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

[Ryan Roberts]

On 04/03/2024 21:04, Barry Song wrote:
> On Tue, Mar 5, 2024 at 1:41 AM David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxx>
>>>>
>>>> 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@xxxxxxxxxx>
>>>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
>>>
>>> Do we need a Fixes tag?

It seems my original question has snowballed a bit. I was conflating this change
with other reports Barry has made where the kernel was panicking (I think?).
Given we are not seeing any incorrect functional behaviour that this change
fixes, I agree we don't need a Fixes tag here.