filesystem isn't willing to tell us the length of a hole,
         * just fall back to PTEs.  Calling get_block 512 times in a loop
         * would be silly.
         */
        if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE) {
                dax_pmd_dbg(&bh, address, "allocated block too small");
                return VM_FAULT_FALLBACK;
        }

        /*
         * If we allocated new storage, make sure no process has any
         * zero pages covering this hole
         */
        if (alloc) {
                loff_t lstart = pgoff << PAGE_SHIFT;
                loff_t lend = lstart + PMD_SIZE - 1; /* inclusive */

                truncate_pagecache_range(inode, lstart, lend);
        }

        i_mmap_lock_read(mapping);

        if (!write && !buffer_mapped(&bh)) {
                spinlock_t *ptl;
                pmd_t entry;
                struct page *zero_page = get_huge_zero_page();

                if (unlikely(!zero_page)) {
                        dax_pmd_dbg(&bh, address, "no zero page");
                        goto fallback;
                }

                ptl = pmd_lock(vma->vm_mm, pmd);
                if (!pmd_none(*pmd)) {
                        spin_unlock(ptl);
                        dax_pmd_dbg(&bh, address, "pmd already present");
                        goto fallback;
                }

                dev_dbg(part_to_dev(bdev->bd_part),
                                "%s: %s addr: %lx pfn: <zero> sect: %llx\n",
                                __func__, current->comm, address,
                                (unsigned long long) to_sector(&bh, inode));

                entry = mk_pmd(zero_page, vma->vm_page_prot);
                entry = pmd_mkhuge(entry);
                set_pmd_at(vma->vm_mm, pmd_addr, pmd, entry);
                result = VM_FAULT_NOPAGE;
                spin_unlock(ptl);
        } else {
                struct blk_dax_ctl dax = {
                        .sector = to_sector(&bh, inode),
                        .size = PMD_SIZE,
                };
                long length = dax_map_atomic(bdev, &dax);

                if (length < 0) {
                        dax_pmd_dbg(&bh, address, "dax-error fallback");
                        goto fallback;
                }
                if (length < PMD_SIZE) {
                        dax_pmd_dbg(&bh, address, "dax-length too small");
                        dax_unmap_atomic(bdev, &dax);
                        goto fallback;
                }
                if (pfn_t_to_pfn(dax.pfn) & PG_PMD_COLOUR) {
                        dax_pmd_dbg(&bh, address, "pfn unaligned");
                        dax_unmap_atomic(bdev, &dax);
                        goto fallback;
                }

                if (!pfn_t_devmap(dax.pfn)) {
                        dax_unmap_atomic(bdev, &dax);
                        dax_pmd_dbg(&bh, address, "pfn not in memmap");
                        goto fallback;
                }
                dax_unmap_atomic(bdev, &dax);

                /*
                 * For PTE faults we insert a radix tree entry for reads, and
                 * leave it clean.  Then on the first write we dirty the radix
                 * tree entry via the dax_pfn_mkwrite() path.  This sequence
                 * allows the dax_pfn_mkwrite() call to be simpler and avoid a
                 * call into get_block() to translate the pgoff to a sector in
                 * order to be able to create a new radix tree entry.
                 *
                 * The PMD path doesn't have an equivalent to
                 * dax_pfn_mkwrite(), though, so for a read followed by a
                 * write we traverse all the way through __dax_pmd_fault()
                 * twice.  This means we can just skip inserting a radix tree
                 * entry completely on the initial read and just wait until
                 * the write to insert a dirty entry.
                 */
                if (write) {
                        error = dax_radix_entry(mapping, pgoff, dax.sector,
                                        true, true);
                        if (error) {
                                dax_pmd_dbg(&bh, address,
                                                "PMD radix insertion failed");
                                goto fallback;
                        }
                }

                dev_dbg(part_to_dev(bdev->bd_part),
                                "%s: %s addr: %lx pfn: %lx sect: %llx\n",
                                __func__, current->comm, address,
                                pfn_t_to_pfn(dax.pfn),
                                (unsigned long long) dax.sector);
                result |= vmf_insert_pfn_pmd(vma, address, pmd,
                                dax.pfn, write);
        }

 out:
        i_mmap_unlock_read(mapping);

        return result;

 fallback:
        count_vm_event(THP_FAULT_FALLBACK);
        result = VM_FAULT_FALLBACK;
        goto out;
}
EXPORT_SYMBOL_GPL(__dax_pmd_fault);

