linux-kernel - Re: [PATCH 4.4 051/268] asm-generic: provide generic_pmdp

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Date:   Mon, 11 Jun 2018 13:55:17 +0100
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Daney <david.daney@...ium.com>,
        David Miller <davem@...emloft.net>,
        "H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Nitin Gupta <nitin.m.gupta@...cle.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vineet Gupta <vgupta@...opsys.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: Re: [PATCH 4.4 051/268] asm-generic: provide
 generic_pmdp_establish()

On Mon, 2018-05-28 at 12:00 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> [ Upstream commit c58f0bb77ed8bf93dfdde762b01cb67eebbdfc29 ]
> 
> Patch series "Do not lose dirty bit on THP pages", v4.
> 
> Vlastimil noted that pmdp_invalidate() is not atomic and we can lose
> dirty and access bits if CPU sets them after pmdp dereference, but
> before set_pmd_at().
> 
> The bug can lead to data loss, but the race window is tiny and I haven't
> seen any reports that suggested that it happens in reality.  So I don't
> think it worth sending it to stable.
[...]
> This patch (of 12):
> 
> This is an implementation of pmdp_establish() that is only suitable for
> an architecture that doesn't have hardware dirty/accessed bits.  In this
> case we can't race with CPU which sets these bits and non-atomic
> approach is fine.
[...]

There's no point in applying just this patch, since it adds a new
function that nothing will call.

I tend to think that since this is fixing a potential data loss, the
whole series should be backported.  But the commit message here says it
shouldn't.

Ben.

-- 
Ben Hutchings, Software Developer                         Codethink Ltd
https://www.codethink.co.uk/                 Dale House, 35 Dale Street
                                     Manchester, M1 2HF, United Kingdom