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SubjectRe: [PATCH 2/3] mm: munlock: fix deadlock in __munlock_pagevec()
On 12/17/2013 01:31 AM, Andrew Morton wrote:
> On Mon, 16 Dec 2013 11:14:15 +0100 Vlastimil Babka <vbabka@suse.cz> wrote:
>
>> Commit 7225522bb ("mm: munlock: batch non-THP page isolation and
>> munlock+putback using pagevec" introduced __munlock_pagevec() to speed up
>> munlock by holding lru_lock over multiple isolated pages. Pages that fail to
>> be isolated are put_back() immediately, also within the lock.
>>
>> This can lead to deadlock when __munlock_pagevec() becomes the holder of the
>> last page pin and put_back() leads to __page_cache_release() which also locks
>> lru_lock. The deadlock has been observed by Sasha Levin using trinity.
>>
>> This patch avoids the deadlock by deferring put_back() operations until
>> lru_lock is released. Another pagevec (which is also used by later phases
>> of the function is reused to gather the pages for put_back() operation.
>>
>> ...
>>
>
> Thanks for fixing this one. I'll cross it off the rather large list of
> recent MM regressions :(

Well I made this one in the first place :/

>> --- a/mm/mlock.c
>> +++ b/mm/mlock.c
>> @@ -295,10 +295,12 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
>> {
>> int i;
>> int nr = pagevec_count(pvec);
>> - int delta_munlocked = -nr;
>> + int delta_munlocked;
>> struct pagevec pvec_putback;
>> int pgrescued = 0;
>>
>> + pagevec_init(&pvec_putback, 0);
>> +
>> /* Phase 1: page isolation */
>> spin_lock_irq(&zone->lru_lock);
>> for (i = 0; i < nr; i++) {
>> @@ -327,16 +329,22 @@ skip_munlock:
>> /*
>> * We won't be munlocking this page in the next phase
>> * but we still need to release the follow_page_mask()
>> - * pin.
>> + * pin. We cannot do it under lru_lock however. If it's
>> + * the last pin, __page_cache_release would deadlock.
>> */
>> + pagevec_add(&pvec_putback, pvec->pages[i]);
>> pvec->pages[i] = NULL;
>> - put_page(page);
>> - delta_munlocked++;
>> }
>> }
>> + delta_munlocked = -nr + pagevec_count(&pvec_putback);
>> __mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
>> spin_unlock_irq(&zone->lru_lock);
>>
>> + /* Now we can release pins of pages that we are not munlocking */
>> + for (i = 0; i < pagevec_count(&pvec_putback); i++) {
>> + put_page(pvec_putback.pages[i]);
>> + }
>> +
>
> We could just do
>
> --- a/mm/mlock.c~mm-munlock-fix-deadlock-in-__munlock_pagevec-fix
> +++ a/mm/mlock.c
> @@ -341,12 +341,9 @@ skip_munlock:
> spin_unlock_irq(&zone->lru_lock);
>
> /* Now we can release pins of pages that we are not munlocking */
> - for (i = 0; i < pagevec_count(&pvec_putback); i++) {
> - put_page(pvec_putback.pages[i]);
> - }
> + pagevec_release(&pvec_putback);
>
> /* Phase 2: page munlock */
> - pagevec_init(&pvec_putback, 0);
> for (i = 0; i < nr; i++) {
> struct page *page = pvec->pages[i];
>

Yeah that looks nicer.

> The lru_add_drain() is unnecessary overhead here. What do you think?

I would expect these isolation failures to be sufficiently rare so that
it doesn't matter. Especially in process exit path which was the
original target of my munlock work. But I don't have any numbers and my
mmtests benchmark for munlock is most likely too simple to trigger this.
But even once per pagevec the drain shouldn't hurt I guess...



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