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From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Qian Cai <cai@lca.pw>
Cc: Matthew Wilcox <willy@infradead.org>,
	akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault()
Date: Tue, 11 Feb 2020 00:21:49 +0300	[thread overview]
Message-ID: <20200210212149.u6zlpi2jefi2vkfg@box> (raw)
In-Reply-To: <1581364697.7365.45.camel@lca.pw>

On Mon, Feb 10, 2020 at 02:58:17PM -0500, Qian Cai wrote:
> On Mon, 2020-02-10 at 21:05 +0300, Kirill A. Shutemov wrote:
> > On Mon, Feb 10, 2020 at 09:25:11AM -0800, Matthew Wilcox wrote:
> > > On Mon, Feb 10, 2020 at 12:00:29PM -0500, Qian Cai wrote:
> > > > @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf,
> > > >  		if (page->index >= max_idx)
> > > >  			goto unlock;
> > > >  
> > > > -		if (file->f_ra.mmap_miss > 0)
> > > > +		if (data_race(file->f_ra.mmap_miss > 0))
> > > >  			file->f_ra.mmap_miss--;
> > > 
> > > How is this safe?  Two threads can each see 1, and then both decrement the
> > > in-memory copy, causing it to end up at -1.
> > 
> > Right, it is bogus.
> > 
> > Below is my completely untested attempt on fix this. It still allows
> > races, but they will only lead to missed accounting, but not underflow.
> 
> Looks good to me. Do you plan to send out an official patch?

Feel free to submit it. After testing.

-- 
 Kirill A. Shutemov


  reply	other threads:[~2020-02-10 21:21 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-10 17:00 [PATCH -next] mm/filemap: fix a data race in filemap_fault() Qian Cai
2020-02-10 17:25 ` Matthew Wilcox
2020-02-10 18:05   ` Kirill A. Shutemov
2020-02-10 19:58     ` Qian Cai
2020-02-10 21:21       ` Kirill A. Shutemov [this message]
2020-02-10 19:20   ` Qian Cai
2020-02-10 19:21     ` Matthew Wilcox
2020-02-10 20:28       ` Qian Cai
2020-02-10 20:44         ` Marco Elver

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