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

[Yin Fengwei]

On 7/19/23 09:52, Yosry Ahmed wrote:
> 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.

Yes. Currently, the order in lru_add_fn() is not correct.

I think we should move folio_test_clear_lru(folio) into

lru locked range. As the lru lock here was expected to

use for sync here. Check the comment in lru_add_fn().


Regards

Yin, Fengwei


>
>> 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
>>>>>