Re: [PATCH v3] mm/vmscan: stop the loop if enough pages have been page_out

[Chen Ridong <chenridong@huaweicloud.com>]

On 2024/10/21 12:44, Barry Song wrote:
> On Fri, Oct 11, 2024 at 7:49 PM chenridong <chenridong@huawei.com> wrote:
>>
>>
>>
>> On 2024/10/11 0:17, Barry Song wrote:
>>> On Thu, Oct 10, 2024 at 4:59 PM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>>>>
>>>> Hi Ridong,
>>>>
>>>> This should be the first version for upstream, and the issue only
>>>> occurred when large folio is spited.
>>>>
>>>> Adding more CCs to see if there's more feedback.
>>>>
>>>>
>>>> On 2024/10/10 16:18, Chen Ridong wrote:
>>>>> From: Chen Ridong <chenridong@huawei.com>
>>>>>
>>>>> An issue was found with the following testing step:
>>>>> 1. Compile with CONFIG_TRANSPARENT_HUGEPAGE=y
>>>>> 2. Mount memcg v1, and create memcg named test_memcg and set
>>>>>      usage_in_bytes=2.1G, memsw.usage_in_bytes=3G.
>>>>> 3. Create a 1G swap file, and allocate 2.2G anon memory in test_memcg.
>>>>>
>>>>> It was found that:
>>>>>
>>>>> cat memory.usage_in_bytes
>>>>> 2144940032
>>>>> cat memory.memsw.usage_in_bytes
>>>>> 2255056896
>>>>>
>>>>> free -h
>>>>>                 total        used        free
>>>>> Mem:           31Gi       2.1Gi        27Gi
>>>>> Swap:         1.0Gi       618Mi       405Mi
>>>>>
>>>>> As shown above, the test_memcg used about 100M swap, but 600M+ swap memory
>>>>> was used, which means that 500M may be wasted because other memcgs can not
>>>>> use these swap memory.
>>>>>
>>>>> It can be explained as follows:
>>>>> 1. When entering shrink_inactive_list, it isolates folios from lru from
>>>>>      tail to head. If it just takes folioN from lru(make it simple).
>>>>>
>>>>>      inactive lru: folio1<->folio2<->folio3...<->folioN-1
>>>>>      isolated list: folioN
>>>>>
>>>>> 2. In shrink_page_list function, if folioN is THP, it may be splited and
>>>>>      added to swap cache folio by folio. After adding to swap cache, it will
>>>>>      submit io to writeback folio to swap, which is asynchronous.
>>>>>      When shrink_page_list is finished, the isolated folios list will be
>>>>>      moved back to the head of inactive lru. The inactive lru may just look
>>>>>      like this, with 512 filioes have been move to the head of inactive lru.
>>>>>
>>>>>      folioN512<->folioN511<->...filioN1<->folio1<->folio2...<->folioN-1
>>>>>
>>>>> 3. When folio writeback io is completed, the folio may be rotated to tail
>>>>>      of lru. The following lru list is expected, with those filioes that have
>>>>>      been added to swap cache are rotated to tail of lru. So those folios
>>>>>      can be reclaimed as soon as possible.
>>>>>
>>>>>      folio1<->folio2<->...<->folioN-1<->filioN1<->...folioN511<->folioN512
>>>>>
>>>>> 4. However, shrink_page_list and folio writeback are asynchronous. If THP
>>>>>      is splited, shrink_page_list loops at least 512 times, which means that
>>>>>      shrink_page_list is not completed but some folios writeback have been
>>>>>      completed, and this may lead to failure to rotate these folios to the
>>>>>      tail of lru. The lru may look likes as below:
>>>
>>> I assume you’re referring to PMD-mapped THP, but your code also modifies
>>> mTHP, which might not be that large. For instance, it could be a 16KB mTHP.
>>>
>>>>>
>>>>>      folioN50<->folioN49<->...filioN1<->folio1<->folio2...<->folioN-1<->
>>>>>      folioN51<->folioN52<->...folioN511<->folioN512
>>>>>
>>>>>      Although those folios (N1-N50) have been finished writing back, they
>>>>>      are still at the head of lru. When isolating folios from lru, it scans
>>>>>      from tail to head, so it is difficult to scan those folios again.
>>>>>
>>>>> What mentioned above may lead to a large number of folios have been added
>>>>> to swap cache but can not be reclaimed in time, which may reduce reclaim
>>>>> efficiency and prevent other memcgs from using this swap memory even if
>>>>> they trigger OOM.
>>>>>
>>>>> To fix this issue, it's better to stop looping if THP has been splited and
>>>>> nr_pageout is greater than nr_to_reclaim.
>>>>>
>>>>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>>>>> ---
>>>>>    mm/vmscan.c | 16 +++++++++++++++-
>>>>>    1 file changed, 15 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>> index 749cdc110c74..fd8ad251eda2 100644
>>>>> --- a/mm/vmscan.c
>>>>> +++ b/mm/vmscan.c
>>>>> @@ -1047,7 +1047,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>>>>        LIST_HEAD(demote_folios);
>>>>>        unsigned int nr_reclaimed = 0;
>>>>>        unsigned int pgactivate = 0;
>>>>> -     bool do_demote_pass;
>>>>> +     bool do_demote_pass, splited = false;
>>>>>        struct swap_iocb *plug = NULL;
>>>>>
>>>>>        folio_batch_init(&free_folios);
>>>>> @@ -1065,6 +1065,16 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>>>>
>>>>>                cond_resched();
>>>>>
>>>>> +             /*
>>>>> +              * If a large folio has been split, many folios are added
>>>>> +              * to folio_list. Looping through the entire list takes
>>>>> +              * too much time, which may prevent folios that have completed
>>>>> +              * writeback from rotateing to the tail of the lru. Just
>>>>> +              * stop looping if nr_pageout is greater than nr_to_reclaim.
>>>>> +              */
>>>>> +             if (unlikely(splited && stat->nr_pageout > sc->nr_to_reclaim))
>>>>> +                     break;
>>>
>>> I’m not entirely sure about the theory behind comparing stat->nr_pageout
>>> with sc->nr_to_reclaim. However, the condition might still hold true even
>>> if you’ve split a relatively small “large folio,” such as 16kB?
>>>
>>
>> Why compare stat->nr_pageout with sc->nr_to_reclaim? It's because if all
>> pages that have been pageout can be reclaimed, then enough pages can be
>> reclaimed when all pages have finished writeback. Thus, it may not have
>> to pageout more.
>>
>> If a small large folio(16 kB) has been split, it may return early
>> without the entire pages in the folio_list being pageout, but I think
>> that is fine. It can pageout more pages the next time it enters
>> shrink_folio_list if there are not enough pages to reclaimed.
>>
>> However, if pages that have been pageout are still at the head of the
>> LRU, it is difficult to scan these pages again. In this case, not only
>> might it "waste" some swap memory but it also has to pageout more pages.
>>
>> Considering the above, I sent this patch. It may not be a perfect
>> solution, but i think it's a good option to consider. And I am wondering
>> if anyone has a better solution.
> 
> Hi Ridong,
> My overall understanding is that you have failed to describe your problem
> particularly I don't understand what your 3 and 4 mean:
> 
>> 3. When folio writeback io is completed, the folio may be rotated to tail
>>    of lru. The following lru list is expected, with those filioes that have
>>    been added to swap cache are rotated to tail of lru. So those folios
>>  can be reclaimed as soon as possible.
>>
>>  folio1<->folio2<->...<->folioN-1<->filioN1<->...folioN511<->folioN512
> 
>  > 4. However, shrink_page_list and folio writeback are asynchronous. If THP
>  >    is splited, shrink_page_list loops at least 512 times, which means that
>  >    shrink_page_list is not completed but some folios writeback have been
>  >    completed, and this may lead to failure to rotate these folios to the
>   >  tail of lru. The lru may look likes as below:
> 
> can you please describe it in a readable approach?
> 
> i feel your below diagram is somehow wrong:
> folio1<->folio2<->...<->folioN-1<->filioN1<->...folioN511<->folioN512
> 
> You mentioned "rotate', how could "rotate" makes:
> folioN512<->folioN511<->...filioN1 in (2)
> become
> filioN1<->...folioN511<->folioN512 in (3).
> 

