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

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Date:   Fri, 06 Jul 2018 13:03:55 -0400
From:   Rik van Riel <riel@...riel.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.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>
Subject: mm,tlb: revert 4647706ebeee?

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?

kind regards,

Rik
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