Re: [PATCH] block: fix bio_alloc_bioset() percpu cache fallback for non-reclaim contexts

[Christoph Hellwig <hch@lst.de>]

On Wed, Jul 08, 2026 at 05:49:21PM +0800, Joseph Qi wrote:
> Looks sane. So commit b520c4eef83d exposed the bug but not introduced it.

Well, there is a bug in virtio_pmem for sure.  But I also think the
bio_alloc_bioset behavior isn't quite optimal for non-sleeping
allocations.  But I'd probably go more for something like this there:

diff --git a/block/bio.c b/block/bio.c
index f2a5f4d0a967..bdca2e60514b 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -555,6 +555,13 @@ struct bio *bio_alloc_bioset(struct block_device *bdev, unsigned short nr_vecs,
 		bio = bio_alloc_percpu_cache(bs);
 	} else {
 		opf &= ~REQ_ALLOC_CACHE;
+	}
+
+	/*
+	 * If the percpu cache was empty, try an slab allocation with optimistic
+	 * GFP_ flags ass well before falling back to the mempool.
+	 */
+	if (!bio) {
 		p = kmem_cache_alloc(bs->bio_slab, gfp);
 		if (p)
 			bio = p + bs->front_pad;