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

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Date:   Wed, 21 Jul 2021 15:48:34 +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



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.