Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page

[Robbie Ko <robbieko@synology.com>]

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.
>>
>> The page ditry bit is cleared, but the dirty bit of the pte still exists,
>> so if mmap continues to write, it will result in data loss.
> Curious, did you observe this data loss? What filesystem? If yes, it seems
> serious enough to
> CC stable and determine a Fixes: tag?

Yes, there is data loss.
I'm using a btrfs environment, but not the following patch
btrfs: implement migratepage callback for data pages
https://git.kernel.org/pub/scm/linux/kernel 
/git/torvalds/linux.git/commit/?h=v5.8-rc5& 
id=f8e6608180a31cc72a23b74969da428da236dbd1


>> We fix the by first remove the migration entry and then clearing
>> the dirty bits of the page, which also clears the pte's dirty bits.
>>
>> Signed-off-by: Robbie Ko <robbieko@synology.com>
>> ---
>>   mm/migrate.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index f37729673558..5c407434b9ba 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -875,10 +875,6 @@ static int writeout(struct address_space *mapping, struct page *page)
>>   		/* No write method for the address space */
>>   		return -EINVAL;
>>   
>> -	if (!clear_page_dirty_for_io(page))
>> -		/* Someone else already triggered a write */
>> -		return -EAGAIN;
>> -
>>   	/*
>>   	 * A dirty page may imply that the underlying filesystem has
>>   	 * the page on some queue. So the page must be clean for
>> @@ -889,6 +885,10 @@ static int writeout(struct address_space *mapping, struct page *page)
>>   	 */
>>   	remove_migration_ptes(page, page, false);
>>   
>> +	if (!clear_page_dirty_for_io(page))
>> +		/* Someone else already triggered a write */
>> +		return -EAGAIN;
>> +
>>   	rc = mapping->a_ops->writepage(page, &wbc);
>>   
>>   	if (rc != AOP_WRITEPAGE_ACTIVATE)
>>
>