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

[Hugh Dickins <hughd@google.com>]

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?

Hugh