linux-kernel - Re: [PATCH v3 12/12] mm/debug_vm_pgtable: Fix corrupted page flag

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Message-ID: <5db75fd6-aeb9-c06f-30ab-839c09a0bc68@arm.com>
Date:   Thu, 22 Jul 2021 09:21:11 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Gavin Shan <gshan@...hat.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, catalin.marinas@....com,
        will@...nel.org, akpm@...ux-foundation.org, chuhu@...hat.com,
        shan.gavin@...il.com
Subject: Re: [PATCH v3 12/12] mm/debug_vm_pgtable: Fix corrupted page flag

Small nit:

s/Fix corrupted page flag/Fix page flag corruption on arm64/

On 7/21/21 5:33 PM, Gavin Shan wrote:
> Hi Anshuman,
> 
> On 7/21/21 8:18 PM, Anshuman Khandual wrote:
>> On 7/19/21 6:36 PM, Gavin Shan wrote:
>>> In page table entry modifying tests, set_xxx_at() are used to populate
>>> the page table entries. On ARM64, PG_arch_1 is set to the target page
>>> flag if execution permission is given. 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.
>>
>> Could you please add comments before each flush_dcache_page() instance
>> explaining why this is needed for arm64 platforms with relevant PG_arch_1
>> context and how this does not have any adverse effect on other platforms ?
>> It should be easy for some one looking at this code after a while to figure
>> out from where flush_dcache_page() came from.
>>
> 
> Good point. I will improve chage log to include the commit ID in v4 where the
> page flag (PG_arch_1) is used and explain how. In that case, it's much clearer
> to understand the reason why we need flush_dcache_page() after set_xxx_at() on
> ARM64.

But also some in code comments where flush_dcache_page() is being called.