在 2020/9/12 上午10:13, Hugh Dickins 写道: > On Fri, 11 Sep 2020, Alex Shi wrote: >> 在 2020/9/10 上午7:16, Hugh Dickins 写道: >>> On Wed, 9 Sep 2020, Alex Shi wrote: >>>> 在 2020/9/9 上午7:41, Hugh Dickins 写道: >>>>> >>>>> [PATCH v18 05/32] mm/thp: remove code path which never got into >>>>> This is a good simplification, but I see no sign that you understand >>>>> why it's valid: it relies on lru_add_page_tail() being called while >>>>> head refcount is frozen to 0: we would not get this far if someone >>>>> else holds a reference to the THP - which they must hold if they have >>>>> isolated the page from its lru (and that's true before or after your >>>>> per-memcg changes - but even truer after those changes, since PageLRU >>>>> can then be flipped without lru_lock at any instant): please explain >>>>> something of this in the commit message. >>>> >>>> Is the following commit log better? >>>> >>>> split_huge_page() will never call on a page which isn't on lru list, so >>>> this code never got a chance to run, and should not be run, to add tail >>>> pages on a lru list which head page isn't there. >>>> >>>> Hugh Dickins' mentioned: >>>> The path should never be called since lru_add_page_tail() being called >>>> while head refcount is frozen to 0: we would not get this far if someone >>>> else holds a reference to the THP - which they must hold if they have >>>> isolated the page from its lru. >>>> >>>> Although the bug was never triggered, it'better be removed for code >>>> correctness, and add a warn for unexpected calling. >>> >>> Not much better, no. split_huge_page() can easily be called for a page >>> which is not on the lru list at the time, >> >> Hi Hugh, >> >> Thanks for comments! >> >> There are some discussion on this point a couple of weeks ago, >> https://lkml.org/lkml/2020/7/9/760 >> >> Matthew Wilcox and Kirill have the following comments, >>> I don't understand how we get to split_huge_page() with a page that's >>> not on an LRU list. Both anonymous and page cache pages should be on >>> an LRU list. What am I missing? >> >> Right, and it's never got removed from LRU during the split. The tail >> pages have to be added to LRU because they now separate from the tail >> page. >> >> -- >> Kirill A. Shutemov > > Yes, those were among the mails that I read through before getting > down to review. I was surprised by their not understanding, but > it was a bit late to reply to that thread. > > Perhaps everybody had been focused on pages which have been and > naturally belong on an LRU list, rather than pages which are on > the LRU list at the instant that split_huge_page() is called. > > There are a number of places where PageLRU gets cleared, and a > number of places where we del_page_from_lru_list(), I think you'll > agree: your patches touch all or most of them. Let's think of a > common one, isolate_lru_pages() used by page reclaim, but the same > would apply to most of the others. > > Then there a number of places where split_huge_page() is called: > I am having difficulty finding any of those which cannot race with > page reclaim, but shall we choose anon THP's deferred_split_scan(), > or shmem THP's shmem_punch_compound()? > > What prevents either of those from calling split_huge_page() at > a time when isolate_lru_pages() has removed the page from LRU? > > But there's no problem in this race, because anyone isolating the > page from LRU must hold their own reference to the page (to prevent > it from being freed independently), and the can_split_huge_page() or > page_ref_freeze() in split_huge_page_to_list() will detect that and > fail the split with -EBUSY (or else succeed and prevent new references > from being acquired). So this case never reaches lru_add_page_tail(). Hi Hugh, Thanks for comments! We are the same page here, we all know split_huge_page_to_list could block them go futher and the code is functionality right. If the comments 'Split start from PageLRU(head), and ...' doesn't make things clear as it's should be, I am glad to see you rewrite and improve them. > >> >>> and I don't know what was the >>> bug which was never triggered. >> >> So the only path to the removed part should be a bug, like sth here, >> https://lkml.org/lkml/2020/7/10/118 >> or >> https://lkml.org/lkml/2020/7/10/972 > > Oh, the use of split_huge_page() in __iommu_dma_alloc_pages() is just > nonsense, I thought it had already been removed - perhaps some debate > over __GFP_COMP held it up. Not something you need worry about in > this patchset. > >> >>> Stick with whatever text you end up with >>> for the combination of 05/32 and 18/32, and I'll rewrite it after. >> >> I am not object to merge them into one, I just don't know how to say >> clear about 2 patches in commit log. As patch 18, TestClearPageLRU >> add the incorrect posibility of remove lru bit during split, that's >> the reason of code path rewrite and a WARN there. > > I did not know that was why you were putting 18/32 in at that > point, it does not mention TestClearPageLRU at all. But the fact > remains that it's a nice cleanup, contains a reassuring WARN if we > got it wrong (and I've suggested a WARN on the other branch too), > it was valid before your changes, and it's valid after your changes. > Please merge it back into the uglier 05/32, and again I'll rewrite > whatever comment you come up with if necessary. I merge them together on the following git branch, and let the commit log to you. :) https://github.com/alexshi/linux.git lruv19 > >>> >>>>> [PATCH v18 06/32] mm/thp: narrow lru locking >>>>> Why? What part does this play in the series? "narrow lru locking" can >>>>> also be described as "widen page cache locking": >>>> >>>> Uh, the page cache locking isn't widen, it's still on the old place. >>> >>> I'm not sure if you're joking there. Perhaps just a misunderstanding. >>> >>> Yes, patch 06/32 does not touch the xa_lock(&mapping->i_pages) and >>> xa_lock(&swap_cache->i_pages) lines (odd how we've arrived at two of >>> those, but please do not get into cleaning it up now); but it removes >>> the spin_lock_irqsave(&pgdata->lru_lock, flags) which used to come >>> before them, and inserts a spin_lock(&pgdat->lru_lock) after them. >>> >>> You call that narrowing the lru locking, okay, but I see it as also >>> pushing the page cache locking outwards: before this patch, page cache >>> lock was taken inside lru_lock; after this patch, page cache lock is >>> taken outside lru_lock. If you cannot see that, then I think you >>> should not have touched this code at all; but it's what we have >>> been testing, and I think we should go forward with it. >>> >>>>> But I wish you could give some reason for it in the commit message! >>>> >>>> It's a head scratch task. Would you like to tell me what's detailed info >>>> should be there? Thanks! >>> >>> So, you don't know why you did it either: then it will be hard to >>> justify. I guess I'll have to write something for it later. I'm >>> strongly tempted just to drop the patch, but expect it will become >>> useful later, for using lock_page_memcg() before getting lru_lock. >>> >> >> I thought the xa_lock and lru_lock relationship was described clear >> in the commit log, > > You say "lru_lock and page cache xa_lock have no reason with current > sequence", but you give no reason for inverting their sequence: > "let's" is not a reason. > >> and still no idea of the move_lock in the chain. > > memcg->move_lock is what's at the heart of lock_page_memcg(), but > as much as possible that tries to avoid the overhead of actually > taking it, since moving memcg is a rare operation. For lock ordering, > see the diagram in mm/rmap.c, which 23/32 updates to match this change. I see. thanks! > > Before this commit: lru_lock > move_lock > i_pages lock was the > expected lock ordering (but it looks as if the lru_lock > move_lock > requirement came from my per-memcg lru_lock patches). > > After this commit: move_lock > i_pages lock > lru_lock is the > required lock ordering, since there are strong reasons (in dirty > writeback) for move_lock > i_pages lock. > >> Please refill them for what I overlooked. > > Will do, but not before reviewing your remaining patches. IIRC, all of comments are accepted and push to https://github.com/alexshi/linux.git lruv19 If you don't minder, could you change everything and send out a new version for further review? > >> Thanks! >> >>>>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx> >>>>> Is that correct? Or Wei Yang suggested some part of it perhaps? >>>> >>>> Yes, we talked a lot to confirm the locking change is safe. >>> >>> Okay, but the patch was written by you, and sent by you to Andrew: >>> that is not a case for "Signed-off-by: Someone Else". >>> >> >> Ok. let's remove his signed-off. >> >>>>> [PATCH v18 27/32] mm/swap.c: optimizing __pagevec_lru_add lru_lock >>>>> Could we please drop this one for the moment? And come back to it later >>>>> when the basic series is safely in. It's a good idea to try sorting >>>>> together those pages which come under the same lock (though my guess is >>>>> that they naturally gather themselves together quite well already); but >>>>> I'm not happy adding 360 bytes to the kernel stack here (and that in >>>>> addition to 192 bytes of horrid pseudo-vma in the shmem swapin case), >>>>> though that could be avoided by making it per-cpu. But I hope there's >>>>> a simpler way of doing it, as efficient, but also useful for the other >>>>> pagevec operations here: perhaps scanning the pagevec for same page-> >>>>> mem_cgroup (and flags node bits), NULLing entries as they are done. >>>>> Another, easily fixed, minor defect in this patch: if I'm reading it >>>>> right, it reverses the order in which the pages are put on the lru? >>>> >>>> this patch could give about 10+% performance gain on my multiple memcg >>>> readtwice testing. fairness locking cost the performance much. >>> >>> Good to know, should have been mentioned. s/fairness/Repeated/ >>> >>> But what was the gain or loss on your multiple memcg readtwice >>> testing without this patch, compared against node-only lru_lock? >>> The 80% gain mentioned before, I presume. So this further >>> optimization can wait until the rest is solid. >> >> the gain based on the patch 26. > > If I understand your brief comment there, you're saying that > in a fixed interval of time, the baseline 5.9-rc did 100 runs, > the patches up to and including 26/32 did 180 runs, then with > 27/32 on top, did 198 runs? Uh, I updated the testing with some new results here: https://lkml.org/lkml/2020/8/26/212 > > That's a good improvement by 27/32, but not essential for getting > the patchset in: I don't think 27/32 is the right way to do it, > so I'd still prefer to hold it back from the "initial offering". I am ok to hold it back. > >> >>> >>>> >>>> I also tried per cpu solution but that cause much trouble of per cpu func >>>> things, and looks no benefit except a bit struct size of stack, so if >>>> stack size still fine. May we could use the solution and improve it better. >>>> like, functionlize, fix the reverse issue etc. >>> >>> I don't know how important the stack depth consideration is nowadays: >>> I still care, maybe others don't, since VMAP_STACK became an option. >>> >>> Yes, please fix the reversal (if I was right on that); and I expect >>> you could use a singly linked list instead of the double. >> >> single linked list is more saving, but do we have to reverse walking to seek >> the head or tail for correct sequence? > > I imagine all you need is to start off with a > for (i = pagevec_count(pvec) - 1; i >= 0; i--) a nice simple solution, thanks! Thanks alex > loop. > >> >>> >>> But I'll look for an alternative - later, once the urgent stuff >>> is completed - and leave the acks on this patch to others. >> >> Ok, looking forward for your new solution! >> >> Thanks >> Alex
