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Fix vdev_queue_aggregate() deadlock

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This deadlock may manifest itself in slightly different ways but
at the core it is caused by a memory allocation blocking on file-
system reclaim in the zio pipeline.  This is normally impossible
because zio_execute() disables filesystem reclaim by setting
PF_FSTRANS on the thread.  However, kmem cache allocations may
still indirectly block on file system reclaim while holding the
critical vq->vq_lock as shown below.

To resolve this issue zio_buf_alloc_flags() is introduced which
allocation flags to be passed.  This can then be used in
vdev_queue_aggregate() with KM_NOSLEEP when allocating the
aggregate IO buffer.  Since aggregating the IO is purely a
performance optimization we want this to either succeed or fail
quickly.  Trying too hard to allocate this memory under the
vq->vq_lock can negatively impact performance and result in
this deadlock.

* z_wr_iss
zio_vdev_io_start
  vdev_queue_io -> Takes vq->vq_lock
    vdev_queue_io_to_issue
      vdev_queue_aggregate
        zio_buf_alloc -> Waiting on spl_kmem_cache process

* z_wr_int
zio_vdev_io_done
  vdev_queue_io_done
    mutex_lock -> Waiting on vq->vq_lock held by z_wr_iss

* txg_sync
spa_sync
  dsl_pool_sync
    zio_wait -> Waiting on zio being handled by z_wr_int

* spl_kmem_cache
spl_cache_grow_work
  kv_alloc
    spl_vmalloc
      ...
      evict
        zpl_evict_inode
          zfs_inactive
            dmu_tx_wait
              txg_wait_open -> Waiting on txg_sync

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #3808
Closes #3867
This commit is contained in:
Brian Behlendorf
2015-12-16 11:28:15 -08:00
parent a8ad3bf02c
commit 6fe53787f3
3 changed files with 22 additions and 1 deletions
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module/zfs/zio.c
+15
View File
@@ -248,6 +248,20 @@ zio_data_buf_alloc(size_t size)
return (kmem_cache_alloc(zio_data_buf_cache[c], KM_PUSHPAGE));
}
/*
* Use zio_buf_alloc_flags when specific allocation flags are needed. e.g.
* passing KM_NOSLEEP when it is acceptable for an allocation to fail.
*/
void *
zio_buf_alloc_flags(size_t size, int flags)
{
size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
return (kmem_cache_alloc(zio_buf_cache[c], flags));
}
void
zio_buf_free(void *buf, size_t size)
{
@@ -3475,6 +3489,7 @@ zbookmark_is_before(const dnode_phys_t *dnp, const zbookmark_phys_t *zb1,
EXPORT_SYMBOL(zio_type_name);
EXPORT_SYMBOL(zio_buf_alloc);
EXPORT_SYMBOL(zio_data_buf_alloc);
EXPORT_SYMBOL(zio_buf_alloc_flags);
EXPORT_SYMBOL(zio_buf_free);
EXPORT_SYMBOL(zio_data_buf_free);