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From: Michal Hocko <mhocko@kernel.org>
To: Vinayak Menon <vinmenon@codeaurora.org>
Cc: minchan@kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm: fix the race between swapin_readahead and SWP_SYNCHRONOUS_IO path
Date: Mon, 2 Sep 2019 15:21:04 +0200	[thread overview]
Message-ID: <20190902132104.GJ14028@dhcp22.suse.cz> (raw)
In-Reply-To: <1567169011-4748-1-git-send-email-vinmenon@codeaurora.org>

On Fri 30-08-19 18:13:31, 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

This sounds like a zram issue, right? Why is a generic swap path changed
then?

> 
> Fix this by reading the swap_count before lookup_swap_cache, which conforms
> with the order in which page is added to swap cache and swap count is
> decremented in do_swap_page. In the race case above, this will let Pb take
> the readahead path and thus pick the proper page from swapcache.
> 
> Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2019-09-02 13:21 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-30 12:43 [PATCH] mm: fix the race between swapin_readahead and SWP_SYNCHRONOUS_IO path Vinayak Menon
2019-09-02 13:21 ` Michal Hocko [this message]
2019-09-03  6:13   ` Vinayak Menon
2019-09-03 11:41     ` Michal Hocko
2019-09-03 12:17       ` Vinayak Menon
2019-09-09  4:05         ` Vinayak Menon
2019-09-09 11:23           ` Michal Hocko
2019-09-09 23:26 ` Minchan Kim
2019-09-10  8:22   ` Vinayak Menon
2019-09-10 17:51     ` Minchan Kim
2019-09-11 10:07       ` Vinayak Menon
2019-09-12 17:14         ` Minchan Kim
2019-09-13  9:05           ` Vinayak Menon
2019-09-16 20:05             ` Minchan Kim
2019-09-17  5:38               ` Vinayak Menon
2019-09-18  1:12                 ` Minchan Kim

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