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Date:   Sat, 20 Mar 2021 03:25:56 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Zhou Guanghui <zhouguanghui1@huawei.com>,
        Zi Yan <ziy@nvidia.com>, Shakeel Butt <shakeelb@google.com>,
        Roman Gushchin <guro@fb.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] mm: page_alloc: fix memcg accounting leak in speculative
 cache lookup
Message-ID: <20210320032556.GD3420@casper.infradead.org>
References: <20210319071547.60973-1-hannes@cmpxchg.org>
 <alpine.LSU.2.11.2103191814040.1043@eggly.anvils>
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On Fri, Mar 19, 2021 at 06:52:58PM -0700, Hugh Dickins wrote:
> > +	/*
> > +	 * Drop the base reference from __alloc_pages and free. In
> > +	 * case there is an outstanding speculative reference, from
> > +	 * e.g. the page cache, it will put and free the page later.
> > +	 */
> > +	if (likely(put_page_testzero(page))) {
> >  		free_the_page(page, order);
> > -	else if (!PageHead(page))
> > +		return;
> > +	}
> > +
> > +	/*
> > +	 * The speculative reference will put and free the page.
> > +	 *
> > +	 * However, if the speculation was into a higher-order page
> > +	 * chunk that isn't marked compound, the other side will know
> > +	 * nothing about our buddy pages and only free the order-0
> > +	 * page at the start of our chunk! We must split off and free
> > +	 * the buddy pages here.
> > +	 *
> > +	 * The buddy pages aren't individually refcounted, so they
> > +	 * can't have any pending speculative references themselves.
> > +	 */
> > +	if (!PageHead(page) && order > 0) {
> 
> The put_page_testzero() has released our reference to the first
> subpage of page: it's now under the control of the racing speculative
> lookup.  So it seems to me unsafe to be checking PageHead(page) here:
> if it was actually a compound page, PageHead might already be cleared
> by now, and we doubly free its tail pages below?  I think we need to
> use a "bool compound = PageHead(page)" on entry to __free_pages().
> 
> Or alternatively, it's wrong to call __free_pages() on a compound
> page anyway, so we should not check PageHead at all, except in a
> WARN_ON_ONCE(PageCompound(page)) at the start?

Alas ...

$ git grep '__free_pages\>.*compound'
drivers/dma-buf/heaps/system_heap.c:            __free_pages(page, compound_order(page));
drivers/dma-buf/heaps/system_heap.c:            __free_pages(p, compound_order(p));
drivers/dma-buf/heaps/system_heap.c:            __free_pages(page, compound_order(page));
mm/huge_memory.c:               __free_pages(zero_page, compound_order(zero_page));
mm/huge_memory.c:               __free_pages(zero_page, compound_order(zero_page));
mm/slub.c:                      __free_pages(page, compound_order(page));

Maybe we should disallow it!

There are a few other places to check:

$ grep -l __GFP_COMP $(git grep -lw __free_pages) | wc -l
24

(assuming the pages are allocated and freed in the same file, which is a
reasonable approximation, but not guaranteed to catch everything.  Many
of these 24 will be false positives, of course.)