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

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Date:   Fri, 11 Aug 2017 16:54:36 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org
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

Michal Hocko wrote:
> 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?

Yes. See http://lkml.kernel.org/r/201708072228.FAJ09347.tOOVOFFQJSHMFL@I-love.SAKURA.ne.jp .