On Tue, Jan 14, 2025 at 11:03 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Jan 14, 2025 at 12:12 AM Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Jan 13, 2025 at 08:17:20AM -0800, Yosry Ahmed wrote:
> > > On Mon, Jan 13, 2025 at 1:35 AM Kirill A. Shutemov
> > > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > > >
> > > > The recently introduced PG_dropbehind allows for freeing folios
> > > > immediately after writeback. Unlike PG_reclaim, it does not need vmscan
> > > > to be involved to get the folio freed.
> > > >
> > > > Instead of using folio_set_reclaim(), use folio_set_dropbehind() in
> > > > lru_deactivate_file().
> > > >
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > ---
> > > > mm/swap.c | 8 +-------
> > > > 1 file changed, 1 insertion(+), 7 deletions(-)
> > > >
> > > > diff --git a/mm/swap.c b/mm/swap.c
> > > > index fc8281ef4241..4eb33b4804a8 100644
> > > > --- a/mm/swap.c
> > > > +++ b/mm/swap.c
> > > > @@ -562,14 +562,8 @@ static void lru_deactivate_file(struct lruvec *lruvec, struct folio *folio)
> > > > folio_clear_referenced(folio);
> > > >
> > > > if (folio_test_writeback(folio) || folio_test_dirty(folio)) {
> > > > - /*
> > > > - * Setting the reclaim flag could race with
> > > > - * folio_end_writeback() and confuse readahead. But the
> > > > - * race window is _really_ small and it's not a critical
> > > > - * problem.
> > > > - */
> > > > lruvec_add_folio(lruvec, folio);
> > > > - folio_set_reclaim(folio);
> > > > + folio_set_dropbehind(folio);
> > > > } else {
> > > > /*
> > > > * The folio's writeback ended while it was in the batch.
> > >
> > > Now there's a difference in behavior here depending on whether or not
> > > the folio is under writeback (or will be written back soon). If it is,
> > > we set PG_dropbehind to get it freed right after, but if writeback has
> > > already ended we put it on the tail of the LRU to be freed later.
> > >
> > > It's a bit counterintuitive to me that folios with pending writeback
> > > get freed faster than folios that completed their writeback already.
> > > Am I missing something?
> >
> > Yeah, it is strange.
> >
> > I think we can drop the writeback/dirty check. Set PG_dropbehind and put
> > the page on the tail of LRU unconditionally. The check was required to
> > avoid confusion with PG_readahead.
> >
> > Comment above the function is not valid anymore.
>
> My read is that we don't put dirty/writeback folios at the tail of the
> LRU because they cannot be freed immediately and we want to give them
> time to be written back before reclaim reaches them. So I don't think
> we want to change that and always put the pages at the tail.
>
> >
> > But the folio that is still dirty under writeback will be freed faster as
> > we get rid of the folio just after writeback is done while clean page can
> > dangle on LRU for a while.
>
> Yeah if we reuse PG_dropbehind then we cannot avoid
> folio_end_writeback() freeing the folio faster than clean ones.
>
> >
> > I don't think we have any convenient place to free clean dropbehind page
> > other than shrink_folio_list(). Or do we?
>
> Not sure tbh. FWIW I am not saying it's necessarily a bad thing to
> free dirty/writeback folios before clean ones when deactivated, it's
> just strange and a behavioral change from today that I wanted to point
> out. Perhaps that's the best we can do for now.
>
> >
> > Looking at shrink_folio_list(), I think we need to bypass page demotion
> > for PG_dropbehind pages.
I agree with Yosry. I don't think lru_deactivate_file() is still
needed -- it was needed only because when truncation fails to free a
dirty/writeback folio, page reclaim can do that quickly. For other
conditions that mapping_evict_folio() returns 0, there isn't much page
reclaim can do, and those conditions are not deactivate_file_folio()
and lru_deactivate_file()'s intentions. So the following should be
enough, and it's a lot cleaner :
diff --git a/mm/truncate.c b/mm/truncate.c
index e2e115adfbc5..12d2aa608517 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -486,7 +486,7 @@ unsigned long mapping_try_invalidate(struct
address_space *mapping,
* of interest and try to speed up its reclaim.
*/
if (!ret) {
- deactivate_file_folio(folio);
+ folio_set_dropbehind(folio)
/* Likely in the lru cache of a remote CPU */
if (nr_failed)
(*nr_failed)++;
Then we can drop deactivate_file_folio() and lru_deactivate_file().
