Re: [PATCH] page_frag: Recover from memory pressure

From: Matthew Wilcox <willy@infradead.org>
To: Vlastimil Babka <vbabka@suse.cz>
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>
Subject: Re: [PATCH] page_frag: Recover from memory pressure
Date: Thu, 5 Nov 2020 12:05:54 +0000	[thread overview]
Message-ID: <20201105120554.GJ17076@casper.infradead.org> (raw)
In-Reply-To: <83d68f28-cae7-012d-0f4b-82960b248bd8@suse.cz>

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 ...

        if (unlikely(!page))
                page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);

        nc->va = page ? page_address(page) : NULL;