linux-kernel - Re: [PATCH] mm: get rid of WARN if failed to cow user pages

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Message-ID: <x49imk3bo1h.fsf@segfault.boston.devel.redhat.com>
Date:   Tue, 18 Feb 2020 16:09:30 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Murphy Zhou <jencce.kernel@...il.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Jia He <justin.he@....com>, Linux MM <linux-mm@...ck.org>,
        "Shutemov\, Kirill" <kirill.shutemov@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: get rid of WARN if failed to cow user pages

Dan Williams <dan.j.williams@...el.com> writes:

> [ drop Ross, add Kirill, linux-mm, and lkml ]
>
> On Tue, Dec 24, 2019 at 9:42 PM Murphy Zhou <jencce.kernel@...il.com> wrote:
>>
>> By running xfstests with fsdax enabled, generic/437 always hits this
>> warning[1] since this commit:
>>
>> commit 83d116c53058d505ddef051e90ab27f57015b025
>> Author: Jia He <justin.he@....com>
>> Date:   Fri Oct 11 22:09:39 2019 +0800
>>
>>     mm: fix double page fault on arm64 if PTE_AF is cleared
>>
>> Looking at the test program[2] generic/437 uses, it's pretty easy
>> to hit this warning. Remove this WARN as it seems not necessary.
>
> This is not sufficient justification. Does this same test fail without
> DAX? If not, why not? At a minimum you need to explain why this is not
> indicating a problem.

I ran into this, too, and Kirill has posted a patch[1] to fix the issue.
Note that it's a potential data corrupter, so just removing the warning
is NOT the right approach.  :)

-Jeff

[1] https://lore.kernel.org/linux-mm/20200218154151.13349-1-kirill.shutemov@linux.intel.com/T/#u