linux-kernel - Re: [RFC PATCH v2 2/2] fs/task_mmu: acquire mmap

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Date:   Tue, 5 Jan 2021 13:20:55 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>, Yu Zhao <yuzhao@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2 2/2] fs/task_mmu: acquire mmap_lock for write on
 soft-dirty cleanup

On Fri, Dec 25, 2020 at 01:25:29AM -0800, Nadav Amit wrote:
> Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes tracking")

Targeting a backport down to 2013 when nothing could wrong in practice
with page_mapcount sounds backwards and unnecessarily risky.

In theory it was already broken and in theory
09854ba94c6aad7886996bfbee2530b3d8a7f4f4 is absolutely perfect and the
previous code of 2013 is completely wrong, but in practice the code
from 2013 worked perfectly until Aug 21 2020.

Since nothing at all could go wrong in soft dirty and uffd-wp until
09854ba94c6aad7886996bfbee2530b3d8a7f4f4, the Fixes need to target
that, definitely not a patch from 2013.

This means the backports will apply clean, they don't need a simple
solution but one that doesn't regress the performance of open source
virtual machines and open source products using clear_refs and uffd-wp
in general.

Thanks,
Andrea