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

[Alex Shi <alex.shi@linux.alibaba.com>]

在 2020/7/21 下午5:26, Alex Shi 写道:
> 
> 
> 在 2020/7/21 上午2:51, Alexander Duyck 写道:
>>> 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.
> 

Hi Johannes,

It looks like lock_page_memcg() could be used to replace lock_page(), which
could change retry into spinlock wait. Would you like to give some comments?

Thank
Alex
> 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);
>