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: Mon, 16 Sep 2019 13:05:55 -0700 [thread overview]
Message-ID: <20190916200555.GA254094@google.com> (raw)
In-Reply-To: <3a500b81-71bb-54bd-9f2f-ab89ee723879@codeaurora.org>
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.
next prev parent reply other threads:[~2019-09-16 20:06 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
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 [this message]
2019-09-17 5:38 ` Vinayak Menon
2019-09-18 1:12 ` Minchan Kim
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