diff mbox series

[1/2] mm/zswap: don't return LRU_SKIP if we have dropped lru lock

Message ID 20240126083015.3557006-1-chengming.zhou@linux.dev
State New
Headers
Series [1/2] mm/zswap: don't return LRU_SKIP if we have dropped lru lock |

Commit Message

Chengming Zhou Jan. 26, 2024, 8:30 a.m. UTC 
  From: Chengming Zhou <zhouchengming@bytedance.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.

Actually we may need to introduce another LRU_STOP to really terminate
the ongoing shrinking scan process, when we encounter a warm page
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@bytedance.com>
---
 mm/zswap.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

Comments

Johannes Weiner Jan. 26, 2024, 2:28 p.m. UTC | #1 
On Fri, Jan 26, 2024 at 08:30:14AM +0000, chengming.zhou@linux.dev wrote:
> From: Chengming Zhou <zhouchengming@bytedance.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.

Good catch. Can you mention the possible consequences in the log?

"Otherwise, the iteration might continue from a cursor position that
was freed while the locks were dropped."?

> Actually we may need to introduce another LRU_STOP to really terminate
> the ongoing shrinking scan process, when we encounter a warm page
> 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@bytedance.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Nhat Pham Jan. 26, 2024, 6:01 p.m. UTC | #2 
On Fri, Jan 26, 2024 at 12:31 AM <chengming.zhou@linux.dev> wrote:
>
> From: Chengming Zhou <zhouchengming@bytedance.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.

> 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@bytedance.com>

Reviewed-by: Nhat Pham <nphamcs@gmail.com>

> ---
>  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
>
Chengming Zhou Jan. 27, 2024, 2:33 p.m. UTC | #3 
On 2024/1/26 22:28, Johannes Weiner wrote:
> On Fri, Jan 26, 2024 at 08:30:14AM +0000, chengming.zhou@linux.dev wrote:
>> From: Chengming Zhou <zhouchengming@bytedance.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.
> 
> Good catch. Can you mention the possible consequences in the log?
> 
> "Otherwise, the iteration might continue from a cursor position that
> was freed while the locks were dropped."?

Good, will do.

> 
>> Actually we may need to introduce another LRU_STOP to really terminate
>> the ongoing shrinking scan process, when we encounter a warm page
>> 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@bytedance.com>
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks.
Chengming Zhou Jan. 27, 2024, 2:37 p.m. UTC | #4 
On 2024/1/27 02:01, Nhat Pham wrote:
> On Fri, Jan 26, 2024 at 12:31 AM <chengming.zhou@linux.dev> wrote:
>>
>> From: Chengming Zhou <zhouchengming@bytedance.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@bytedance.com>
> 
> Reviewed-by: Nhat Pham <nphamcs@gmail.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
>>
diff mbox series

Patch

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;
 	}