linux-kernel - Re: [PATCH 2/2] mm, oom: fix potential data corruption when oom

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Message-ID: <20170811070938.GA30811@dhcp22.suse.cz>
Date:   Fri, 11 Aug 2017 09:09:38 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     akpm@...ux-foundation.org, andrea@...nel.org, kirill@...temov.name,
        oleg@...hat.com, wenwei.tww@...baba-inc.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm, oom: fix potential data corruption when
 oom_reaper races with writer

On Fri 11-08-17 11:28:52, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > +/*
> > + * Checks whether a page fault on the given mm is still reliable.
> > + * This is no longer true if the oom reaper started to reap the
> > + * address space which is reflected by MMF_UNSTABLE flag set in
> > + * the mm. At that moment any !shared mapping would lose the content
> > + * and could cause a memory corruption (zero pages instead of the
> > + * original content).
> > + *
> > + * User should call this before establishing a page table entry for
> > + * a !shared mapping and under the proper page table lock.
> > + *
> > + * Return 0 when the PF is safe VM_FAULT_SIGBUS otherwise.
> > + */
> > +static inline int check_stable_address_space(struct mm_struct *mm)
> > +{
> > +	if (unlikely(test_bit(MMF_UNSTABLE, &mm->flags)))
> > +		return VM_FAULT_SIGBUS;
> > +	return 0;
> > +}
> > +
> 
> Will you explain the mechanism why random values are written instead of zeros
> so that this patch can actually fix the race problem?

I am not sure what you mean here. Were you able to see a write with an
unexpected content?

-- 
Michal Hocko
SUSE Labs