Re: [PATCH RFC 1/1] mm/swap: queue reclaimable folio to local rotate batch when !folio_test_lru()

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Feb 14, 2024 at 05:54:56PM +0800, Chengming Zhou wrote:
> On 2024/2/13 16:49, Yosry Ahmed wrote:
> > On Fri, Feb 9, 2024 at 4:00 AM <chengming.zhou@linux.dev> wrote:
> >>
> >> From: Chengming Zhou <zhouchengming@bytedance.com>
> >>
> >> All LRU move interfaces have a problem that it has no effect if the
> >> folio is isolated from LRU (in cpu batch or isolated by shrinker).
> >> Since it can't move/change folio LRU status when it's isolated, mostly
> >> just clear the folio flag and do nothing in this case.
> >>
> >> In our case, a written back and reclaimable folio won't be rotated to
> >> the tail of inactive list, since it's still in cpu lru_add batch. It
> >> may cause the delayed reclaim of this folio and evict other folios.
> >>
> >> This patch changes to queue the reclaimable folio to cpu rotate batch
> >> even when !folio_test_lru(), hoping it will likely be handled after
> >> the lru_add batch which will put folio on the LRU list first, so
> >> will be rotated to the tail successfully when handle rotate batch.
> > 
> > It seems to me that it is totally up to chance whether the lru_add
> > batch is handled first, especially that there may be problems if it
> > isn't.
> 
> You're right, I just don't know better solution :)
> 
> > 
> >>
> >> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> >> ---
> >>  mm/swap.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/mm/swap.c b/mm/swap.c
> >> index cd8f0150ba3a..d304731e47cf 100644
> >> --- a/mm/swap.c
> >> +++ b/mm/swap.c
> >> @@ -236,7 +236,8 @@ static void folio_batch_add_and_move(struct folio_batch *fbatch,
> >>
> >>  static void lru_move_tail_fn(struct lruvec *lruvec, struct folio *folio)
> >>  {
> >> -       if (!folio_test_unevictable(folio)) {
> >> +       if (!folio_test_locked(folio) && !folio_test_dirty(folio) &&
> >> +           !folio_test_unevictable(folio) && !folio_test_active(folio)) {
> > 
> > What are these conditions based on? I assume we want to check if the
> > folio is locked because we no longer check that it is on the LRUs, so
> > we want to check that no one else is operating on it, but I am not
> > sure that's enough.
> 
> These conditions are used for checking whether the folio should be reclaimed/rotated
> at this point. Like we shouldn't reclaim it if it has been dirtied or actived.

This should be explained somewhere, a comment or in the commit message.
 
> lru_move_tail_fn() will only be called after we isolate this folio successfully
> in folio_batch_move_lru(), so if other path has isolated this folio (cpu batch
> or reclaim shrinker), this function will not be called.

Interesting, why are we checking if the folio is locked here then?

> 
> > 
> >>                 lruvec_del_folio(lruvec, folio);
> >>                 folio_clear_active(folio);
> >>                 lruvec_add_folio_tail(lruvec, folio);
> >> @@ -254,7 +255,7 @@ static void lru_move_tail_fn(struct lruvec *lruvec, struct folio *folio)
> >>  void folio_rotate_reclaimable(struct folio *folio)
> >>  {
> >>         if (!folio_test_locked(folio) && !folio_test_dirty(folio) &&
> >> -           !folio_test_unevictable(folio) && folio_test_lru(folio)) {
> >> +           !folio_test_unevictable(folio) && !folio_test_active(folio)) {
> > 
> > I am not sure it is safe to continue with a folio that is not on the
> > LRUs. It could be isolated for other purposes, and we end up adding it
> > to an LRU nonetheless. Also, folio_batch_move_lru() will do
> 
> This shouldn't happen since lru_move_tail_fn() will only be called if
> folio_test_clear_lru() successfully in folio_batch_move_lru().

I see, so this is where we hope lru_add batch gets handled first. I need
to think about this some more, let's also see what others like Yu say.

Thanks!