Reduce need for contiguous memory for ioctls

We've had cases where we trigger an OOM despite having memory freely
available on the system. For example, here, we had about 21GB free:

    kernel: Node 0 Normal: 2418758*4kB (UME) 1549533*8kB (UE) 0*16kB
    0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB =
    22071296kB

The problem being, all the memory is in 4K and 8K contiguous regions,
but the allocation request was for a 16K contiguous region:

    kernel: SafeExecutors-4 invoked oom-killer:
    gfp_mask=0x42dc0(GFP_KERNEL|__GFP_NOWARN|__GFP_COMP|__GFP_ZERO),
    order=2, oom_score_adj=0

The offending allocation came from this call trace:

    kernel: Call Trace:
    kernel:  dump_stack+0x57/0x7a
    kernel:  dump_header+0x4f/0x1e1
    kernel:  oom_kill_process.cold.33+0xb/0x10
    kernel:  out_of_memory+0x1ad/0x490
    kernel:  __alloc_pages_slowpath+0xd55/0xe40
    kernel:  __alloc_pages_nodemask+0x2df/0x330
    kernel:  kmalloc_large_node+0x42/0x90
    kernel:  __kmalloc_node+0x25a/0x320
    kernel:  ? spl_kmem_free_impl+0x21/0x30 [spl]
    kernel:  spl_kmem_alloc_impl+0xa5/0x100 [spl]
    kernel:  spl_kmem_zalloc+0x19/0x20 [spl]
    kernel:  zfsdev_ioctl+0x2b/0xe0 [zfs]
    kernel:  do_vfs_ioctl+0xa9/0x640
    kernel:  ? __audit_syscall_entry+0xdd/0x130
    kernel:  ksys_ioctl+0x67/0x90
    kernel:  __x64_sys_ioctl+0x1a/0x20
    kernel:  do_syscall_64+0x5e/0x200
    kernel:  entry_SYSCALL_64_after_hwframe+0x44/0xa9
    kernel: RIP: 0033:0x7fdca3674317

The problem is, for each ioctl that ZFS makes, it has to allocate a
zfs_cmd_t structure, which is 13744 bytes in size (on my system):

    sdb> sizeof zfs_cmd
    (size_t)13744

This size, coupled with the fact that we currently allocate it with
kmem_zalloc, means we need a 16K contiguous region of memory to satisfy
the request.

The solution taken by this change, is to use "vmem" instead of "kmem" to
do the allocation, such that we don't necessarily need a contiguous 16K
memory region to satisfy the allocation.

Arguably, a better solution would be not to require such a large
allocation to begin with (e.g. reduce the size of the zfs_cmd_t
structure), but that'd be a much larger change than this "one liner".
Thus, I've opted for this approach for now; we can always circle back
and attempt to reduce the size of the structure in the future.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes #14474

diff --git a/module/os/freebsd/zfs/kmod_core.c b/module/os/freebsd/zfs/kmod_core.c
index a1b1c8e4e6d9bf138f34acc7a5732b1b9e7e00f3..f4c87013dbf085b755ef1c112f83fb87b1770bd3 100644 (file)
--- a/module/os/freebsd/zfs/kmod_core.c
+++ b/module/os/freebsd/zfs/kmod_core.c
@@ -142,7 +142,7 @@ zfsdev_ioctl(struct cdev *dev, ulong_t zcmd, caddr_t arg, int flag,
                return (EINVAL);
 
        uaddr = (void *)zp->zfs_cmd;
-       zc = kmem_zalloc(sizeof (zfs_cmd_t), KM_SLEEP);
+       zc = vmem_zalloc(sizeof (zfs_cmd_t), KM_SLEEP);
 #ifdef ZFS_LEGACY_SUPPORT
        /*
         * Remap ioctl code for legacy user binaries
@@ -150,10 +150,10 @@ zfsdev_ioctl(struct cdev *dev, ulong_t zcmd, caddr_t arg, int flag,
        if (zp->zfs_ioctl_version == ZFS_IOCVER_LEGACY) {
                vecnum = zfs_ioctl_legacy_to_ozfs(vecnum);
                if (vecnum < 0) {
-                       kmem_free(zc, sizeof (zfs_cmd_t));
+                       vmem_free(zc, sizeof (zfs_cmd_t));
                        return (ENOTSUP);
                }
-               zcl = kmem_zalloc(sizeof (zfs_cmd_legacy_t), KM_SLEEP);
+               zcl = vmem_zalloc(sizeof (zfs_cmd_legacy_t), KM_SLEEP);
                if (copyin(uaddr, zcl, sizeof (zfs_cmd_legacy_t))) {
                        error = SET_ERROR(EFAULT);
                        goto out;
@@ -180,9 +180,9 @@ zfsdev_ioctl(struct cdev *dev, ulong_t zcmd, caddr_t arg, int flag,
 out:
 #ifdef ZFS_LEGACY_SUPPORT
        if (zcl)
-               kmem_free(zcl, sizeof (zfs_cmd_legacy_t));
+               vmem_free(zcl, sizeof (zfs_cmd_legacy_t));
 #endif
-       kmem_free(zc, sizeof (zfs_cmd_t));
+       vmem_free(zc, sizeof (zfs_cmd_t));
        MPASS(tsd_get(rrw_tsd_key) == NULL);
        return (error);
 }

diff --git a/module/os/linux/zfs/zfs_ioctl_os.c b/module/os/linux/zfs/zfs_ioctl_os.c
index f8d1777b07a6f3593beaa884241261b0682b35f3..f068f544f0ecf47caa2a34353a335d4766e71413 100644 (file)
--- a/module/os/linux/zfs/zfs_ioctl_os.c
+++ b/module/os/linux/zfs/zfs_ioctl_os.c
@@ -135,7 +135,7 @@ zfsdev_ioctl(struct file *filp, unsigned cmd, unsigned long arg)
 
        vecnum = cmd - ZFS_IOC_FIRST;
 
-       zc = kmem_zalloc(sizeof (zfs_cmd_t), KM_SLEEP);
+       zc = vmem_zalloc(sizeof (zfs_cmd_t), KM_SLEEP);
 
        if (ddi_copyin((void *)(uintptr_t)arg, zc, sizeof (zfs_cmd_t), 0)) {
                error = -SET_ERROR(EFAULT);
@@ -146,7 +146,7 @@ zfsdev_ioctl(struct file *filp, unsigned cmd, unsigned long arg)
        if (error == 0 && rc != 0)
                error = -SET_ERROR(EFAULT);
 out:
-       kmem_free(zc, sizeof (zfs_cmd_t));
+       vmem_free(zc, sizeof (zfs_cmd_t));
        return (error);
 
 }