block: try slab allocation in bio_alloc_bioset() before mempool

When the per-CPU bio cache is enabled but empty, bio_alloc_percpu_cache()
returns NULL and bio_alloc_bioset() falls straight through to the mempool
fallback:

    if (unlikely(!bio)) {
        if (!(saved_gfp & __GFP_DIRECT_RECLAIM))
            return NULL;
        ...
    }

For non-sleeping allocations (no __GFP_DIRECT_RECLAIM) this returns NULL
without ever attempting a slab allocation, even when there is plenty of
free memory.

Commit b520c4e ("block: split bio_alloc_bioset more clearly into a
fast and slowpath") introduced this. Before it, a percpu cache miss fell
through to mempool_alloc(), which attempted the underlying slab allocation
first and only failed when that slab allocation failed. The restructuring
dropped the slab attempt that non-sleeping callers of a cache-enabled
bioset (such as the default fs_bio_set used by bio_alloc()) relied on.

Try a slab allocation with optimistic GFP_ flags before falling back to
the mempool whenever the bio is still NULL, so both the cache-empty and
non-cache paths share the same slab attempt. This restores the previous
behavior for non-sleeping allocations.

Fixes: b520c4e ("block: split bio_alloc_bioset more clearly into a fast and slowpath")
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260709020145.4011533-1-joseph.qi@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: josephhz

block/bio.c

@@ -555,6 +555,14 @@ struct bio *bio_alloc_bioset(struct block_device *bdev, unsigned short nr_vecs,
 		bio = bio_alloc_percpu_cache(bs);
 	} else {
 		opf &= ~REQ_ALLOC_CACHE;
+	}
+
+	/*
+	 * For a bioset without a percpu cache, or when the percpu cache was
+	 * empty, try a slab allocation with optimistic GFP_ flags before
+	 * falling back to the mempool.
+	 */
+	if (!bio) {
 		p = kmem_cache_alloc(bs->bio_slab, gfp);
 		if (p)
 			bio = p + bs->front_pad;