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From: Minchan Kim <minchan@kernel.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Vinayak Menon <vinmenon@codeaurora.org>,
	linux-mm@kvack.org, kernel-team@android.com
Subject: Re: [PATCH] mm: do not free shared swap slots
Date: Thu, 31 Oct 2019 07:46:08 -0700	[thread overview]
Message-ID: <20191031144608.GA128849@google.com> (raw)
In-Reply-To: <20191030102409.GZ31513@dhcp22.suse.cz>

On Wed, Oct 30, 2019 at 11:24:09AM +0100, Michal Hocko wrote:
> On Tue 29-10-19 17:24:47, Minchan Kim wrote:
> > On Tue, Oct 22, 2019 at 04:51:34PM +0530, Vinayak Menon wrote:
> > > The following race is observed due to which a processes faulting
> > > on a swap entry, finds the page neither in swapcache nor swap. This
> > > causes zram to give a zero filled page that gets mapped to the
> > > process, resulting in a user space crash later.
> > > 
> > > Consider parent and child processes Pa and Pb sharing the same swap
> > > slot with swap_count 2. Swap is on zram with SWP_SYNCHRONOUS_IO set.
> > > Virtual address 'VA' of Pa and Pb points to the shared swap entry.
> > > 
> > > Pa                                       Pb
> > > 
> > > fault on VA                              fault on VA
> > > do_swap_page                             do_swap_page
> > > lookup_swap_cache fails                  lookup_swap_cache fails
> > >                                          Pb scheduled out
> > > swapin_readahead (deletes zram entry)
> > > swap_free (makes swap_count 1)
> > >                                          Pb scheduled in
> > >                                          swap_readpage (swap_count == 1)
> > >                                          Takes SWP_SYNCHRONOUS_IO path
> > >                                          zram enrty absent
> > >                                          zram gives a zero filled page
> > > 
> > > Fix this by making sure that swap slot is freed only when swap count
> > > drops down to one.
> > > 
> > > Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
> > > Suggested-by: Minchan Kim <minchan@google.com>
> > > Cc: Michal Hocko <mhocko@suse.com>
> > Acked-by: Minchan Kim <minchan@kernel.org>
> > 
> > -stable material from v4.15.
> 
> Which commit does this fix? Fixes: tag would be highly appreciated.

Oops, I missed to mention it.
Fixes: aa8d22a11da9 (mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference)

> 
> -- 
> Michal Hocko
> SUSE Labs


  reply	other threads:[~2019-10-31 14:46 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-22 11:21 [PATCH] mm: do not free shared swap slots Vinayak Menon
2019-10-30  0:24 ` Minchan Kim
2019-10-30 10:24   ` Michal Hocko
2019-10-31 14:46     ` Minchan Kim [this message]
2019-10-31 17:15       ` Michal Hocko

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