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: Tue, 3 Sep 2019 13:41:09 +0200 [thread overview]
Message-ID: <20190903114109.GR14028@dhcp22.suse.cz> (raw)
In-Reply-To: <79303914-d6a6-011a-150f-74488c8e12f2@codeaurora.org>
On Tue 03-09-19 11:43:16, Vinayak Menon wrote:
> Hi Michal,
>
> Thanks for reviewing this.
>
>
> On 9/2/2019 6:51 PM, Michal Hocko wrote:
> > 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?
>
>
> I think zram entry being deleted by Pa and zram giving out a zeroed page to Pb is normal.
Isn't that a data loss? The race you mentioned shouldn't be possible
with the standard swap storage AFAIU. If that is really the case then
the zram needs a fix rather than a generic path. Or at least a very good
explanation why the generic path is a preferred way.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2019-09-03 11:41 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-30 12:43 Vinayak Menon
2019-09-02 13:21 ` Michal Hocko
2019-09-03 6:13 ` Vinayak Menon
2019-09-03 11:41 ` Michal Hocko [this message]
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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