Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 18 Nov 2020 11:46:54 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     akpm@linux-foundation.org
Cc:     Dongli Zhang <dongli.zhang@oracle.com>, linux-mm@kvack.org,
        netdev@vger.kernel.org, willy@infradead.org,
        aruna.ramakrishna@oracle.com, bert.barbe@oracle.com,
        rama.nichanamatlu@oracle.com, venkat.x.venkatsubra@oracle.com,
        manjunath.b.patil@oracle.com, joe.jin@oracle.com,
        srinivas.eeda@oracle.com, stable@vger.kernel.org,
        linux-kernel@vger.kernel.org, davem@davemloft.net,
        edumazet@google.com, vbabka@suse.cz
Subject: Re: [PATCH v3 1/1] page_frag: Recover from memory pressure
Message-ID: <20201118114654.3435f76c@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net>
In-Reply-To: <20201115201029.11903-1-dongli.zhang@oracle.com>
References: <20201115201029.11903-1-dongli.zhang@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.