[PATCH] virtio-pmem: use GFP_NOIO for flush bio allocation

[Joseph Qi <joseph.qi@linux.alibaba.com>]

async_pmem_flush() allocates a child bio for the flush with GFP_ATOMIC.
This runs from pmem_submit_bio(), a ->submit_bio callback that executes
in a sleepable context, so there is no atomicity requirement here.

bio_alloc() only guarantees success when __GFP_DIRECT_RECLAIM is set,
because that is what lets it fall back to the mempool reserve. With
GFP_ATOMIC the reclaim bit is absent, so the allocation can fail and
return -ENOMEM whenever the fast paths (percpu cache and slab) are
exhausted, which is common right after boot. A flush is issued from
filesystem writeback and must not fail on a transient allocation
shortage, otherwise the device can appear unmountable:

  Buffer I/O error on dev pmem0, logical block 0, lost sync page write

Use GFP_NOIO instead. It keeps __GFP_DIRECT_RECLAIM so the mempool and
rescuer machinery guarantee forward progress, while avoiding recursion
back into the filesystem and block layer during writeback.

Fixes: 6e84200c0a29 ("virtio-pmem: Add virtio pmem driver")
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
---
 drivers/nvdimm/nd_virtio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
index 4176046627beb..081370aac6317 100644
--- a/drivers/nvdimm/nd_virtio.c
+++ b/drivers/nvdimm/nd_virtio.c
@@ -117,7 +117,7 @@ int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
 	if (bio && bio->bi_iter.bi_sector != -1) {
 		struct bio *child = bio_alloc(bio->bi_bdev, 0,
 					      REQ_OP_WRITE | REQ_PREFLUSH,
-					      GFP_ATOMIC);
+					      GFP_NOIO);
 
 		if (!child)
 			return -ENOMEM;
-- 
2.39.3