Re: [PATCH] page_frag: Recover from memory pressure

[Eric Dumazet <erdnetdev@gmail.com>]

On 11/5/20 5:21 AM, Matthew Wilcox (Oracle) wrote:
> When the machine is under extreme memory pressure, the page_frag allocator
> signals this to the networking stack by marking allocations with the
> 'pfmemalloc' flag, which causes non-essential packets to be dropped.
> Unfortunately, even after the machine recovers from the low memory
> condition, the page continues to be used by the page_frag allocator,
> so all allocations from this page will continue to be dropped.
> 
> Fix this by freeing and re-allocating the page instead of recycling it.
> 
> Reported-by: Dongli Zhang <dongli.zhang@oracle.com>
> Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
> Cc: Bert Barbe <bert.barbe@oracle.com>
> Cc: Rama Nichanamatlu <rama.nichanamatlu@oracle.com>
> Cc: Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>
> Cc: Manjunath Patil <manjunath.b.patil@oracle.com>
> Cc: Joe Jin <joe.jin@oracle.com>
> Cc: SRINIVAS <srinivas.eeda@oracle.com>
> Cc: stable@vger.kernel.org
> Fixes: 79930f5892e ("net: do not deplete pfmemalloc reserve")

Your patch looks fine, although this Fixes: tag seems incorrect.

79930f5892e ("net: do not deplete pfmemalloc reserve") was propagating
the page pfmemalloc status into the skb, and seems correct to me.

The bug was the page_frag_alloc() was keeping a problematic page for
an arbitrary period of time ?

> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/page_alloc.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 778e815130a6..631546ae1c53 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5139,6 +5139,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
>  
>  		if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
>  			goto refill;
> +		if (nc->pfmemalloc) {

                if (unlikely(nc->pfmemalloc)) {

> +			free_the_page(page, compound_order(page));
> +			goto refill;
> +		}
>  
>  #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>  		/* if size can vary use size else just use PAGE_SIZE */
>