nvdimm: virtio_pmem: stop allocating child flush bio

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>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
index 28f086c4a187..d929d83abf3b 100644
--- a/include/linux/libnvdimm.h
+++ b/include/linux/libnvdimm.h
@@ -126,6 +126,15 @@ struct nd_mapping_desc {
 struct bio;
 struct resource;
 struct nd_region;
+
+/*
+ * Provider flush callback return values:
+ *   0: flush completed synchronously
+ *  <0: flush failed
+ *  >0: flush completion was queued and @bio will be completed later
+ */
+#define NVDIMM_FLUSH_ASYNC 1
+
 struct nd_region_desc {
 	struct resource *res;
 	struct nd_mapping_desc *mapping;