Re: [PATCH] bcache: fix available bucket accounting in bucket allocation

[Robert Pang]

On Mon, Nov 2, 2026 at 11:04 PM Robert Pang <robertpang@xxxxxxxxxx> wrote:
>
> In bch_bucket_alloc(), a cache set's avail_nbuckets is decremented whenever a
> bucket is allocated, assuming that the bucket becomes unavailable once
> allocated. However, when a bucket is allocated but the GC mark is unset or
> reclaimable, it is available still. This false counter decrement leads to an
> inflated in-use measurement, which triggers cache bypasses once CUTOFF_CACHE_ADD
> is reached, persisting until the next garbage collection cycle.
>
> Fix this by ensuring avail_nbuckets is only decremented when the allocated
> bucket is already marked and not in the GC_MARK_RECLAIMABLE state.
>
> Signed-off-by: Robert Pang <robertpang@xxxxxxxxxx>
> ---
>  drivers/md/bcache/alloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
> index 3ec240ec685d..2d0cb9b414ac 100644
> --- a/drivers/md/bcache/alloc.c
> +++ b/drivers/md/bcache/alloc.c
> @@ -446,7 +446,8 @@ long bch_bucket_alloc(struct cache *ca, unsigned int reserve, bool wait)
>                 b->prio = INITIAL_PRIO;
>         }
>
> -       if (ca->set->avail_nbuckets > 0) {
> +       if (ca->set->avail_nbuckets > 0 &&
> +           GC_MARK(b) && GC_MARK(b) != GC_MARK_RECLAIMABLE) {
>                 ca->set->avail_nbuckets--;
>                 bch_update_bucket_in_use(ca->set, &ca->set->gc_stats);
>         }
> --
> 2.56.0.398.g882cbad275-goog
>

Hi all,

Here is some more context on this fix. This issue was observed during
the investigation of [1]. We believe this is a latent issue exposed by
longer garbage collection durations following commit 009073806fbc
("bcache: reduce gc latency by processing less nodes and sleep less
time") to reduce front IO tail latency during GC.

Best regards,
Robert Pang

[1] https://lore.kernel.org/linux-bcache/CAJhEC04+1tu_vGVNfc0TicEb9T2wPpFPLqmQRqX0G_k7M3sBwQ@xxxxxxxxxxxxxx/T/#m0be3c1a4a63427ed7b233a3f753a55fcded69d05