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

[Robbie Ko]

Vlastimil Babka æ 2020/7/14 äå5:46 åé:
> On 7/13/20 3:57 AM, Robbie Ko wrote:
> > Vlastimil Babka æ 2020/7/10 äå11:31 åé:
> > > On 7/9/20 4:48 AM, robbieko wrote:
> > > > From: Robbie Ko <robbieko@xxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxxx>
> > > > ---
> > > >    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)