Re: [PATCH v3 1/1] page_frag: Recover from memory pressure

[Eric Dumazet]

On Sun, Nov 15, 2020 at 9:16 PM Dongli Zhang <dongli.zhang@xxxxxxxxxx> wrote:
>
> The ethernet driver may allocate skb (and skb->data) via napi_alloc_skb().
> This ends up to page_frag_alloc() to allocate skb->data from
> page_frag_cache->va.
>
> During the memory pressure, page_frag_cache->va may be allocated as
> pfmemalloc page. As a result, the skb->pfmemalloc is always true as
> skb->data is from page_frag_cache->va. The skb will be dropped if the
> sock (receiver) does not have SOCK_MEMALLOC. This is expected behaviour
> under memory pressure.
...
> References: https://lore.kernel.org/lkml/20201103193239.1807-1-dongli.zhang@xxxxxxxxxx/
> References: https://lore.kernel.org/linux-mm/20201105042140.5253-1-willy@xxxxxxxxxxxxx/
> Suggested-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Aruna Ramakrishna <aruna.ramakrishna@xxxxxxxxxx>
> Cc: Bert Barbe <bert.barbe@xxxxxxxxxx>
> Cc: Rama Nichanamatlu <rama.nichanamatlu@xxxxxxxxxx>
> Cc: Venkat Venkatsubra <venkat.x.venkatsubra@xxxxxxxxxx>
> Cc: Manjunath Patil <manjunath.b.patil@xxxxxxxxxx>
> Cc: Joe Jin <joe.jin@xxxxxxxxxx>
> Cc: SRINIVAS <srinivas.eeda@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 79930f5892e ("net: do not deplete pfmemalloc reserve")
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> Changed since v1:
>   - change author from Matthew to Dongli
>   - Add references to all prior discussions
>   - Add more details to commit message
> Changed since v2:
>   - add unlikely (suggested by Eric Dumazet)
>
>  mm/page_alloc.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 23f5066bd4a5..91129ce75ed4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5103,6 +5103,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
>                 if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
>                         goto refill;
>
> +               if (unlikely(nc->pfmemalloc)) {
> +                       free_the_page(page, compound_order(page));
> +                       goto refill;
> +               }
> +

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Thanks !