From: Alex Shi <alex.shi@linux.alibaba.com>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
mgorman@techsingularity.net, tj@kernel.org,
khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com,
willy@infradead.org, hannes@cmpxchg.org, lkp@intel.com,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
cgroups@vger.kernel.org, shakeelb@google.com,
iamjoonsoo.kim@lge.com, richard.weiyang@gmail.com,
kirill@shutemov.name, alexander.duyck@gmail.com,
rong.a.chen@intel.com, mhocko@suse.com, vdavydov.dev@gmail.com,
shy828301@gmail.com, vbabka@suse.cz, minchan@kernel.org,
cai@lca.pw
Subject: Re: [PATCH v18 00/32] per memcg lru_lock: reviews
Date: Sun, 13 Sep 2020 22:22:48 +0800 [thread overview]
Message-ID: <2fed13c0-1d78-fa92-b607-e997b1bc5fb2@linux.alibaba.com> (raw)
In-Reply-To: <alpine.LSU.2.11.2009112216260.23961@eggly.anvils>
在 2020/9/12 下午4:38, Hugh Dickins 写道:
> On Wed, 9 Sep 2020, Alex Shi wrote:
>> 在 2020/9/9 上午7:41, Hugh Dickins 写道:
>>>
>>> The use of lock_page_memcg() in __munlock_pagevec() in 20/32,
>>> introduced in patchset v17, looks good but it isn't: I was lucky that
>>> systemd at reboot did some munlocking that exposed the problem to lockdep.
>>> The first time into the loop, lock_page_memcg() is done before lru_lock
>>> (as 06/32 has allowed); but the second time around the loop, it is done
>>> while still holding lru_lock.
>>
>> I don't know the details of lockdep show. Just wondering could it possible
>> to solid the move_lock/lru_lock sequence?
>> or try other blocking way which mentioned in commit_charge()?
>>
>>>
>>> lock_page_memcg() really needs to be absorbed into (a variant of)
>>> relock_page_lruvec(), and I do have that (it's awkward because of
>>> the different ways in which the IRQ flags are handled). And out of
>>> curiosity, I've also tried using that in mm/swap.c too, instead of the
>>> TestClearPageLRU technique: lockdep is happy, but an update_lru_size()
>>> warning showed that it cannot safely be mixed with the TestClearPageLRU
>>> technique (that I'd left in isolate_lru_page()). So I'll stash away
>>> that relock_page_lruvec(), and consider what's best for mm/mlock.c:
>>> now that I've posted these comments so far, that's my priority, then
>>> to get the result under testing again, before resuming these comments.
>>
>> No idea of your solution, but looking forward for your good news! :)
>
> Yes, it is good news, and simpler than anything suggested above.
Awesome!
>
> The main difficulties will probably be to look good in the 80 columns
> (I know that limit has been lifted recently, but some of us use xterms
> side by side), and to explain it.
>
> mm/mlock.c has not been kept up-to-date very well: and in particular,
> you have taken too seriously that "Serialize with any parallel
> __split_huge_page_refcount()..." comment that you updated to two
> comments "Serialize split tail pages in __split_huge_page_tail()...".
>
> Delete them! The original comment was by Vlastimil for v3.14 in 2014.
> But Kirill redesigned THP refcounting for v4.5 in 2016: that's when
> __split_huge_page_refcount() went away. And with the new refcounting,
> the THP splitting races that lru_lock protected munlock_vma_page()
> and __munlock_pagevec() from: those races have become impossible.
>
> Or maybe there never was such a race in __munlock_pagevec(): you
> have added the comment there, assuming lru_lock was for that purpose,
> but that was probably just the convenient place to take it,
> to cover all the del_page_from_lru()s.
>
> Observe how split_huge_page_to_list() uses unmap_page() to remove
> all pmds and all ptes for the huge page being split, and remap_page()
> only replaces the migration entries (used for anon but not for shmem
> or file) after doing all of the __split_huge_page_tail()s, before
> unlocking any of the pages. Recall that munlock_vma_page() and
> __munlock_pagevec() are being applied to pages found mapped
> into userspace, by ptes or pmd: there are none of those while
> __split_huge_page_tail() is being used, so no race to protect from.
>
> (Could a newly detached tail be freshly faulted into userspace just
> before __split_huge_page() has reached the head? Not quite, the
> fault has to wait to get the tail's page lock. But even if it
> could, how would that be a problem for __munlock_pagevec()?)
>
> There's lots more that could be said: for example, PageMlocked will
> always be clear on the THP head during __split_huge_page_tail(),
> because the last unmap of a PageMlocked page does clear_page_mlock().
> But that's not required to prove the case, it's just another argument
> against the "Serialize" comment you have in __munlock_pagevec().
