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

[Robbie Ko <robbieko@synology.com>]

Vlastimil Babka 於 2020/7/14 下午5:46 寫道:
> 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.
>>>>
>>>> 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
> And the kernel is otherwise upstream? Which version?
> Anyway we better let btrfs guys know (+CC) even if the fix is in MM code.

Kernel verion is 4.4.
I think this is a bug that has been around for a long time.

I think the problem is not limited to btrfs, as long as other fs
have not implemented the migrationpage, they will encounter
the problem. (Eg ecryptfs, fat, nfs...)

>> 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
> That's a new commit, so if this is really affecting upstream btrfs pre-5.8 we
> should either backport that commit, or your fix (after review).
>
>>>> 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)
>>>>
>