Michal Hocko <mhocko@xxxxxxxx> writes: > On Wed 22-11-23 02:39:15, Yosry Ahmed wrote: >> On Wed, Nov 22, 2023 at 2:09 AM Michal Hocko <mhocko@xxxxxxxx> wrote: >> > >> > On Wed 22-11-23 09:52:42, Michal Hocko wrote: >> > > On Tue 21-11-23 22:44:32, Yosry Ahmed wrote: >> > > > On Tue, Nov 21, 2023 at 10:41 PM Liu Shixin <liushixin2@xxxxxxxxxx> wrote: >> > > > > >> > > > > >> > > > > On 2023/11/21 21:00, Michal Hocko wrote: >> > > > > > On Tue 21-11-23 17:06:24, Liu Shixin wrote: >> > > > > > >> > > > > > However, in swapcache_only mode, the scan count still increased when scan >> > > > > > non-swapcache pages because there are large number of non-swapcache pages >> > > > > > and rare swapcache pages in swapcache_only mode, and if the non-swapcache >> > > > > > is skipped and do not count, the scan of pages in isolate_lru_folios() can >> > > > > > eventually lead to hung task, just as Sachin reported [2]. >> > > > > > I find this paragraph really confusing! I guess what you meant to say is >> > > > > > that a real swapcache_only is problematic because it can end up not >> > > > > > making any progress, correct? >> > > > > This paragraph is going to explain why checking swapcache_only after scan += nr_pages; >> > > > > > >> > > > > > AFAIU you have addressed that problem by making swapcache_only anon LRU >> > > > > > specific, right? That would be certainly more robust as you can still >> > > > > > reclaim from file LRUs. I cannot say I like that because swapcache_only >> > > > > > is a bit confusing and I do not think we want to grow more special >> > > > > > purpose reclaim types. Would it be possible/reasonable to instead put >> > > > > > swapcache pages on the file LRU instead? >> > > > > It looks like a good idea, but I'm not sure if it's possible. I can try it, is there anything to >> > > > > pay attention to? >> > > > >> > > > I think this might be more intrusive than we think. Every time a page >> > > > is added to or removed from the swap cache, we will need to move it >> > > > between LRUs. All pages on the anon LRU will need to go through the >> > > > file LRU before being reclaimed. I think this might be too big of a >> > > > change to achieve this patch's goal. >> > > >> > > TBH I am not really sure how complex that might turn out to be. >> > > Swapcache tends to be full of subtle issues. So you might be right but >> > > it would be better to know _why_ this is not possible before we end up >> > > phising for couple of swapcache pages on potentially huge anon LRU to >> > > isolate them. Think of TB sized machines in this context. >> > >> > Forgot to mention that it is not really far fetched from comparing this >> > to MADV_FREE pages. Those are anonymous but we do not want to keep them >> > on anon LRU because we want to age them indepdendent on the swap >> > availability as they are just dropped during reclaim. Not too much >> > different from swapcache pages. There are more constrains on those but >> > fundamentally this is the same problem, no? >> >> I agree it's not a first, but swap cache pages are more complicated >> because they can go back and forth, unlike MADV_FREE pages which >> usually go on a one way ticket AFAICT. > > Yes swapcache pages are indeed more complicated but most of the time > they just go away as well, no? When we swapin a page, we will put it in swapcache too. And the page can be in that state for long time if there is more than 50% free space in the swap device. > MADV_FREE can be reinitiated if they are > written as well. So fundamentally they are not that different. > [snip] -- Best Regards, Huang, Ying
