Re: [PATCH v2] block: try slab allocation in bio_alloc_bioset() before mempool

[Joseph Qi]

Gentle ping...

On 7/9/26 10:01 AM, Joseph Qi wrote:
> 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 b520c4eef83d ("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: b520c4eef83d ("block: split bio_alloc_bioset more clearly into a fast and slowpath")
> Suggested-by: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
> ---
>  block/bio.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index f2a5f4d0a9672..982f2e47a4218 100644
> --- a/block/bio.c
> +++ b/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;