linux-kernel - Re: [PATCH 1/2] mm/zswap: don't return LRU

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Date: Sat, 27 Jan 2024 22:37:07 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Nhat Pham <nphamcs@...il.com>
Cc: hannes@...xchg.org, yosryahmed@...gle.com, akpm@...ux-foundation.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH 1/2] mm/zswap: don't return LRU_SKIP if we have dropped
 lru lock

On 2024/1/27 02:01, Nhat Pham wrote:
> On Fri, Jan 26, 2024 at 12:31 AM <chengming.zhou@...ux.dev> wrote:
>>
>> From: Chengming Zhou <zhouchengming@...edance.com>
>>
>> LRU_SKIP can only be returned if we don't ever dropped lru lock, or
>> we need to return LRU_RETRY to restart from the head of lru list.
> 
> Ooops. You're right! I just double checked and only LRU_REMOVED_RETRY
> and LRU_RETRY indicate we might have dropped the lock. My bad.
> 
>>
>> Actually we may need to introduce another LRU_STOP to really terminate
>> the ongoing shrinking scan process, when we encounter a warm page
> 
> Yup. This is what I was trying (and failing) to do. To be honest, this
> needs to be even stronger: short-circuit ALL concurrent/ongoing zswap
> shrinker scan processes that are touching this memcg (as they will
> also shrink into warmer regions going forward). But that's a bit more
> engineering to do. LRU_STOP, which stops this scan process, would be a
> good place to start.

Good suggestion, will look into that more later.

> 
>> already in the swap cache. The current list_lru implementation
>> doesn't have this function to early break from __list_lru_walk_one.
>>
>> Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> 
> Reviewed-by: Nhat Pham <nphamcs@...il.com>

Thanks.

> 
>> ---
>>  mm/zswap.c | 4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 00e90b9b5417..81cb3790e0dd 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -901,10 +901,8 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
>>                  * into the warmer region. We should terminate shrinking (if we're in the dynamic
>>                  * shrinker context).
>>                  */
>> -               if (writeback_result == -EEXIST && encountered_page_in_swapcache) {
>> -                       ret = LRU_SKIP;
>> +               if (writeback_result == -EEXIST && encountered_page_in_swapcache)
>>                         *encountered_page_in_swapcache = true;
>> -               }
>>
>>                 goto put_unlock;
>>         }
>> --
>> 2.40.1
>>