Re: [PATCH v3 2/3] mm: swap: drop dropbehind swap cache folios on writeback completion

[Barry Song]

On Thu, Aug 20, 2026 at 11:38 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Aug 20, 2026 at 07:36:42PM +0800, Barry Song wrote:
> > > +void swap_writeback_dropbehind_folio(struct folio *folio)
> > > +{
> > > +       llist_add((struct llist_node *)&folio->lru,
> > > +                 raw_cpu_ptr(&swap_dropbehind_llist));
> > > +       queue_work(swap_dropbehind_wq, &swap_dropbehind_work);
> > > +}
> > > +
> >
> > This is good. For file dropbehind, I see both F2FS [1] and block
> > devices [2] reinventing workqueues in their own filesystems or
> > drivers, while folio_end_dropbehind() simply benefits from its safe,
> > sleepable context. This seems a bit odd.
> >
> > So I'm really curious whether this could be extended to file-backed
> > folios as well, so that those filesystems and device drivers don't
> > have to deal with this themselves.
>
> You should probably both be aware of Tal's series:
>
> https://lore.kernel.org/linux-mm/20260730-blk-dontcache-v7-0-3e8e6850068d@xxxxxxxxxxxx/

Yes, thanks. I even asked a question there, but nobody has answered it yet. :-)

https://lore.kernel.org/linux-mm/CAGsJ_4wUvLoKGafR3U-ji1kUUaz6K_QdN6QxMT-apSaO5tXp0A@xxxxxxxxxxxxxx/

>
> In fact, I'd base this series on Tal's.  It would be considerably
> easier.

I assume there are two things which might be helpful?
1.  bio_in_atomic() helper
2.  bio_set_flag(bio, BIO_COMPLETE_IN_TASK);

Then, if this flag is set, we move the bio completion to a workqueue.
Right now,  Alexandre's approach is moving the folio dropbehind to workqueue
but not moving bio completion to workqueue.

+ if (swap_dropbehind) {
+     swap_writeback_dropbehind_folio(folio);
+     return;
+ }

Not sure which approach is better, but moving bio completion to a
workqueue seems to have less coupling with the LRU and related code.

Maybe we can reuse Tal's code, for example, bio_complete_in_task()?