I am sorry for any confusion.

If THP is split, filioN1, filioN2, filioN3, ...filioN512 are committed
to writeback one by one. it assumed that filioN1,
filioN2,filioN3,...filioN512 are completed in order.

Orignal:
folioN512<->folioN511<->...filioN1<->folio1<->folio2...<->folioN-1

filioN1 is finished, filioN1 is rotated to the tail of LRU:
folioN512<->folioN511<->...filioN2<->folio1<->folio2...<->folioN-1<->folioN1

filioN2 is finished:
folioN512<->folioN511<->...filioN3<->folio1<->folio2...<->folioN-1<->folioN1<->folioN2

filioN3 is finished:
folioN512<->folioN511<->...filioN4<->folio1<->folio2...<->folioN-1<->folioN1<->folioN2<->filioN3

...

filioN512 is finished:
folio1<->folio2<->...<->folioN-1<->filioN1<->...folioN511<->folioN512

When the filios are finished, the LRU might just like this:
folio1<->folio2<->...<->folioN-1<->filioN1<->...folioN511<->folioN512

> btw, writeback isn't always async. it could be sync for zram and sync_io
> swap. in that case, your patch might change the order of LRU. i mean,
> for example, while a mTHP becomes cold, we always reclaim all of them,
> but not part of them and put back part of small folios to the head of lru.
> 

Yes, This can be changed.
Although it may put back part of small folios to the head of lru, it can
return in time from shrink_folio_list without causing much additional I/O.

If you have understood this issue, do you have any suggestions to fix
it? My patch may not be a perfect way to fix this issue.

Best regards,
Ridong