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

[Yin Fengwei]

On 7/19/23 10:22, Yosry Ahmed wrote:
> On Tue, Jul 18, 2023 at 7:10 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>
>>
>>
>> On 7/19/23 10:00, Yosry Ahmed wrote:
>>> On Tue, Jul 18, 2023 at 6:57 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>
>>>>
>>>> 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().
>>>
>>> Right, I am wondering if it would be better to just revert
>>> 2262ace60713 and rely on the memory barrier and operations ordering
>>> instead of the lru lock. The lru lock is heavily contended as-is, so
>>> avoiding using it where possible is preferable I assume.
>> My understanding is set_lru after add folio to lru list is correct.
>> Once folio_set_lru(), others can do isolation of this folio. But if
>> this folio is not in lru list yet, what could happen? It's not required
>> to hold lru lock to do isolation.
> 
> IIUC, clearing PG_lru is an atomic lockless operation to make sure no
> one else is isolating the folio, but then you need to hold the lruvec
> lock and actually delete the folio from the lru to complete its
> isolation. This is my read of folio_isolate_lru().
> 
> Anyway, whether we rely on the lruvec lock or memory barrier +
> operation ordering doesn't make a huge difference (I think?). The code
> seemed to work with the latter before mlock_count was introduced.
> 
> If we decide to go with the latter, I can integrate the fix into the
> refresh of my mlock_count rework RFC series (as it would be dependent
> on that series). If we decide to go with the lruvec, then it can be
> done as part of this series, or separately.
Let's wait the response from Huge and Yu. :).

> 
> Thanks.
> 
>>
>>>
>>>>
>>>>
>>>> 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
>>>>>>>>>