On Mon, 16 Dec 2013 11:14:15 +0100 Vlastimil Babka <vbabka@xxxxxxx> 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 :(
> --- 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];
The lru_add_drain() is unnecessary overhead here. What do you think?
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