linux-kernel - Re: [PATCH] x86/mm: Make pud_present() check _PAGE_PROTNONE and _PAGE

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Message-ID: <2e7a04cf-80cb-58c1-7344-2f8422ed7d31@arm.com>
Date:   Fri, 20 Mar 2020 18:52:57 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH] x86/mm: Make pud_present() check _PAGE_PROTNONE and
 _PAGE_PSE as well



On 03/20/2020 05:17 PM, Kirill A. Shutemov wrote:
> On Fri, Mar 20, 2020 at 08:53:16AM +0530, Anshuman Khandual wrote:
>>
>>
>> On 03/18/2020 10:31 AM, Anshuman Khandual wrote:
>>> pud_present() should also check _PAGE_PROTNONE and _PAGE_PSE bits like in
>>> case pmd_present(). This makes a PUD entry test positive for pud_present()
>>> after getting invalidated with pud_mknotpresent(), hence standardizing the
>>> semantics with PMD helpers.
>>>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: Borislav Petkov <bp@...en8.de>
>>> Cc: "H. Peter Anvin" <hpa@...or.com>
>>> Cc: Dave Hansen <dave.hansen@...el.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: x86@...nel.org
>>> Cc: linux-mm@...ck.org
>>> Cc: linux-kernel@...r.kernel.org
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>> ---
>>> Even though pud_mknotpresent() is not used any where currently, there is
>>> a discrepancy between PMD and PUD.
>>>
>>> WARN_ON(!pud_present(pud_mknotpresent(pud_mkhuge(pud)))) -> Fail
>>> WARN_ON(!pmd_present(pmd_mknotpresent(pmd_mkhuge(pmd)))) -> Pass
>>>
>>> Though pud_mknotpresent() currently clears _PAGE_PROTNONE, pud_present()
>>> does not check it. This change fixes both inconsistencies.
>>>
>>> This has been build and boot tested on x86.
>>
>> Adding Kirill and Dan.
>>
>> +Cc: Kirill A. Shutemov <kirill@...temov.name>
>> +Cc: Dan Williams <dan.j.williams@...el.com>
> 
> Or we can just drop the pud_mknotpresent(). There's no users AFAICS and
> only x86 provides it.

Yes that will be an option but IMHO fixing pud_present() here might be
a better choice because,

(1) pud_mknotpresent() with fixed pud_present() might be required later
(2) PMD & PUD will be exact same (THP is supported on either level)

Nonetheless, I am happy to go either way.