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

[Vlastimil Babka <vbabka@suse.cz>]

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?).

> +
> +	if (page_evictable(page)) {
> +		lru = page_lru(page);
> +		update_page_reclaim_stat(lruvec, page_is_file_cache(page),
> +					 PageActive(page));
> +		if (was_unevictable)
> +			count_vm_event(UNEVICTABLE_PGRESCUED);
> +	} else {
> +		lru = LRU_UNEVICTABLE;
> +		ClearPageActive(page);
> +		SetPageUnevictable(page);
> +		if (!was_unevictable)
> +			count_vm_event(UNEVICTABLE_PGCULLED);
> +	}
> +
>  	add_page_to_lru_list(page, lruvec, lru);
> -	update_page_reclaim_stat(lruvec, file, active);
>  	trace_mm_lru_insertion(page, lru);
>  }
>  

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