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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Cheers,

David / dhildenb