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nvdimm: virtio_pmem: stop allocating child flush bio

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pmem_submit_bio() passes the parent bio to nvdimm_flush() for
REQ_FUA. For virtio-pmem this makes async_pmem_flush() allocate
and submit a child PREFLUSH bio chained to the parent.

That child allocation is in the block submit path. Making it
blocking with GFP_NOIO can consume the same global bio mempool that
submit_bio() uses, while making it GFP_ATOMIC can fail under
pressure. A forced failure of the child allocation produced:

virtio_pmem: forcing child bio allocation failure for test
Buffer I/O error on dev pmem0, logical block 0, lost sync page write
EXT4-fs (pmem0): I/O error while writing superblock
EXT4-fs (pmem0): mount failed

Avoid the child bio without turning REQ_FUA into a synchronous
submit-path wait. Let provider flush callbacks return
NVDIMM_FLUSH_ASYNC after taking ownership of parent bio completion.
pmem_submit_bio() returns in that case, and virtio-pmem queues an
ordered WQ_MEM_RECLAIM work item that runs the existing host flush
path and completes the parent bio.

This keeps the asynchronous completion model of the child-bio path
while removing the child bio allocation from the submit path.

Signed-off-by: Li Chen <me@linux.beauty>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260630092338.2094628-5-me@linux.beauty>
This commit is contained in:
Li Chen 2026-06-30 17:23:29 +08:00 • committed by Michael S. Tsirkin
commit 40f356e610
6 changed files with 73 additions and 18 deletions
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