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

Alex Shi
2020-07-19T03:55:45Z

在 2020/7/18 上午4:30, Alexander Duyck 写道:
> On Fri, Jul 10, 2020 at 5:59 PM Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org> wrote:
>>
>> This patch reorder the isolation steps during munlock, move the lru lock
>> to guard each pages, unfold __munlock_isolate_lru_page func, to do the
>> preparation for lru lock change.
>>
>> __split_huge_page_refcount doesn't exist, but we still have to guard
>> PageMlocked and PageLRU for tail page in __split_huge_page_tail.
>>
>> [lkp-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org: found a sleeping function bug ... at mm/rmap.c]
>> Signed-off-by: Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>
>> Cc: Kirill A. Shutemov <kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org>
>> Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
>> Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
>> Cc: Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
>> Cc: Hugh Dickins <hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>> Cc: linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org
>> Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>> ---
>> mm/mlock.c | 93 ++++++++++++++++++++++++++++++++++----------------------------
>> 1 file changed, 51 insertions(+), 42 deletions(-)
>>
>> diff --git a/mm/mlock.c b/mm/mlock.c
>> index 228ba5a8e0a5..0bdde88b4438 100644
>> --- a/mm/mlock.c
>> +++ b/mm/mlock.c
>> @@ -103,25 +103,6 @@ void mlock_vma_page(struct page *page)
>> }
>>
>> /*
>> - * Isolate a page from LRU with optional get_page() pin.
>> - * Assumes lru_lock already held and page already pinned.
>> - */
>> -static bool __munlock_isolate_lru_page(struct page *page, bool getpage)
>> -{
>> - if (TestClearPageLRU(page)) {
>> - struct lruvec *lruvec;
>> -
>> - lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
>> - if (getpage)
>> - get_page(page);
>> - del_page_from_lru_list(page, lruvec, page_lru(page));
>> - return true;
>> - }
>> -
>> - return false;
>> -}
>> -
>> -/*
>> * Finish munlock after successful page isolation
>> *
>> * Page must be locked. This is a wrapper for try_to_munlock()
>> @@ -181,6 +162,7 @@ static void __munlock_isolation_failed(struct page *page)
>> unsigned int munlock_vma_page(struct page *page)
>> {
>> int nr_pages;
>> + bool clearlru = false;
>> pg_data_t *pgdat = page_pgdat(page);
>>
>> /* For try_to_munlock() and to serialize with page migration */
>> @@ -189,32 +171,42 @@ 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().
>> */
>> + get_page(page);
>
> I don't think this get_page() call needs to be up here. It could be
> left down before we delete the page from the LRU list as it is really
> needed to take a reference on the page before we call
> __munlock_isolated_page(), or at least that is the way it looks to me.
> By doing that you can avoid a bunch of cleanup in these exception
> cases.

Uh, It seems unlikely for !page->_refcount, and then got to release_pages(),
if so, get_page do could move down.
Thanks

>
>> + clearlru = TestClearPageLRU(page);
>
> I'm not sure I fully understand the reason for moving this here. By
> clearing this flag before you clear Mlocked does this give you some
> sort of extra protection? I don't see how since Mlocked doesn't
> necessarily imply the page is on LRU.
>

Above comments give a reason for the lru_lock usage,
>> + * 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().

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.

>> spin_lock_irq(&pgdat->lru_lock);
>>
>> if (!TestClearPageMlocked(page)) {
>> - /* Potentially, PTE-mapped THP: do not skip the rest PTEs */
>> - nr_pages = 1;
>> - goto unlock_out;
>> + if (clearlru)
>> + SetPageLRU(page);
>> + /*
>> + * Potentially, PTE-mapped THP: do not skip the rest PTEs
>> + * Reuse lock as memory barrier for release_pages racing.
>> + */
>> + spin_unlock_irq(&pgdat->lru_lock);
>> + put_page(page);
>> + return 0;
>> }
>>
>> nr_pages = hpage_nr_pages(page);
>> __mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
>>
>> - if (__munlock_isolate_lru_page(page, true)) {
>> + if (clearlru) {
>> + struct lruvec *lruvec;
>> +
>
> You could just place the get_page() call here.
>
>> + lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
>> + del_page_from_lru_list(page, lruvec, page_lru(page));
>> spin_unlock_irq(&pgdat->lru_lock);
>> __munlock_isolated_page(page);
>> - goto out;
>> + } else {
>> + spin_unlock_irq(&pgdat->lru_lock);
>> + put_page(page);
>> + __munlock_isolation_failed(page);
>
> If you move the get_page() as I suggested above there wouldn't be a
> need for the put_page(). It then becomes possible to simplify the code
> a bit by merging the unlock paths and doing an if/else with the
> __munlock functions like so:
> if (clearlru) {
> ...
> del_page_from_lru..
> }
>
> spin_unlock_irq()
>
> if (clearlru)
> __munlock_isolated_page();
> else
> __munlock_isolated_failed();
>
>> }
>> - __munlock_isolation_failed(page);
>> -
>> -unlock_out:
>> - spin_unlock_irq(&pgdat->lru_lock);
>>
>> -out:
>> return nr_pages - 1;
>> }
>>
>> @@ -297,34 +289,51 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
>> 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];
>> + struct lruvec *lruvec;
>> + bool clearlru;
>>
>> - 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))
>> - continue;
>> - else
>> - __munlock_isolation_failed(page);
>> - } else {
>> + clearlru = TestClearPageLRU(page);
>> + spin_lock_irq(&zone->zone_pgdat->lru_lock);
>
> I still don't see what you are gaining by moving the bit test up to
> this point. Seems like it would be better left below with the lock
> just being used to prevent a possible race while you are pulling the
> page out of the LRU list.
>

the same reason as above comments mentained __split_huge_page_tail()
issue.

>> +
>> + if (!TestClearPageMlocked(page)) {
>> delta_munlocked++;
>> + if (clearlru)
>> + SetPageLRU(page);
>> + goto putback;
>> + }
>> +
>> + if (!clearlru) {
>> + __munlock_isolation_failed(page);
>> + goto putback;
>> }
>
> With the other function you were processing this outside of the lock,
> here you are doing it inside. It would probably make more sense here
> to follow similar logic and take care of the del_page_from_lru_list
> ifr clealru is set, unlock, and then if clearlru is set continue else
> track the isolation failure. That way you can avoid having to use as
> many jump labels.
>
>> /*
>> + * Isolate this page.
>> + * We already have pin from follow_page_mask()
>> + * so we can spare the get_page() here.
>> + */
>> + lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
>> + del_page_from_lru_list(page, lruvec, page_lru(page));
>> + spin_unlock_irq(&zone->zone_pgdat->lru_lock);
>> + continue;
>> +
>> + /*
>> * We won't be munlocking this page in the next phase
>> * but we still need to release the follow_page_mask()
>> * pin. We cannot do it under lru_lock however. If it's
>> * the last pin, __page_cache_release() would deadlock.
>> */
>> +putback:
>> + spin_unlock_irq(&zone->zone_pgdat->lru_lock);
>> pagevec_add(&pvec_putback, pvec->pages[i]);
>> pvec->pages[i] = NULL;
>> }
>> + /* tempary disable irq, will remove later */
>> + local_irq_disable();
>> __mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
>> - spin_unlock_irq(&zone->zone_pgdat->lru_lock);
>> + local_irq_enable();
>>
>> /* Now we can release pins of pages that we are not munlocking */
>> pagevec_release(&pvec_putback);
>> --
>> 1.8.3.1
>>
>>