linux-kernel - Re: [PATCH v2] x86/mm: Fix PAT bit missing from page protection modify mask

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Date:   Thu, 8 Jun 2023 07:15:56 +0200
From:   Juergen Gross <jgross@...e.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "janusz.krzysztofik@...ux.intel.com" 
        <janusz.krzysztofik@...ux.intel.com>, "bp@...en8.de" <bp@...en8.de>
Cc:     "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi.shyti@...ux.intel.com" <andi.shyti@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>
Subject: Re: [PATCH v2] x86/mm: Fix PAT bit missing from page protection
 modify mask

On 07.06.23 23:12, Edgecombe, Rick P wrote:
> On Wed, 2023-06-07 at 19:11 +0200, Janusz Krzysztofik wrote:
>> On Wednesday, 7 June 2023 17:31:24 CEST Dave Hansen wrote:
>>> On 6/7/23 08:23, Janusz Krzysztofik wrote:
>>>>
>>>> Extend bitmask used by pgprot_modify() for selecting bits to be
>>>> preserved
>>>> with _PAGE_PAT bit.  However, since that bit can be reused as
>>>> _PAGE_PSE,
>>>> and the _PAGE_CHG_MASK symbol, primarly used by pte_modify(), is
>>>> likely
>>>> intentionally defined with that bit not set, keep that symbol
>>>> unchanged.
>>>
>>> I'm really having a hard time parsing what that last sentence is
>>> saying.
>>>
>>> Could you try again, please?
>>
>> OK, but then I need to get my doubts addressed by someone first,
>> otherwise I'm
>> not able to provide a better justification from my heart.
>>
>> The issue needs to be fixed by including _PAGE_PAT bit into a bitmask
>> used
>> by pgprot_modify() for selecting bits to be preserved.  We can either
>> do
>> that internally to pgprot_modify() (my initial proposal, which my
>> poorly
>> worded paragraph was still trying to describe and justify), or by
>> making
>> _PAGE_PAT a part of _PAGE_CHG_MASK, as suggested by Borislav and
>> reflected in
>> my v2 changelog.  But for the latter, I think we need to make sure
>> that we
>> don't break other users of _PAGE_CHG_MASK.  Maybe Borislav can
>> confirm that's
>> the case.
>>
>> Since _PAGE_PAT is the same as _PAGE_PSE, _HPAGE_CHG_MASK -- a huge
>> pmds'
>> counterpart of _PAGE_CHG_MASK, introduced by commit c489f1257b8c
>> ("thp: add
>> pmd_modify"), defined as (_PAGE_CHG_MASK | _PAGE_PSE) -- will no
>> longer differ
>> from _PAGE_CHG_MASK as soon as we add _PAGE_PAT bit to the latter.
>> If such
>> modification of _PAGE_CHG_MASK was irrelevant to its users then one
>> may ask
>> why a new symbol was introduced instead of reusing the existing one
>> with that
>> otherwise irrelevant bit (_PAGE_PSE in that case) added.  I've
>> initially
>> assumed that keeping _PAGE_CHG_MASK without _PAGE_PSE (vel _PAGE_PAT)
>> included
>> into it was intentional for some reason.  Maybe Johannes Weiner, the
>> author of
>> that patch (adding him to Cc:), could shed more light on that.
> 
> So since _PAGE_PSE is actually the same value as _PAGE_PAT, you don't
> actually need to have _PAGE_PSE in _HPAGE_CHG_MASK in order to get
> functional correctness. Is that right?
> 
> I think it is still a little hidden (even before this) and I wonder
> about separating out the common bits into, like, _COMMON_PAGE_CHG_MASK
> or something. Then setting specific PAGE and HPAGE bits (like
> _PAGE_PAT, _PAGE_PSE and _PAGE_PAT_LARGE) in their specific define.
> Would it be more readable that way?

I'd go that route. I don't think we should rely on _PAGE_PSE == _PAGE_PAT
here.


Juergen

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