Re: Patch "mm, mlock, vmscan: no more skipping pagevecs" has been added to the 4.14-stable tree

On Tue, May 1, 2018 at 4:52 PM gregkh@linuxfoundation.org wrote:

This is a note to let you know that I've just added the patch titled

 mm, mlock, vmscan: no more skipping pagevecs

to the 4.14-stable tree which can be found at:

http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: mm-mlock-vmscan-no-more-skipping-pagevecs.patch and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

I would advise not to include this patch in the previous stable trees. This patch does not fix any critical bug and for some very specific microbenchmarks on very large machines, it has shown performance regression. So, I don't think it is worth the risk.

From foo@baz Tue May 1 16:18:19 PDT 2018 From: Shakeel Butt shakeelb@google.com Date: Wed, 21 Feb 2018 14:45:28 -0800 Subject: mm, mlock, vmscan: no more skipping pagevecs

From: Shakeel Butt shakeelb@google.com

[ Upstream commit 9c4e6b1a7027f102990c0395296015a812525f4d ]

When a thread mlocks an address space backed either by file pages which are currently not present in memory or swapped out anon pages (not in swapcache), a new page is allocated and 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.

This patch puts all the pages, even unevictable, to the pagevecs and on the drain, the pages will be added on their LRUs correctly by checking their evictability. This resolves the mlocked 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.

However there is still one place which makes a page evictable and does PageLRU check on that page without LRU lock and needs special attention. TestClearPageMlocked() and isolate_lru_page() in clear_page_mlock().

     #0: __pagevec_lru_add_fn        #1: clear_page_mlock

     SetPageLRU()                    if (!TestClearPageMlocked())
                                       return
     smp_mb() // <--required
                                     // inside does PageLRU
     if (!PageMlocked())             if (isolate_lru_page())
       move to evictable LRU           putback_lru_page()
     else
       move to unevictable LRU

In '#1', TestClearPageMlocked() provides full memory barrier semantics and thus the PageLRU check (inside isolate_lru_page) can not be reordered before it.

In '#0', without explicit memory barrier, the PageMlocked() check can be reordered before SetPageLRU(). If that happens, '#0' can put a page in unevictable LRU and '#1' might have just cleared the Mlocked bit of that page but fails to isolate as PageLRU fails as '#0' still hasn't set PageLRU bit of that page. That page will be stranded on the unevictable LRU.

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.

Link: http://lkml.kernel.org/r/20171121211241.18877-1-shakeelb@google.com Signed-off-by: Shakeel Butt shakeelb@google.com Acked-by: Vlastimil Babka vbabka@suse.cz Cc: Jérôme Glisse jglisse@redhat.com Cc: Huang Ying ying.huang@intel.com Cc: Tim Chen tim.c.chen@linux.intel.com Cc: Michal Hocko mhocko@kernel.org Cc: Greg Thelen gthelen@google.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Balbir Singh bsingharora@gmail.com Cc: Minchan Kim minchan@kernel.org Cc: Shaohua Li shli@fb.com Cc: Jan Kara jack@suse.cz Cc: Nicholas Piggin npiggin@gmail.com Cc: Dan Williams dan.j.williams@intel.com Cc: Mel Gorman mgorman@suse.de Cc: Hugh Dickins hughd@google.com Cc: Vlastimil Babka vbabka@suse.cz Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

---

include/linux/swap.h | 2 - mm/mlock.c | 6 +++ mm/swap.c | 82

+++++++++++++++++++++++++++++----------------------

mm/vmscan.c | 59 ------------------------------------ 4 files changed, 54 insertions(+), 95 deletions(-)

--- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -326,8 +326,6 @@ extern void deactivate_file_page(struct extern void mark_page_lazyfree(struct page *page); extern void swap_setup(void);

-extern void add_page_to_unevictable_list(struct page *page);

• extern void lru_cache_add_active_or_unevictable(struct page *page, struct vm_area_struct

*vma);

--- a/mm/mlock.c +++ b/mm/mlock.c @@ -64,6 +64,12 @@ void clear_page_mlock(struct page *page) mod_zone_page_state(page_zone(page), NR_MLOCK, -hpage_nr_pages(page)); count_vm_event(UNEVICTABLE_PGCLEARED);

•   /*
  

•    * The previous TestClearPageMlocked() corresponds to the smp_mb()
  

•    * in __pagevec_lru_add_fn().
  

•    *
  

•    * See __pagevec_lru_add_fn for more explanation.
  

•    */
     if (!isolate_lru_page(page)) {
             putback_lru_page(page);
     } else {
  

--- a/mm/swap.c +++ b/mm/swap.c @@ -446,30 +446,6 @@ void lru_cache_add(struct page *page) }

/**

• • add_page_to_unevictable_list - add a page to the unevictable list
• • @page: the page to be added to the unevictable list
• • Add page directly to its zone's unevictable list. To avoid races with
• • tasks that might be making the page evictable, through eg. munlock,
• • munmap or exit, while it's not on the lru, we want to add the page
• • while it's locked or otherwise "invisible" to other tasks. This is
• • difficult to do when using the pagevec cache, so bypass that.
• */

-void add_page_to_unevictable_list(struct page *page) -{

•   struct pglist_data *pgdat = page_pgdat(page);
  

