Re: [PATCH] mm, mlock, vmscan: no more skipping pagevecs

[Shakeel Butt <shakeelb@google.com>]

On Tue, Nov 21, 2017 at 7:06 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Tue, Nov 21, 2017 at 01:39:57PM +0100, Vlastimil Babka wrote:
>> On 11/04/2017 11:43 PM, Shakeel Butt wrote:
>> > When a thread mlocks an address space backed by file, a new
>> > page is allocated (assuming file page is not in memory), added
>> > to the local pagevec (lru_add_pvec), I/O is triggered and the
>> > thread then sleeps on the page. On I/O completion, the thread
>> > can wake on a different CPU, the mlock syscall will then sets
>> > the PageMlocked() bit of the page but will not be able to put
>> > that page in unevictable LRU as the page is on the pagevec of
>> > a different CPU. Even on drain, that page will go to evictable
>> > LRU because the PageMlocked() bit is not checked on pagevec
>> > drain.
>> >
>> > The page will eventually go to right LRU on reclaim but the
>> > LRU stats will remain skewed for a long time.
>> >
>> > However, this issue does not happen for anon pages on swap
>> > because unlike file pages, anon pages are not added to pagevec
>> > until they have been fully swapped in. Also the fault handler
>> > uses vm_flags to set the PageMlocked() bit of such anon pages
>> > even before returning to mlock() syscall and mlocked pages will
>> > skip pagevecs and directly be put into unevictable LRU. No such
>> > luck for file pages.
>> >
>> > One way to resolve this issue, is to somehow plumb vm_flags from
>> > filemap_fault() to add_to_page_cache_lru() which will then skip
>> > the pagevec for pages of VM_LOCKED vma and directly put them to
>> > unevictable LRU. However this patch took a different approach.
>> >
>> > All the pages, even unevictable, will be added to the pagevecs
>> > and on the drain, the pages will be added on their LRUs correctly
>> > by checking their evictability. This resolves the mlocked file
>> > pages on pagevec of other CPUs issue because when those pagevecs
>> > will be drained, the mlocked file pages will go to unevictable
>> > LRU. Also this makes the race with munlock easier to resolve
>> > because the pagevec drains happen in LRU lock.
>> >
>> > There is one (good) side effect though. Without this patch, the
>> > pages allocated for System V shared memory segment are added to
>> > evictable LRUs even after shmctl(SHM_LOCK) on that segment. This
>> > patch will correctly put such pages to unevictable LRU.
>> >
>> > Signed-off-by: Shakeel Butt <shakeelb@google.com>
>>
>> I like the approach in general, as it seems to make the code simpler,
>> and the diffstats support that. I found no bugs, but I can't say that
>> with certainty that there aren't any, though. This code is rather
>> tricky. But it should be enough for an ack, so.
>>
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>
>> A question below, though.
>>
>> ...
>>
>> > @@ -883,15 +855,41 @@ void lru_add_page_tail(struct page *page, struct page *page_tail,
>> >  static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
>> >                              void *arg)
>> >  {
>> > -   int file = page_is_file_cache(page);
>> > -   int active = PageActive(page);
>> > -   enum lru_list lru = page_lru(page);
>> > +   enum lru_list lru;
>> > +   int was_unevictable = TestClearPageUnevictable(page);
>> >
>> >     VM_BUG_ON_PAGE(PageLRU(page), page);
>> >
>> >     SetPageLRU(page);
>> > +   /*
>> > +    * Page becomes evictable in two ways:
>> > +    * 1) Within LRU lock [munlock_vma_pages() and __munlock_pagevec()].
>> > +    * 2) Before acquiring LRU lock to put the page to correct LRU and then
>> > +    *   a) do PageLRU check with lock [check_move_unevictable_pages]
>> > +    *   b) do PageLRU check before lock [isolate_lru_page]
>> > +    *
>> > +    * (1) & (2a) are ok as LRU lock will serialize them. For (2b), if the
>> > +    * other thread does not observe our setting of PG_lru and fails
>> > +    * isolation, the following page_evictable() check will make us put
>> > +    * the page in correct LRU.
>> > +    */
>> > +   smp_mb();
>>
>> Could you elaborate on the purpose of smp_mb() here? Previously there
>> was "The other side is TestClearPageMlocked() or shmem_lock()" in
>> putback_lru_page(), which seems rather unclear to me (neither has an
>> explicit barrier?).
>
> The TestClearPageMlocked() is an RMW operation with return value, and
> thus an implicit full barrier (see Documentation/atomic_bitops.txt).
>
> The ordering is between putback and munlock:
>
> #0                         #1
> list_add(&page->lru,...)   if (TestClearPageMlock())
> SetPageLRU()                 __munlock_isolate_lru_page()
> smp_mb()
> if (page_evictable())
>   rescue
>
> The scenario that the barrier prevents from happening is:
>
> list_add(&page->lru,...)
> if (page_evictable())
>    rescue
>                            if (TestClearPageMlock())
>                                __munlock_isolate_lru_page() // FAILS on !PageLRU
> SetPageLRU()
>
> and now an evictable page is stranded on the unevictable LRU.
>
> The barrier guarantees that if #0 doesn't see the page evictable yet,
> #1 WILL see the PageLRU and succeed in isolation and rescue.
>
> Shakeel, please don't drop that "the other side" comment. You mention
> the places that make the page evictable - which is great, and please
> keep that as well - but for barriers it's always good to know exactly
> which operation guarantees the ordering on the other side. In fact, it
> would be great if you could add comments to the TestClearPageMlocked()
> sites that mention how they order against the smp_mb() in LRU putback.

Johannes, I have a question. The example you presented is valid before
this patch as '#0' was happening outside LRU lock. This patch moves
'#0' inside LRU lock and '#1' was already in LRU lock therefore no
issue for this particular scenario. However there is still a
TestClearPageMlocked() in clear_page_mlock() which happens outside LRU
lock and same issue which you have explained can happen even with this
patch (but without smp_mb()).

So, "the other side" for smp_mb() after this patch will only be the
TestClearPageMlock() in clear_page_mlock() because all other
TestClearPageMlocked() instances are serialized by LRU lock. Please
let me know if I missed something.

Thanks,
Shakeel

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