>
> So, no need for the problematic lock_page_memcg(page) there in
> __munlock_pagevec(), nor to lock (or relock) lruvec just below it.
> __munlock_pagevec() still needs lru_lock to del_page_from_lru_list(),
> of course, but that must be done after your TestClearPageMlocked has
> stabilized page->memcg. Use relock_page_lruvec_irq() here? I suppose
> that will be easiest, but notice how __munlock_pagevec_fill() has
> already made sure that all the pages in the pagevec are from the same
> zone (and it cannot do the same for memcg without locking page memcg);
> so some of relock's work will be redundant.
It sounds reasonable for me.
>
> Otherwise, I'm much happier with your mm/mlock.c since looking at it
> in more detail: a couple of nits though - drop the clear_page_mlock()
> hunk from 25/32 - kernel style says do it the way you are undoing by
> - if (!isolate_lru_page(page)) {
> + if (!isolate_lru_page(page))
> putback_lru_page(page);
> - } else {
> + else {
> I don't always follow that over-braced style when making changes,
> but you should not touch otherwise untouched code just to make it
> go against the approved style. And in munlock_vma_page(),
> - if (!TestClearPageMlocked(page)) {
> + if (!TestClearPageMlocked(page))
> /* Potentially, PTE-mapped THP: do not skip the rest PTEs */
> - nr_pages = 1;
> - goto unlock_out;
> - }
> + return 0;
> please restore the braces: with that comment line in there,
> the compiler does not need the braces, but the human eye does.
Yes, That's better to keep the brace there.
Thanks
Alex
next prev parent reply other threads:[~2020-09-13 14:24 UTC|newest]
Thread overview: 102+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-24 12:54 [PATCH v18 00/32] per memcg lru_lock Alex Shi
2020-08-24 12:54 ` [PATCH v18 01/32] mm/memcg: warning on !memcg after readahead page charged Alex Shi
2020-08-24 12:54 ` [PATCH v18 02/32] mm/memcg: bail out early from swap accounting when memcg is disabled Alex Shi
2020-08-24 12:54 ` [PATCH v18 03/32] mm/thp: move lru_add_page_tail func to huge_memory.c Alex Shi
2020-08-24 12:54 ` [PATCH v18 04/32] mm/thp: clean up lru_add_page_tail Alex Shi
2020-08-24 12:54 ` [PATCH v18 05/32] mm/thp: remove code path which never got into Alex Shi
2020-08-24 12:54 ` [PATCH v18 06/32] mm/thp: narrow lru locking Alex Shi
2020-09-10 13:49 ` Matthew Wilcox
2020-09-11 3:37 ` Alex Shi
2020-09-13 15:27 ` Matthew Wilcox
2020-09-19 1:00 ` Hugh Dickins
2020-08-24 12:54 ` [PATCH v18 07/32] mm/swap.c: stop deactivate_file_page if page not on lru Alex Shi
2020-08-24 12:54 ` [PATCH v18 08/32] mm/vmscan: remove unnecessary lruvec adding Alex Shi
2020-08-24 12:54 ` [PATCH v18 09/32] mm/page_idle: no unlikely double check for idle page counting Alex Shi
2020-08-24 12:54 ` [PATCH v18 10/32] mm/compaction: rename compact_deferred as compact_should_defer Alex Shi
2020-08-24 12:54 ` [PATCH v18 11/32] mm/memcg: add debug checking in lock_page_memcg Alex Shi
2020-08-24 12:54 ` [PATCH v18 12/32] mm/memcg: optimize mem_cgroup_page_lruvec Alex Shi
2020-08-24 12:54 ` [PATCH v18 13/32] mm/swap.c: fold vm event PGROTATED into pagevec_move_tail_fn Alex Shi
2020-08-24 12:54 ` [PATCH v18 14/32] mm/lru: move lru_lock holding in func lru_note_cost_page Alex Shi
2020-08-24 12:54 ` [PATCH v18 15/32] mm/lru: move lock into lru_note_cost Alex Shi
2020-09-21 21:36 ` Hugh Dickins
2020-09-21 22:03 ` Hugh Dickins
2020-09-22 3:39 ` Alex Shi
2020-09-22 3:38 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 16/32] mm/lru: introduce TestClearPageLRU Alex Shi
2020-09-21 23:16 ` Hugh Dickins
