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

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Date:   Wed, 18 Nov 2020 11:46:54 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     akpm@...ux-foundation.org
Cc:     Dongli Zhang <dongli.zhang@...cle.com>, linux-mm@...ck.org,
        netdev@...r.kernel.org, willy@...radead.org,
        aruna.ramakrishna@...cle.com, bert.barbe@...cle.com,
        rama.nichanamatlu@...cle.com, venkat.x.venkatsubra@...cle.com,
        manjunath.b.patil@...cle.com, joe.jin@...cle.com,
        srinivas.eeda@...cle.com, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, davem@...emloft.net,
        edumazet@...gle.com, vbabka@...e.cz
Subject: Re: [PATCH v3 1/1] page_frag: Recover from memory pressure

On Sun, 15 Nov 2020 12:10:29 -0800 Dongli Zhang 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.
> 
> However, once kernel is not under memory pressure any longer (suppose large
> amount of memory pages are just reclaimed), the page_frag_alloc() may still
> re-use the prior pfmemalloc page_frag_cache->va to allocate skb->data. As a
> result, the skb->pfmemalloc is always true unless page_frag_cache->va is
> re-allocated, even if the kernel is not under memory pressure any longer.
> 
> Here is how kernel runs into issue.
> 
> 1. The kernel is under memory pressure and allocation of
> PAGE_FRAG_CACHE_MAX_ORDER in __page_frag_cache_refill() will fail. Instead,
> the pfmemalloc page is allocated for page_frag_cache->va.
> 
> 2: All skb->data from page_frag_cache->va (pfmemalloc) will have
> skb->pfmemalloc=true. The skb will always be dropped by sock without
> SOCK_MEMALLOC. This is an expected behaviour.
> 
> 3. Suppose a large amount of pages are reclaimed and kernel is not under
> memory pressure any longer. We expect skb->pfmemalloc drop will not happen.
> 
> 4. Unfortunately, page_frag_alloc() does not proactively re-allocate
> page_frag_alloc->va and will always re-use the prior pfmemalloc page. The
> skb->pfmemalloc is always true even kernel is not under memory pressure any
> longer.
> 
> Fix this by freeing and re-allocating the page instead of recycling it.

Andrew, are you taking this via -mm or should I put it in net? 
I'm sending a PR to Linus tomorrow.