From: Minchan Kim <minchan@kernel.org>
To: Vinayak Menon <vinmenon@codeaurora.org>
Cc: linux-mm@kvack.org
Subject: Re: [PATCH] mm: fix the race between swapin_readahead and SWP_SYNCHRONOUS_IO path
Date: Tue, 17 Sep 2019 18:12:00 -0700 [thread overview]
Message-ID: <20190918011200.GA159757@google.com> (raw)
In-Reply-To: <4788d556-1b53-8d3e-121c-de2c286bac43@codeaurora.org>
On Tue, Sep 17, 2019 at 11:08:49AM +0530, Vinayak Menon wrote:
>
> On 9/17/2019 1:35 AM, Minchan Kim wrote:
> > Hi Vinayak,
> >
> > On Fri, Sep 13, 2019 at 02:35:41PM +0530, Vinayak Menon wrote:
> >> On 9/12/2019 10:44 PM, Minchan Kim wrote:
> >>> Hi Vinayak,
> >>>
> >>> On Wed, Sep 11, 2019 at 03:37:23PM +0530, Vinayak Menon wrote:
> >>>
> >>> < snip >
> >>>
> >>>>>> Can swapcache check be done like below, before taking the SWP_SYNCHRONOUS_IO path, as an alternative ?
> >>>>> With your approach, what prevent below scenario?
> >>>>>
> >>>>> A B
> >>>>>
> >>>>> do_swap_page
> >>>>> SWP_SYNCHRONOUS_IO && __swap_count == 1
> >>>> As shrink_page_list is picking the page from LRU and B is trying to read from swap simultaneously, I assume someone had read
> >>>>
> >>>> the page from swap prior to B, when its swap_count was say 2 (for it to be reclaimed by shrink_page_list now)
> >>> It could happen after B saw __swap_count == 1. Think about forking new process.
> >>> In that case, swap_count is 2 and the forked process will access the page(it
> >>> ends up freeing zram slot but the page would be swap cache. However, B process
> >>> doesn't know it).
> >>
> >> Okay, so when B has read __swap_count == 1, it means that it has taken down_read on mmap_sem in fault path
> >>
> >> already. This means fork will not be able to proceed which needs to have down_write on parent's mmap_sem ?
> >>
> > You are exactly right. However, I still believe better option to solve
> > the issue is to check swap_count and delte only if swap_count == 1
> > in swap_slot_free_notify because it's zram specific issue and more safe
> > without depending other lock scheme.
>
>
> Sure. Let me know if you want me to post a patch for that.
>
Please post a patch.
Thanks, Vinayak!
prev parent reply other threads:[~2019-09-18 1:12 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
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 [this message]
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