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

[Yin, Fengwei]

On 7/19/2023 10:26 PM, Hugh Dickins 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).
> 
> 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?
Considering following sequence:
    CPU1 (migration)                            CPU2 (mlock)

    isolation page (clear lru)                  mlock_pte_range
        try_to_migrate                              -> take_pte_lock
            try_to_migrate_one                      munlock_folio
                pvmw -> take pte lock                   __munlock_folio if batch full
                                                           folio_wait_clear_lru

deadlock may happen.


Regards
Yin, Fengwei

> 
> Hugh