Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle large folio

[Yin, Fengwei]

On 7/19/2023 11:44 PM, Yosry Ahmed wrote:
> On Wed, Jul 19, 2023 at 7:26 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>>
>> On Wed, 19 Jul 2023, Yin Fengwei wrote:
>>>>>>>>>>>> Could this also happen against normal 4K page? I mean when user try to munlock
>>>>>>>>>>>> a normal 4K page and this 4K page is isolated. So it become unevictable page?
>>>>>>>>>>> Looks like it can be possible. If cpu 1 is in __munlock_folio() and
>>>>>>>>>>> cpu 2 is isolating the folio for any purpose:
>>>>>>>>>>>
>>>>>>>>>>> cpu1                        cpu2
>>>>>>>>>>>                             isolate folio
>>>>>>>>>>> folio_test_clear_lru() // 0
>>>>>>>>>>>                             putback folio // add to unevictable list
>>>>>>>>>>> folio_test_clear_mlocked()
>>>>>>>>                                folio_set_lru()
>>> Let's wait the response from Huge and Yu. :).
>>
>> I haven't been able to give it enough thought, but I suspect you are right:
>> that the current __munlock_folio() is deficient when folio_test_clear_lru()
>> fails.
>>
>> (Though it has not been reported as a problem in practice: perhaps because
>> so few places try to isolate from the unevictable "list".)
>>
>> I forget what my order of development was, but it's likely that I first
>> wrote the version for our own internal kernel - which used our original
>> lruvec locking, which did not depend on getting PG_lru first (having got
>> lru_lock, it checked memcg, then tried again if that had changed).
> 
> Right. Just holding the lruvec lock without clearing PG_lru would not
> protect against memcg movement in this case.
> 
>>
>> I was uneasy with the PG_lru aspect of upstream lru_lock implementation,
>> but it turned out to work okay - elsewhere; but it looks as if I missed
>> its implication when adapting __munlock_page() for upstream.
>>
>> If I were trying to fix this __munlock_folio() race myself (sorry, I'm
>> not), I would first look at that aspect: instead of folio_test_clear_lru()
>> behaving always like a trylock, could "folio_wait_clear_lru()" or whatever
>> spin waiting for PG_lru here?
> 
> +Matthew Wilcox
> 
> It seems to me that before 70dea5346ea3 ("mm/swap: convert lru_add to
> a folio_batch"), __pagevec_lru_add_fn() (aka lru_add_fn()) used to do
> folio_set_lru() before checking folio_evictable(). While this is
> probably extraneous since folio_batch_move_lru() will set it again
> afterwards, it's probably harmless given that the lruvec lock is held
> throughout (so no one can complete the folio isolation anyway), and
> given that there were no problems introduced by this extra
> folio_set_lru() as far as I can tell.
After checking related code, Yes. Looks fine if we move folio_set_lru()
before if (folio_evictable(folio)) in lru_add_fn() because of holding
lru lock.

> 
> If we restore folio_set_lru() to lru_add_fn(), and revert 2262ace60713
> ("mm/munlock:
> delete smp_mb() from __pagevec_lru_add_fn()") to restore the strict
> ordering between manipulating PG_lru and PG_mlocked, I suppose we can
> get away without having to spin. Again, that would only be possible if
> reworking mlock_count [1] is acceptable. Otherwise, we can't clear
> PG_mlocked before PG_lru in __munlock_folio().
What about following change to move mlocked operation before check lru
in __munlock_folio()?

diff --git a/mm/mlock.c b/mm/mlock.c
index 0a0c996c5c21..514f0d5bfbfd 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -122,7 +122,9 @@ static struct lruvec *__mlock_new_folio(struct folio *folio, struct lruvec *lruv
 static struct lruvec *__munlock_folio(struct folio *folio, struct lruvec *lruvec)
 {
        int nr_pages = folio_nr_pages(folio);
-       bool isolated = false;
+       bool isolated = false, mlocked = true;
+
+       mlocked = folio_test_clear_mlocked(folio);

        if (!folio_test_clear_lru(folio))
                goto munlock;
@@ -134,13 +136,17 @@ static struct lruvec *__munlock_folio(struct folio *folio, struct lruvec *lruvec
                /* Then mlock_count is maintained, but might undercount */
                if (folio->mlock_count)
                        folio->mlock_count--;
-               if (folio->mlock_count)
+               if (folio->mlock_count) {
+                       if (mlocked)
+                               folio_set_mlocked(folio);
                        goto out;
+               }
        }
        /* else assume that was the last mlock: reclaim will fix it if not */

 munlock:
-       if (folio_test_clear_mlocked(folio)) {
+       if (mlocked) {
                __zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages);
                if (isolated || !folio_test_unevictable(folio))
                        __count_vm_events(UNEVICTABLE_PGMUNLOCKED, nr_pages);


> 
> I am not saying this is necessarily better than spinning, just a note
> (and perhaps selfishly making [1] more appealing ;)).
> 
> [1]https://lore.kernel.org/lkml/20230618065719.1363271-1-yosryahmed@xxxxxxxxxx/
> 
>>
>> Hugh