2020-09-22 3:53 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 17/32] mm/compaction: do page isolation first in compaction Alex Shi
2020-09-21 23:49 ` Hugh Dickins
2020-09-22 4:57 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 18/32] mm/thp: add tail pages into lru anyway in split_huge_page() Alex Shi
2020-08-24 12:54 ` [PATCH v18 19/32] mm/swap.c: serialize memcg changes in pagevec_lru_move_fn Alex Shi
2020-09-22 0:42 ` Hugh Dickins
2020-09-22 5:00 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 20/32] mm/lru: replace pgdat lru_lock with lruvec lock Alex Shi
2020-09-22 5:27 ` Hugh Dickins
2020-09-22 8:58 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 21/32] mm/lru: introduce the relock_page_lruvec function Alex Shi
2020-09-22 5:40 ` Hugh Dickins
2020-08-24 12:54 ` [PATCH v18 22/32] mm/vmscan: use relock for move_pages_to_lru Alex Shi
2020-09-22 5:44 ` Hugh Dickins
2020-09-23 1:55 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 23/32] mm/lru: revise the comments of lru_lock Alex Shi
2020-09-22 5:48 ` Hugh Dickins
2020-08-24 12:54 ` [PATCH v18 24/32] mm/pgdat: remove pgdat lru_lock Alex Shi
2020-09-22 5:53 ` Hugh Dickins
2020-09-23 1:55 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 25/32] mm/mlock: remove lru_lock on TestClearPageMlocked in munlock_vma_page Alex Shi
2020-08-26 5:52 ` Alex Shi
2020-09-22 6:13 ` Hugh Dickins
2020-09-23 1:58 ` Alex Shi
2020-08-24 12:54 ` [PATCH v18 26/32] mm/mlock: remove __munlock_isolate_lru_page Alex Shi
2020-08-24 12:55 ` [PATCH v18 27/32] mm/swap.c: optimizing __pagevec_lru_add lru_lock Alex Shi
2020-08-26 9:07 ` Alex Shi
2020-08-24 12:55 ` [PATCH v18 28/32] mm/compaction: Drop locked from isolate_migratepages_block Alex Shi
2020-08-24 12:55 ` [PATCH v18 29/32] mm: Identify compound pages sooner in isolate_migratepages_block Alex Shi
2020-08-24 12:55 ` [PATCH v18 30/32] mm: Drop use of test_and_set_skip in favor of just setting skip Alex Shi
2020-08-24 12:55 ` [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru_pages Alex Shi
2020-09-09 1:01 ` Matthew Wilcox
2020-09-09 15:43 ` Alexander Duyck
2020-09-09 17:07 ` Matthew Wilcox
2020-09-09 18:24 ` Hugh Dickins
2020-09-09 20:15 ` Matthew Wilcox
2020-09-09 21:05 ` Hugh Dickins
2020-09-09 21:17 ` Alexander Duyck
2020-08-24 12:55 ` [PATCH v18 32/32] mm: Split release_pages work into 3 passes Alex Shi
2020-08-24 18:42 ` [PATCH v18 00/32] per memcg lru_lock Andrew Morton
2020-08-24 19:50 ` Qian Cai
2020-08-24 20:24 ` Hugh Dickins
2020-08-25 1:56 ` Daniel Jordan
2020-08-25 3:26 ` Alex Shi
2020-08-25 11:39 ` Matthew Wilcox
2020-08-26 1:19 ` Daniel Jordan
2020-08-26 8:59 ` Alex Shi
2020-08-28 1:40 ` Daniel Jordan
2020-08-28 5:22 ` Alex Shi
2020-09-09 2:44 ` Aaron Lu
2020-09-09 11:40 ` Michal Hocko
2020-08-25 8:52 ` Alex Shi
2020-08-25 13:00 ` Alex Shi
2020-08-27 7:01 ` Hugh Dickins
2020-08-27 12:20 ` Race between freeing and waking page Matthew Wilcox
2020-09-08 23:41 ` [PATCH v18 00/32] per memcg lru_lock: reviews Hugh Dickins
2020-09-09 2:24 ` Wei Yang
2020-09-09 15:08 ` Alex Shi
2020-09-09 23:16 ` Hugh Dickins
2020-09-11 2:50 ` Alex Shi
2020-09-12 2:13 ` Hugh Dickins
2020-09-13 14:21 ` Alex Shi
2020-09-15 8:21 ` Hugh Dickins
2020-09-15 16:58 ` Daniel Jordan
2020-09-17 2:37 ` Alex Shi
2020-09-17 14:35 ` Daniel Jordan
2020-09-17 15:39 ` Alexander Duyck
2020-09-17 16:48 ` Daniel Jordan
2020-09-12 8:38 ` Hugh Dickins
2020-09-13 14:22 ` Alex Shi [this message]
2020-09-09 16:11 ` Alexander Duyck
2020-09-10 0:32 ` Hugh Dickins
2020-09-10 14:24 ` Alexander Duyck
2020-09-12 5:12 ` Hugh Dickins
2020-08-25 7:21 ` [PATCH v18 00/32] per memcg lru_lock Michal Hocko
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