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Subject: Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page
To:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Vlastimil Babka <vbabka@suse.cz>
Cc:     linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
        linux-btrfs@vger.kernel.org, Roman Gushchin <guro@fb.com>,
        David Sterba <dsterba@suse.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
References: <20200709024808.18466-1-robbieko@synology.com>
 <859c810e-376e-5e8b-e8a5-0da3f83315d1@suse.cz>
 <80b55fcf-def1-8a83-8f53-a22f2be56244@synology.com>
 <433e26b0-5201-129a-4afe-4881e42781fa@suse.cz>
 <20200714101951.6osakxdgbhrnfrbd@box>
From:   Robbie Ko <robbieko@synology.com>
Message-ID: <a7bb68ef-9b33-3ed7-8eff-91feb2223d80@synology.com>
Date:   Wed, 15 Jul 2020 10:45:39 +0800
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Kirill A. Shutemov 於 2020/7/14 下午6:19 寫道:
> On Tue, Jul 14, 2020 at 11:46:12AM +0200, Vlastimil Babka wrote:
>> On 7/13/20 3:57 AM, Robbie Ko wrote:
>>> Vlastimil Babka 於 2020/7/10 下午11:31 寫道:
>>>> On 7/9/20 4:48 AM, robbieko wrote:
>>>>> From: Robbie Ko <robbieko@synology.com>
>>>>>
>>>>> When a migrate page occurs, we first create a migration entry
>>>>> to replace the original pte, and then go to fallback_migrate_page
>>>>> to execute a writeout if the migratepage is not supported.
>>>>>
>>>>> In the writeout, we will clear the dirty bit of the page and use
>>>>> page_mkclean to clear the dirty bit along with the corresponding pte,
>>>>> but page_mkclean does not support migration entry.
> I don't follow the scenario.
>
> When we establish migration entries with try_to_unmap(), it transfers
> dirty bit from PTE to the page.

Sorry, I mean is _PAGE_RW with pte_write

When we establish migration entries with try_to_unmap(),
we create a migration entry, and if pte_write we set it to SWP_MIGRATION_WRITE,
which will replace the migration entry with the original pte.

When migratepage,  we go to fallback_migrate_page to execute a writeout
if the migratepage is not supported.

In the writeout, we call clear_page_dirty_for_io to  clear the dirty bit of the page
and use page_mkclean to clear pte _PAGE_RW with pte_wrprotect in page_mkclean_one.

However, page_mkclean_one does not support migration entries, so the
migration entry is still SWP_MIGRATION_WRITE.

In writeout, then we call remove_migration_ptes to remove the migration entry,
because it is still SWP_MIGRATION_WRITE so set _PAGE_RW to pte via pte_mkwrite.

Therefore, subsequent mmap wirte will not trigger page_mkwrite to cause data loss.