linux-kernel - Re: mm,tlb: revert 4647706ebeee?

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Message-Id: <20180709171356.87d834e125f06e0cdaa72f85@linux-foundation.org>
Date:   Mon, 9 Jul 2018 17:13:56 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Rik van Riel <riel@...riel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...e.com>,
        "kirill.shutemov" <kirill.shutemov@...ux.intel.com>,
        Minchan Kim <minchan@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        kernel-team <kernel-team@...com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: mm,tlb: revert 4647706ebeee?

On Sun, 8 Jul 2018 01:25:38 +1000 Nicholas Piggin <npiggin@...il.com> wrote:

> On Fri, 06 Jul 2018 13:03:55 -0400
> Rik van Riel <riel@...riel.com> wrote:
> 
> > Hello,
> > 
> > It looks like last summer, there were 2 sets of patches
> > in flight to fix the issue of simultaneous mprotect/madvise
> > calls unmapping PTEs, and some pages not being flushed from
> > the TLB before returning to userspace.
> > 
> > Minchan posted these patches:
> > 56236a59556c ("mm: refactor TLB gathering API")
> > 99baac21e458 ("mm: fix MADV_[FREE|DONTNEED] TLB flush miss problem")
> > 
> > Around the same time, Mel posted:
> > 4647706ebeee ("mm: always flush VMA ranges affected by zap_page_range")
> > 
> > They both appear to solve the same bug.
> > 
> > Only one of the two solutions is needed.
> > 
> > However, 4647706ebeee appears to introduce extra TLB
> > flushes - one per VMA, instead of one over the entire
> > range unmapped, and also extra flushes when there are
> > no simultaneous unmappers of the same mm.
> > 
> > For that reason, it seems like we should revert
> > 4647706ebeee and keep only Minchan's solution in
> > the kernel.
> > 
> > Am I overlooking any reason why we should not revert
> > 4647706ebeee?
> 
> Yes I think so. Discussed here recently:
> 
> https://marc.info/?l=linux-mm&m=152878780528037&w=2

Unclear if that was an ack ;)

> Actually we realized that powerpc does not implement the mmu
> gather flushing quite right so it needs a fix before this
> revert. But I propose the revert for next merge window.

Yes, I have Rik's patch for 4.19-rc1.  I added yourself, Aneesh and
Nadav to cc so you'll see it fly past.  If poss, please do get this all
tested before the time comes and let me know?