Re: [RFC] mm/migrate: make sure folio_unlock() before folio_wait_writeback()

[Byungchul Park <byungchul@sk.com>]

On Thu, Oct 02, 2025 at 01:38:59PM +0200, David Hildenbrand wrote:
> > To simplify the scenario:
> > 
> 
> Just curious, where is the __folio_start_writeback() to complete the
> picture?

ext4_end_io_end() was running as a wq worker after the io completion.

DEPT report can tell that the following scenario happened with
__folio_start_writeback() called far earlier, at least, before
folio_test_writeback() was seen as true, but unfortunately DEPT doesn't
capture the exact location of __folio_start_writeback().

	Byungchul

> >     context X (wq worker)     context Y (process context)
> > 
> >                               migrate_pages_batch()
> >     ext4_end_io_end()           ...
> >       ...                       migrate_folio_unmap()
> >       ext4_get_inode_loc()        ...
> >         ...                       folio_lock() // hold the folio lock
> >         bdev_getblk()             ...
> >           ...                     folio_wait_writeback() // wait forever
> >           __find_get_block_slow()
> >             ...                           ...
> >             folio_lock() // wait forever
> >             folio_unlock()      migrate_folio_undo_src()
> >                                   ...
> >       ...                         folio_unlock() // never reachable
> >       ext4_finish_bio()
> >       ...
> >       folio_end_writeback() // never reachable
> > 
> 
> But aren't you implying that it should from this point on be disallowed
> to call folio_wait_writeback() with the folio lock held? That sounds ...
> a bit wrong.
> 
> Note that it is currently explicitly allowed: folio_wait_writeback()
> documents "If the folio is not locked, writeback may start again after
> writeback has finished.". So there is no way to prevent writeback from
> immediately starting again.
> 
> In particular, wouldn't we have to fixup other callsites to make this
> consistent and then VM_WARN_ON_ONCE() assert that in folio_wait_writeback()?
> 
> Of course, as we've never seen this deadlock before in practice, I do
> wonder if something else prevents it?
> 
> If it's a real issue, I wonder if a trylock on the writeback path could
> be an option.
> 
> --
> Cheers
> 
> David / dhildenb
>