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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>