linux-kernel - Re: Re: [PATCH] [PATCH] mm: disable preemption before swapcache

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Date:   Wed, 25 Jul 2018 12:34:16 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "zhaowuyun@...gtech.com" <zhaowuyun@...gtech.com>
Cc:     mgorman <mgorman@...hsingularity.net>,
        akpm <akpm@...ux-foundation.org>, minchan <minchan@...nel.org>,
        vinmenon <vinmenon@...eaurora.org>, hannes <hannes@...xchg.org>,
        "hillf.zj" <hillf.zj@...baba-inc.com>,
        linux-mm <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: Re: [PATCH] [PATCH] mm: disable preemption before swapcache_free

On Wed 25-07-18 17:53:07, zhaowuyun@...gtech.com wrote:
> >[Please do not top post - thank you]
> >[CC Hugh - the original patch was http://lkml.kernel.org/r/2018072514375722198958@wingtech.com]
> >
> >On Wed 25-07-18 15:57:55, zhaowuyun@...gtech.com wrote:
> >> That is a BUG we found in mm/vmscan.c at KERNEL VERSION 4.9.82
> >
> >The code is quite similar in the current tree as well.
> >
> >> Sumary is TASK A (normal priority) doing __remove_mapping page preempted by TASK B (RT priority) doing __read_swap_cache_async,
> >> the TASK A preempted before swapcache_free, left SWAP_HAS_CACHE flag in the swap cache,
> >> the TASK B which doing __read_swap_cache_async, will not success at swapcache_prepare(entry) because the swap cache was exist, then it will loop forever because it is a RT thread...
> >> the spin lock unlocked before swapcache_free, so disable preemption until swapcache_free executed ...
> >
> >OK, I see your point now. I have missed the lock is dropped before
> >swapcache_free. How can preemption disabling prevent this race to happen
> >while the code is preempted by an IRQ?
> >--
> >Michal Hocko
> >SUSE Labs 
> 
> Hi Michal,
> 
> The action what processes __read_swap_cache_async is on the process context, so I think disable preemption is enough.

So what you are saying is that no IRQ or other non-process contexts will
not loop in __read_swap_cache_async so the live lock is not possible?
-- 
Michal Hocko
SUSE Labs