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Subject: Re: [PATCH] page_frag: Recover from memory pressure
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-mm@kvack.org, netdev@vger.kernel.org,
        Dongli Zhang <dongli.zhang@oracle.com>,
        Aruna Ramakrishna <aruna.ramakrishna@oracle.com>,
        Bert Barbe <bert.barbe@oracle.com>,
        Rama Nichanamatlu <rama.nichanamatlu@oracle.com>,
        Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>,
        Manjunath Patil <manjunath.b.patil@oracle.com>,
        Joe Jin <joe.jin@oracle.com>,
        SRINIVAS <srinivas.eeda@oracle.com>, stable@vger.kernel.org,
        Jann Horn <jannh@google.com>
References: <20201105042140.5253-1-willy@infradead.org>
 <83d68f28-cae7-012d-0f4b-82960b248bd8@suse.cz>
 <20201105120554.GJ17076@casper.infradead.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <bd404987-6c26-ae7d-55cf-be74b432c3ec@suse.cz>
Date:   Thu, 5 Nov 2020 13:09:15 +0100
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On 11/5/20 1:05 PM, Matthew Wilcox wrote:
> On Thu, Nov 05, 2020 at 12:56:43PM +0100, Vlastimil Babka wrote:
>> > +++ 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) {
>> > +			free_the_page(page, compound_order(page));
>> > +			goto refill;
>> 
>> Theoretically the refill can fail and we return NULL while leaving nc->va
>> pointing to a freed page, so I think you should set nc->va to NULL.
>> 
>> Geez, can't the same thing already happen after we sub the nc->pagecnt_bias
>> from page ref, and last users of the page fragments then return them and dec
>> the ref to zero and the page gets freed?
> 
> I don't think you read __page_frag_cache_refill() closely enough ...

Or rather not at all, sorry :) somehow I just saw "ah here we call the page 
allocator".

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> 
>          if (unlikely(!page))
>                  page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
> 
>          nc->va = page ? page_address(page) : NULL;
> 
>