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

[Chengming Zhou <chengming.zhou@linux.dev>]

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.

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.

> 
>>                 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().

Thanks.