/**
 * dax_pmd_fault - handle a PMD fault on a DAX file
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * When a page fault occurs, filesystems may call this helper in their
 * pmd_fault handler for DAX files.
 */
int dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                        pmd_t *pmd, unsigned int flags, get_block_t get_block)
{
        int result;
        struct super_block *sb = file_inode(vma->vm_file)->i_sb;

        if (flags & FAULT_FLAG_WRITE) {
                sb_start_pagefault(sb);
                file_update_time(vma->vm_file);
        }
        result = __dax_pmd_fault(vma, address, pmd, flags, get_block);
        if (flags & FAULT_FLAG_WRITE)
                sb_end_pagefault(sb);

        return result;
}
EXPORT_SYMBOL_GPL(dax_pmd_fault);
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

/**
 * dax_pfn_mkwrite - handle first write to DAX page
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 */
int dax_pfn_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        struct file *file = vma->vm_file;
        int error;

        /*
         * We pass NO_SECTOR to dax_radix_entry() because we expect that a
         * RADIX_DAX_PTE entry already exists in the radix tree from a
         * previous call to __dax_fault().  We just want to look up that PTE
         * entry using vmf->pgoff and make sure the dirty tag is set.  This
         * saves us from having to make a call to get_block() here to look
         * up the sector.
         */
        error = dax_radix_entry(file->f_mapping, vmf->pgoff, NO_SECTOR, false,
                        true);

        if (error == -ENOMEM)
                return VM_FAULT_OOM;
        if (error)
                return VM_FAULT_SIGBUS;
        return VM_FAULT_NOPAGE;
}
EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);

static bool dax_range_is_aligned(struct block_device *bdev,
                                 unsigned int offset, unsigned int length)
{
        unsigned short sector_size = bdev_logical_block_size(bdev);

        if (!IS_ALIGNED(offset, sector_size))
                return false;
        if (!IS_ALIGNED(length, sector_size))
                return false;

        return true;
}

int __dax_zero_page_range(struct block_device *bdev, sector_t sector,
                unsigned int offset, unsigned int length)
{
        struct blk_dax_ctl dax = {
                .sector         = sector,
                .size           = PAGE_SIZE,
        };

        if (dax_range_is_aligned(bdev, offset, length)) {
                sector_t start_sector = dax.sector + (offset >> 9);

                return blkdev_issue_zeroout(bdev, start_sector,
                                length >> 9, GFP_NOFS, true);
        } else {
                if (dax_map_atomic(bdev, &dax) < 0)
                        return PTR_ERR(dax.addr);
                clear_pmem(dax.addr + offset, length);
                wmb_pmem();
                dax_unmap_atomic(bdev, &dax);
        }
        return 0;
}
EXPORT_SYMBOL_GPL(__dax_zero_page_range);

/**
 * dax_zero_page_range - zero a range within a page of a DAX file
 * @inode: The file being truncated
 * @from: The file offset that is being truncated to
 * @length: The number of bytes to zero
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * This function can be called by a filesystem when it is zeroing part of a
 * page in a DAX file.  This is intended for hole-punch operations.  If
 * you are truncating a file, the helper function dax_truncate_page() may be
 * more convenient.
 */
int dax_zero_page_range(struct inode *inode, loff_t from, unsigned length,
                                                        get_block_t get_block)
{
        struct buffer_head bh;
        pgoff_t index = from >> PAGE_SHIFT;
        unsigned offset = from & (PAGE_SIZE-1);
        int err;

        /* Block boundary? Nothing to do */
        if (!length)
                return 0;
        BUG_ON((offset + length) > PAGE_SIZE);

        memset(&bh, 0, sizeof(bh));
        bh.b_bdev = inode->i_sb->s_bdev;
        bh.b_size = PAGE_SIZE;
        err = get_block(inode, index, &bh, 0);
        if (err < 0 || !buffer_written(&bh))
                return err;

        return __dax_zero_page_range(bh.b_bdev, to_sector(&bh, inode),
                        offset, length);
}
EXPORT_SYMBOL_GPL(dax_zero_page_range);

/**
 * dax_truncate_page - handle a partial page being truncated in a DAX file
 * @inode: The file being truncated
 * @from: The file offset that is being truncated to
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * Similar to block_truncate_page(), this function can be called by a
 * filesystem when it is truncating a DAX file to handle the partial page.
 */
int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
{
        unsigned length = PAGE_ALIGN(from) - from;
        return dax_zero_page_range(inode, from, length, get_block);
}
EXPORT_SYMBOL_GPL(dax_truncate_page);