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@xxxxxxxxxx>
> 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")
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@xxxxxxxxxxxxx>
> ---
> 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 */
>
