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Reply-To: Gavin Shan <gshan@redhat.com>
Subject: Re: [PATCH v3 12/12] mm/debug_vm_pgtable: Fix corrupted page flag
To:     Anshuman Khandual <anshuman.khandual@arm.com>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org, catalin.marinas@arm.com,
        will@kernel.org, akpm@linux-foundation.org, chuhu@redhat.com,
        shan.gavin@gmail.com
References: <20210719130613.334901-1-gshan@redhat.com>
 <20210719130613.334901-13-gshan@redhat.com>
 <57cb2f04-b3f2-2df4-3d9b-0b430b9c9f3e@arm.com>
From:   Gavin Shan <gshan@redhat.com>
Message-ID: <8157142c-58e0-44c4-5cdb-76fff4a07210@redhat.com>
Date:   Wed, 21 Jul 2021 22:03:39 +1000
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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.

Thanks,
Gavin