Re: [PATCH v16 16/22] mm/mlock: reorder isolation sequence during munlock

[Alex Shi]

å 2020/7/21 äå2:51, Alexander Duyck åé:
>> Look into the __split_huge_page_tail, there is a tiny gap between tail page
>> get PG_mlocked, and it is added into lru list.
>> The TestClearPageLRU could blocked memcg changes of the page from stopping
>> isolate_lru_page.
> I get that there is a gap between the two in __split_huge_page_tail.
> My concern is more the fact that you are pulling the bit testing
> outside of the locked region when I don't think it needs to be. The
> lock is being taken unconditionally, so why pull the testing out when
> you could just do it inside the lock anyway? My worry is that you
> might be addressing __split_huge_page_tail but in the process you
> might be introducing a new race with something like
> __pagevec_lru_add_fn.

Yes, the page maybe interfered by clear_page_mlock and add pages to wrong lru
list.

> 
> If I am not mistaken the Mlocked flag can still be cleared regardless
> of if the LRU bit is set or not. So you can still clear the LRU bit
> before you pull the page out of the list, but it can be done after
> clearing the Mlocked flag instead of before you have even taken the
> LRU lock. In that way it would function more similar to how you
> handled pagevec_lru_move_fn() as all this function is really doing is
> moving the pages out of the unevictable list into one of the other LRU
> lists anyway since the Mlocked flag was cleared.
> 

Without the lru bit guard, the page may be moved between memcgs, luckly,
lock_page would stop the mem_cgroup_move_account with BUSY state cost.
whole new change would like the following, I will testing/resend again.

Thanks!
Alex

@@ -182,7 +179,7 @@ static void __munlock_isolation_failed(struct page *page)
 unsigned int munlock_vma_page(struct page *page)
 {
        int nr_pages;
-       pg_data_t *pgdat = page_pgdat(page);
+       struct lruvec *lruvec;

        /* For try_to_munlock() and to serialize with page migration */
        BUG_ON(!PageLocked(page));
@@ -190,11 +187,11 @@ unsigned int munlock_vma_page(struct page *page)
        VM_BUG_ON_PAGE(PageTail(page), page);

        /*
-        * Serialize with any parallel __split_huge_page_refcount() which
+        * Serialize split tail pages in __split_huge_page_tail() which
         * might otherwise copy PageMlocked to part of the tail pages before
         * we clear it in the head page. It also stabilizes hpage_nr_pages().
         */
-       spin_lock_irq(&pgdat->lru_lock);
+       lruvec = lock_page_lruvec_irq(page);

        if (!TestClearPageMlocked(page)) {
                /* Potentially, PTE-mapped THP: do not skip the rest PTEs */
@@ -205,15 +202,15 @@ unsigned int munlock_vma_page(struct page *page)
        nr_pages = hpage_nr_pages(page);
        __mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);

-       if (__munlock_isolate_lru_page(page, true)) {
-               spin_unlock_irq(&pgdat->lru_lock);
+       if (__munlock_isolate_lru_page(page, lruvec, true)) {
+               unlock_page_lruvec_irq(lruvec);
                __munlock_isolated_page(page);
                goto out;
        }
        __munlock_isolation_failed(page);

 unlock_out:
-       spin_unlock_irq(&pgdat->lru_lock);
+       unlock_page_lruvec_irq(lruvec);

 out:
        return nr_pages - 1;
@@ -293,23 +290,27 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
        int nr = pagevec_count(pvec);
        int delta_munlocked = -nr;
        struct pagevec pvec_putback;
+       struct lruvec *lruvec = NULL;
        int pgrescued = 0;

        pagevec_init(&pvec_putback);

        /* Phase 1: page isolation */
-       spin_lock_irq(&zone->zone_pgdat->lru_lock);
        for (i = 0; i < nr; i++) {
                struct page *page = pvec->pages[i];

+               /* block memcg change in mem_cgroup_move_account */
+               lock_page(page);
+               lruvec = relock_page_lruvec_irq(page, lruvec);
                if (TestClearPageMlocked(page)) {
                        /*
                         * We already have pin from follow_page_mask()
                         * so we can spare the get_page() here.
                         */
-                       if (__munlock_isolate_lru_page(page, false))
+                       if (__munlock_isolate_lru_page(page, lruvec, false)) {
+                               unlock_page(page);
                                continue;
-                       else
+                       } else
                                __munlock_isolation_failed(page);
                } else {
                        delta_munlocked++;
@@ -321,11 +322,14 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
                 * pin. We cannot do it under lru_lock however. If it's
                 * the last pin, __page_cache_release() would deadlock.
                 */
+               unlock_page(page);
                pagevec_add(&pvec_putback, pvec->pages[i]);
                pvec->pages[i] = NULL;
        }
-       __mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
-       spin_unlock_irq(&zone->zone_pgdat->lru_lock);
+       if (lruvec) {
+               __mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
+               unlock_page_lruvec_irq(lruvec);
+       }

        /* Now we can release pins of pages that we are not munlocking */
        pagevec_release(&pvec_putback);