linux-kernel - Re: [PATCH v4 12/12] mm/debug_vm_pgtable: Fix corrupted page flag

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Date:   Wed, 28 Jul 2021 15:35:37 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Gavin Shan <gshan@...hat.com>
Cc:     shan.gavin@...il.com, chuhu@...hat.com, akpm@...ux-foundation.org,
        will@...nel.org, catalin.marinas@....com, cai@....pw,
        aneesh.kumar@...ux.ibm.com, gerald.schaefer@...ux.ibm.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 12/12] mm/debug_vm_pgtable: Fix corrupted page flag



On 7/28/21 1:23 PM, Christophe Leroy wrote:
> Gavin Shan <gshan@...hat.com> a écrit :
> 
>> In page table entry modifying tests, set_xxx_at() are used to populate
>> the page table entries. On ARM64, PG_arch_1 (PG_dcache_clean) flag is
>> set to the target page flag if execution permission is given. The logic
>> exits since commit 4f04d8f00545 ("arm64: MMU definitions"). The page
>> flag is kept when the page is free'd to buddy's free area list. However,
>> it will trigger page checking failure when it's pulled from the buddy's
>> free area list, as the following warning messages indicate.
>>
>>    BUG: Bad page state in process memhog  pfn:08000
>>    page:0000000015c0a628 refcount:0 mapcount:0 \
>>         mapping:0000000000000000 index:0x1 pfn:0x8000
>>    flags: 0x7ffff8000000800(arch_1|node=0|zone=0|lastcpupid=0xfffff)
>>    raw: 07ffff8000000800 dead000000000100 dead000000000122 0000000000000000
>>    raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
>>    page dumped because: PAGE_FLAGS_CHECK_AT_PREP flag(s) set
>>
>> This fixes the issue by clearing PG_arch_1 through flush_dcache_page()
>> after set_xxx_at() is called. For architectures other than ARM64, the
>> unexpected overhead of cache flushing is acceptable.
>>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
> 
> Maybe a Fixes: tag would be good to have

Agreed.

Fixes: a5c3b9ffb0f4 ("mm/debug_vm_pgtable: add tests validating advanced arch page table helpers")

> 
> And would it be possible to have this fix as first patch of the series so that it can be applied to stable without applying the whole series ?
Changing the allocation scheme does solve another problem (using non-owned pages)
but is achieved via the entire series applied. But this particular patch could be
moved to the beginning without much problem.