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

[Yosry Ahmed]

On Tue, Jul 18, 2023 at 6:32 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Jul 18, 2023 at 4:47 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
> >
> >
> >
> > On 7/19/23 06:48, Yosry Ahmed wrote:
> > > On Sun, Jul 16, 2023 at 6:58 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
> > >>
> > >>
> > >>
> > >> On 7/17/23 08:35, Yu Zhao wrote:
> > >>> On Sun, Jul 16, 2023 at 6:00 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
> > >>>>
> > >>>> On 7/15/2023 2:06 PM, Yu Zhao wrote:
> > >>>>> There is a problem here that I didn't have the time to elaborate: we
> > >>>>> can't mlock() a folio that is within the range but not fully mapped
> > >>>>> because this folio can be on the deferred split queue. When the split
> > >>>>> happens, those unmapped folios (not mapped by this vma but are mapped
> > >>>>> into other vmas) will be stranded on the unevictable lru.
> > >>>>
> > >>>> This should be fine unless I missed something. During large folio split,
> > >>>> the unmap_folio() will be migrate(anon)/unmap(file) folio. Folio will be
> > >>>> munlocked in unmap_folio(). So the head/tail pages will be evictable always.
> > >>>
> > >>> It's close but not entirely accurate: munlock can fail on isolated folios.
> > >> Yes. The munlock just clear PG_mlocked bit but with PG_unevictable left.
> > >>
> > >> 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()
> > Yes. Yu showed this sequence to me in another email. I thought the putback_lru()
> > could correct the none-mlocked but unevictable folio. But it doesn't because
> > of this race.
>
> (+Hugh Dickins for vis)
>
> Yu, I am not familiar with the split_folio() case, so I am not sure it
> is the same exact race I stated above.
>
> Can you confirm whether or not doing folio_test_clear_mlocked() before
> folio_test_clear_lru() would fix the race you are referring to? IIUC,
> in this case, we make sure we clear PG_mlocked before we try to to
> clear PG_lru. If we fail to clear it, then someone else have the folio
> isolated after we clear PG_mlocked, so we can be sure that when they
> put the folio back it will be correctly made evictable.
>
> Is my understanding correct?

Hmm, actually this might not be enough. In folio_add_lru() we will
call folio_batch_add_and_move(), which calls lru_add_fn() and *then*
sets PG_lru. Since we check folio_evictable() in lru_add_fn(), the
race can still happen:


cpu1                              cpu2
                                      folio_evictable() //false
folio_test_clear_mlocked()
folio_test_clear_lru() //false
                                      folio_set_lru()

Relying on PG_lru for synchronization might not be enough with the
current code. We might need to revert 2262ace60713 ("mm/munlock:
delete smp_mb() from __pagevec_lru_add_fn()").

Sorry for going back and forth here, I am thinking out loud.

>
> If yes, I can add this fix to my next version of the RFC series to
> rework mlock_count. It would be a lot more complicated with the
> current implementation (as I stated in a previous email).
>
> >
> > >
> > >
> > > The page would be stranded on the unevictable list in this case, no?
> > > Maybe we should only try to isolate the page (clear PG_lru) after we
> > > possibly clear PG_mlocked? In this case if we fail to isolate we know
> > > for sure that whoever has the page isolated will observe that
> > > PG_mlocked is clear and correctly make the page evictable.
> > >
> > > This probably would be complicated with the current implementation, as
> > > we first need to decrement mlock_count to determine if we want to
> > > clear PG_mlocked, and to do so we need to isolate the page as
> > > mlock_count overlays page->lru. With the proposal in [1] to rework
> > > mlock_count, it might be much simpler as far as I can tell. I intend
> > > to refresh this proposal soon-ish.
> > >
> > > [1]https://lore.kernel.org/lkml/20230618065719.1363271-1-yosryahmed@xxxxxxxxxx/
> > >
> > >>
> > >>
> > >> Regards
> > >> Yin, Fengwei
> > >>