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

Byungchul Park
2025-10-03T01:02:37Z

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?
>
> > 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

Thank you for the information. I was wrong then.

That means we shouldn't allow folio_lock() but try lock, while heading
for writeback end in terms of dependency.

Byungchul

> 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
>