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

Shakeel Butt
2017-11-16T01:45:57Z

Ping, really appreciate comments on this patch.

On Sat, Nov 4, 2017 at 3:43 PM, Shakeel Butt <shakeelb@google.com> 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>
> ---
> include/linux/swap.h | 2 --
> mm/swap.c | 68 +++++++++++++++++++++++++---------------------------
> mm/vmscan.c | 59 +--------------------------------------------
> 3 files changed, 34 insertions(+), 95 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index f02fb5db8914..9b31d04914eb 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -326,8 +326,6 @@ extern void deactivate_file_page(struct page *page);
> 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);
>
> diff --git a/mm/swap.c b/mm/swap.c
> index a77d68f2c1b6..776fb33e81d3 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -445,30 +445,6 @@ void lru_cache_add(struct page *page)
> __lru_cache_add(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
> @@ -484,13 +460,9 @@ void lru_cache_add_active_or_unevictable(struct page *page,
> {
> 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(struct page *page,
> hpage_nr_pages(page));
> count_vm_event(UNEVICTABLE_PGMLOCKED);
> }
> - add_page_to_unevictable_list(page);
> + lru_cache_add(page);
> }
>
> /*
> @@ -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();
> +
> + 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);
> }
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eb2f0315b8c0..b171da71eadf 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -787,64 +787,7 @@ int remove_mapping(struct address_space *mapping, struct page *page)
> */
> 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 */
> }
>
> --
> 2.15.0.403.gc27cc4dac6-goog
>