Re: [PATCH 4/4] netfs: Fix folio_queue ENOMEM in writeback by adding a mempool

[Zhou, Yun]

On 7/27/2026 9:07 PM, David Howells wrote:
> Fix the handling of folio_queue allocation failure in writeback by adding a
> mempool and passing in gfp_t flags to the rolling buffer functions that
> allocate memory, using the mempool if gfp != GFP_KERNEL.
>
> This is then extended upwards and the gfp to be used for a request is stored
> in the netfs_io_request struct and is then used for both requests and
> subrequests, eliminating the sleeping loops there.

Have you considered splitting this into multiple patches based on 
different functionalities?

> @@ -200,12 +205,14 @@ struct netfs_io_subrequest *netfs_alloc_subrequest(struct netfs_io_request *rreq
>          mempool_t *mempool = rreq->netfs_ops->subrequest_pool ?: &netfs_subrequest_pool;
>          struct kmem_cache *cache = mempool->pool_data;
>
> -       for (;;) {
> -               subreq = mempool_alloc(rreq->netfs_ops->subrequest_pool ?: &netfs_subrequest_pool,
> -                                      GFP_KERNEL);
> -               if (subreq)
> -                       break;
> -               msleep(10);
> +       if (rreq->gfp == GFP_KERNEL) {

Direct equality checks are not reliable; using !(rreq->gfp & GFP_NOFS) 
for the check is more robust.

> +               subreq = mempool->alloc(rreq->gfp, mempool->pool_data);
> +               if (!subreq)
> +                       return ERR_PTR(-ENOMEM);

Since some callers check for success by testing !NULL, we cannot return 
ENOMEM.

221         subreq = netfs_alloc_subrequest(rreq);
222         if (!subreq) {
223             ret = -ENOMEM;
224             break;

> +       } else {
> +               subreq = mempool_alloc(mempool, rreq->gfp);
> +               if (!subreq)
> +                       return NULL;

Redundant check. If it is added to mirror the GFP_KERNEL path above, 
returning the same error code would be better for the caller to handle. 
Or perhaps this is an intentional design?