Re: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio

[David Hildenbrand <david@redhat.com>]

On 04.03.24 21:42, Barry Song wrote:
> On Tue, Mar 5, 2024 at 3:27 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 04.03.24 14:03, Ryan Roberts wrote:
>>> On 04/03/2024 12:41, David Hildenbrand wrote:
>>>> On 04.03.24 13:20, Ryan Roberts wrote:
>>>>> Hi Barry,
>>>>>
>>>>> On 04/03/2024 10:37, Barry Song wrote:
>>>>>> From: Barry Song <v-songbaohua@oppo.com>
>>>>>>
>>>>>> page_vma_mapped_walk() within try_to_unmap_one() races with other
>>>>>> PTEs modification such as break-before-make, while iterating PTEs
>>>>>> of a large folio, it will only begin to acquire PTL after it gets
>>>>>> a valid(present) PTE. break-before-make intermediately sets PTEs
>>>>>> to pte_none. Thus, a large folio's PTEs might be partially skipped
>>>>>> in try_to_unmap_one().
>>>>>
>>>>> I just want to check my understanding here - I think the problem occurs for
>>>>> PTE-mapped, PMD-sized folios as well as smaller-than-PMD-size large folios? Now
>>>>> that I've had a look at the code and have a better understanding, I think that
>>>>> must be the case? And therefore this problem exists independently of my work to
>>>>> support swap-out of mTHP? (From your previous report I was under the impression
>>>>> that it only affected mTHP).
>>>>>
>>>>> Its just that the problem is becoming more pronounced because with mTHP,
>>>>> PTE-mapped large folios are much more common?
>>>>
>>>> That is my understanding.
>>>>
>>>>>
>>>>>> For example, for an anon folio, after try_to_unmap_one(), we may
>>>>>> have PTE0 present, while PTE1 ~ PTE(nr_pages - 1) are swap entries.
>>>>>> So folio will be still mapped, the folio fails to be reclaimed.
>>>>>> What’s even more worrying is, its PTEs are no longer in a unified
>>>>>> state. This might lead to accident folio_split() afterwards. And
>>>>>> since a part of PTEs are now swap entries, accessing them will
>>>>>> incur page fault - do_swap_page.
>>>>>> It creates both anxiety and more expense. While we can't avoid
>>>>>> userspace's unmap to break up unified PTEs such as CONT-PTE for
>>>>>> a large folio, we can indeed keep away from kernel's breaking up
>>>>>> them due to its code design.
>>>>>> This patch is holding PTL from PTE0, thus, the folio will either
>>>>>> be entirely reclaimed or entirely kept. On the other hand, this
>>>>>> approach doesn't increase PTL contention. Even w/o the patch,
>>>>>> page_vma_mapped_walk() will always get PTL after it sometimes
>>>>>> skips one or two PTEs because intermediate break-before-makes
>>>>>> are short, according to test. Of course, even w/o this patch,
>>>>>> the vast majority of try_to_unmap_one still can get PTL from
>>>>>> PTE0. This patch makes the number 100%.
>>>>>> The other option is that we can give up in try_to_unmap_one
>>>>>> once we find PTE0 is not the first entry we get PTL, we call
>>>>>> page_vma_mapped_walk_done() to end the iteration at this case.
>>>>>> This will keep the unified PTEs while the folio isn't reclaimed.
>>>>>> The result is quite similar with small folios with one PTE -
>>>>>> either entirely reclaimed or entirely kept.
>>>>>> Reclaiming large folios by holding PTL from PTE0 seems a better
>>>>>> option comparing to giving up after detecting PTL begins from
>>>>>> non-PTE0.
>>>>>>
>>>>
>>>> I'm sure that wall of text can be formatted in a better way :) . Also, I think
>>>> we can drop some of the details,
>>>>
>>>> If you need some inspiration, I can give it a shot.
>>>>
>>>>>> Cc: Hugh Dickins <hughd@google.com>
>>>>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>>>>
>>>>> Do we need a Fixes tag?
>>>>>
>>>>
>>>> What would be the description of the problem we are fixing?
>>>>
>>>> 1) failing to unmap?
>>>>
>>>> That can happen with small folios as well IIUC.
>>>>
>>>> 2) Putting the large folio on the deferred split queue?
>>>>
>>>> That sounds more reasonable.
>>>
>>> Isn't the real problem today that we can end up writng a THP to the swap file
>>> (so 2M more IO and space used) but we can't remove it from memory, so no actual
>>> reclaim happens? Although I guess your (2) is really just another way of saying
>>> that.
>>
>> The same could happen with small folios I believe? We might end up
>> running into the
>>
>> folio_mapped()
>>
>> after the try_to_unmap().
>>
>> Note that the actual I/O does not happen during add_to_swap(), but
>> during the pageout() call when we find the folio to be dirty.
>>
>> So there would not actually be more I/O. Only swap space would be
>> reserved, that would be used later when not running into the race.
> 
> I am not worried about small folios at all as they have only one PTE.
> so the PTE is either completely unmapped or completely mapped.
> 
> In terms of large folios, it is a different story. for example, a large
> folio with 16 PTEs with CONT-PTE, we will have
> 
> 1. unfolded CONT-PTE, eg. PTE0 present, PTE1-PTE15 swap entries
> 
> 2. page faults on PTE1-PTE15 after try_to_unmap if we access them.
> 
> This is totally useless PF and can be avoided if we can try_to_unmap
> properly at the beginning.
> 
> 3. potential need to split a large folio afterwards. for example, MADV_PAGEOUT,
> MADV_FREE might split it after finding it is not completely mapped.
> 
> For small folios, we don't have any concern on the above issues.

Right, but when we talk about "Fixes:", what exactly are we consider 
"really broken" above and what is "undesired"?

(a) is there a correctness issue? I don't think so.

(b) is there a real performance issue? I'd like to understand.

After all, we've been living with that ever since we supported THP_SWAP, 
correct? "something does not work ideally in some corner cases" might be 
reasonable to handle here (and I really think we should), but might not 
be worth a "Fixes:".

So if we could clarify that, it would be great.

-- 
Cheers,

David / dhildenb