On 2024/10/21 12:44, Barry Song wrote: > On Fri, Oct 11, 2024 at 7:49 PM chenridong <chenridong@xxxxxxxxxx> wrote: >> >> >> >> On 2024/10/11 0:17, Barry Song wrote: >>> On Thu, Oct 10, 2024 at 4:59 PM Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> 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@xxxxxxxxxx> >>>>> >>>>> 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@xxxxxxxxxx> >>>>> --- >>>>> 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