•   struct lruvec *lruvec;
  

•   spin_lock_irq(&pgdat->lru_lock);
  

•   lruvec = mem_cgroup_page_lruvec(page, pgdat);
  

•   ClearPageActive(page);
  

•   SetPageUnevictable(page);
  

•   SetPageLRU(page);
  

•   add_page_to_lru_list(page, lruvec, LRU_UNEVICTABLE);
  

•   spin_unlock_irq(&pgdat->lru_lock);
  

-}

-/**

• lru_cache_add_active_or_unevictable
• @page: the page to be added to LRU
• @vma: vma in which page is mapped for determining reclaimability

@@ -484,13 +460,9 @@ void lru_cache_add_active_or_unevictable { VM_BUG_ON_PAGE(PageLRU(page), page);

•   if (likely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) !=
  

VM_LOCKED)) {

•   if (likely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) !=
  

VM_LOCKED))

             SetPageActive(page);

•           lru_cache_add(page);
  

•           return;
  

•   }
  

•   if (!TestSetPageMlocked(page)) {
  

•   else if (!TestSetPageMlocked(page)) {
             /*
              * We use the irq-unsafe __mod_zone_page_stat because this
              * counter is not modified from interrupt context, and
  

the pte

@@ -500,7 +472,7 @@ void lru_cache_add_active_or_unevictable hpage_nr_pages(page)); count_vm_event(UNEVICTABLE_PGMLOCKED); }

•   add_page_to_unevictable_list(page);
  

•   lru_cache_add(page);
  

  }

/* @@ -883,15 +855,55 @@ void lru_add_page_tail(struct page *page 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 [clear_page_mlock]
  

•    *
  

•    * (1) & (2a) are ok as LRU lock will serialize them. For (2b),
  

we need

•    * following strict ordering:
  

•    *
  

•    * #0: __pagevec_lru_add_fn             #1: clear_page_mlock
  

•    *
  

•    * SetPageLRU()                         TestClearPageMlocked()
  

•    * smp_mb() // explicit ordering        // above provides strict
  

•    *                                      // ordering
  

•    * PageMlocked()                        PageLRU()
  

•    *
  

•    *
  

•    * if '#1' does not observe setting of PG_lru by '#0' and fails
  

•    * isolation, the explicit barrier will make sure that
  

page_evictable

•    * check will put the page in correct LRU. Without smp_mb(),
  

SetPageLRU

•    * can be reordered after PageMlocked check and can make '#1' to
  

fail

•    * the isolation of the page whose Mlocked bit is cleared (#0 is
  

also

•    * looking at the same page) and the evictable page will be
  

stranded

•    * in an unevictable LRU.
  

•    */
  

•   smp_mb();
  

•   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);
  

  }

--- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -790,64 +790,7 @@ int remove_mapping(struct address_space */ void putback_lru_page(struct page *page) {

•   bool is_unevictable;
  

•   int was_unevictable = PageUnevictable(page);
  

•   VM_BUG_ON_PAGE(PageLRU(page), page);
  

-redo:

•   ClearPageUnevictable(page);
  

•   if (page_evictable(page)) {
  

•           /*
  

•            * For evictable pages, we can use the cache.
  

•            * In event of a race, worst case is we end up with an
  

•            * unevictable page on [in]active list.
  

•            * We know how to handle that.
  

•            */
  

•           is_unevictable = false;
  

•           lru_cache_add(page);
  

•   } else {
  

•           /*
  

•            * Put unevictable pages directly on zone's unevictable
  

•            * list.
  

•            */
  

•           is_unevictable = true;
  

•           add_page_to_unevictable_list(page);
  

•           /*
  

•            * When racing with an mlock or AS_UNEVICTABLE clearing
  

•            * (page is unlocked) make sure that if the other thread
  

•            * does not observe our setting of PG_lru and fails
  

•            * isolation/check_move_unevictable_pages,
  

•            * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
  

•            * the page back to the evictable list.
  

•            *
  

•            * The other side is TestClearPageMlocked() or
  

shmem_lock().

•            */
  

•           smp_mb();
  

•   }
  

•   /*
  

•    * page's status can change while we move it among lru. If an
  

evictable

•    * page is on unevictable list, it never be freed. To avoid that,
  

•    * check after we added it to the list, again.
  

•    */
  

•   if (is_unevictable && page_evictable(page)) {
  

•           if (!isolate_lru_page(page)) {
  

•                   put_page(page);
  

•                   goto redo;
  

•           }
  

•           /* This means someone else dropped this page from LRU
  

•            * So, it will be freed or putback to LRU again. There is
  

•            * nothing to do here.
  

•            */
  

•   }
  

•   if (was_unevictable && !is_unevictable)
  

•           count_vm_event(UNEVICTABLE_PGRESCUED);
  

•   else if (!was_unevictable && is_unevictable)
  

•           count_vm_event(UNEVICTABLE_PGCULLED);
  

•   lru_cache_add(page);
     put_page(page);         /* drop ref from isolate */
  

  }

Patches currently in stable-queue which might be from shakeelb@google.com

are

queue-4.14/mm-slab-memcg_link-the-slab-s-kmem_cache.patch queue-4.14/mm-mlock-vmscan-no-more-skipping-pagevecs.patch