Re: [PATCH] mm: Do not start/end writeback for pages stored in zswap

[Usama Arif <usamaarif642@gmail.com>]

On 10/06/2024 18:31, Yosry Ahmed wrote:
> On Mon, Jun 10, 2024 at 7:31 AM Usama Arif <usamaarif642@gmail.com> wrote:
>> start/end writeback combination incorrectly increments NR_WRITTEN
>> counter, eventhough the pages aren't written to disk. Pages successfully
>> stored in zswap should just unlock folio and return from writepage.
>>
>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>> ---
>>   mm/page_io.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/mm/page_io.c b/mm/page_io.c
>> index a360857cf75d..501784d79977 100644
>> --- a/mm/page_io.c
>> +++ b/mm/page_io.c
>> @@ -196,9 +196,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
>>                  return ret;
>>          }
>>          if (zswap_store(folio)) {
>> -               folio_start_writeback(folio);
>>                  folio_unlock(folio);
>> -               folio_end_writeback(folio);
> Removing these calls will have several effects, I am not really sure it's safe.
>
> 1. As you note in the commit log, NR_WRITTEN stats (and apparently
> others) will no longer be updated. While this may make sense, it's a
> user-visible change. I am not sure if anyone relies on this.

Thanks for the review.

This patch would correct NR_WRITTEN stat, so I think its a good thing? 
But yeah as you said for people relying on that stat, suddenly this 
number would be lowered if they pick up a kernel with this patch, not 
sure how such changes would be dealt with in the kernel.

> 2. folio_end_writeback() calls folio_rotate_reclaimable() after
> writeback completes to put a folio that has been marked with
> PG_reclaim at the tail of the LRU, to be reclaimed first next time. Do
> we get this call through other paths now?

We could add

if (folio_test_reclaim(folio)) {
         folio_clear_reclaim(folio);
         folio_rotate_reclaimable(folio);
     }

to if zswap_store is successful to fix this?

> 3. If I remember correctly, there was some sort of state machine where
> folios go from dirty to writeback to clean. I am not sure what happens
> if we take the writeback phase out of the equation.
>
> Overall, the change seems like it will special case the folios written
> to zswap vs. to disk further, and we may end up missing important
> things (like folio_rotate_reclaimable()). I would like to see a much
> stronger argument for why it is safe and justified tbh :)
>
The patch came about from zero page swap optimization series 
(https://lore.kernel.org/all/ZmcITDhdBzUGEHuY@casper.infradead.org/), 
where Matthew pointed out that NR_WRITTEN would be wrong with the way I 
was doing it.

Overall, I thought it would be good to have consistency with how 
zeropages and zswap pages would be dealt with, and have a more correct 
NR_WRITTEN stat.

In the next revision of the zero page patch, I will just add 
folio_rotate_reclaimable after folio_unlock if folio is zero filled.

>>                  return 0;
>>          }
>>          if (!mem_cgroup_zswap_writeback_enabled(folio_memcg(folio))) {
>> --
>> 2.43.0
>>