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

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Message-ID: <bd404987-6c26-ae7d-55cf-be74b432c3ec@suse.cz>
Date:   Thu, 5 Nov 2020 13:09:15 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, netdev@...r.kernel.org,
        Dongli Zhang <dongli.zhang@...cle.com>,
        Aruna Ramakrishna <aruna.ramakrishna@...cle.com>,
        Bert Barbe <bert.barbe@...cle.com>,
        Rama Nichanamatlu <rama.nichanamatlu@...cle.com>,
        Venkat Venkatsubra <venkat.x.venkatsubra@...cle.com>,
        Manjunath Patil <manjunath.b.patil@...cle.com>,
        Joe Jin <joe.jin@...cle.com>,
        SRINIVAS <srinivas.eeda@...cle.com>, stable@...r.kernel.org,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] page_frag: Recover from memory pressure

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@...e.cz>

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