linux-kernel - Re: [PATCH] arch, mm: introduce arch_tlb_gather_mmu

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Date:   Thu, 14 Dec 2017 15:05:04 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>,
        Minchan Kim <minchan@...nel.org>,
        Andrea Argangeli <andrea@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch, mm: introduce arch_tlb_gather_mmu_exit

On Tue 05-12-17 20:14:37, Michal Hocko wrote:
> On Tue 05-12-17 10:31:12, Linus Torvalds wrote:
> > On Tue, Dec 5, 2017 at 6:58 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > >
> > > This all is nice but tlb_gather users are not aware of that and this can
> > > actually cause some real problems. E.g. the oom_reaper tries to reap the
> > > whole address space but it might race with threads accessing the memory [1].
> > > It is possible that soft-dirty handling might suffer from the same
> > > problem [2] as soon as it starts supporting the feature.
> > 
> > So we fixed the oom reaper to just do proper TLB invalidates in commit
> > 687cb0884a71 ("mm, oom_reaper: gather each vma to prevent leaking TLB
> > entry").
> > 
> > So now "fullmm" should be the expected "exit" case, and it all should
> > be unambiguous.
> > 
> > Do we really have any reason to apply this patch any more?
> 
> Well, the point was the clarity. The bad behavior came as a surprise for
> the oom reaper and as Minchan mentioned we would see a similar problem
> with soft-dirty bits as soon as they are supported on arm64 or
> potentially other architectures which might do special handling for exit
> case.

I am not going to push this patch if it is considered pointless but I
haven't heard back anything to the above argument.
-- 
Michal Hocko
SUSE Labs