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

[Ryan Roberts]

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@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?
>>
> 
> 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.

> 
>>> ---
>>>   mm/vmscan.c | 11 +++++++++++
>>>   1 file changed, 11 insertions(+)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 0b888a2afa58..e4722fbbcd0c 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -1270,6 +1270,17 @@ static unsigned int shrink_folio_list(struct list_head
>>> *folio_list,
>>>                 if (folio_test_pmd_mappable(folio))
>>>                   flags |= TTU_SPLIT_HUGE_PMD;
>>> +            /*
>>> +             * if page table lock is not held from the first PTE of
>>> +             * a large folio, some PTEs might be skipped because of
>>> +             * races with break-before-make, for example, PTEs can
>>> +             * be pte_none intermediately, thus one or more PTEs
>>> +             * might be skipped in try_to_unmap_one, we might result
>>> +             * in a large folio is partially mapped and partially
>>> +             * unmapped after try_to_unmap
>>> +             */
>>> +            if (folio_test_large(folio))
>>> +                flags |= TTU_SYNC;
>>
>> This looks sensible to me after thinking about it for a while. But I also have a
>> gut feeling that there might be some more subtleties that are going over my
>> head, since I'm not expert in this area. So will leave others to provide R-b :)
>>
> 
> As we are seeing more such problems with lockless PT walks, maybe we really want
> some other special value (nonswap entry?) to indicate that a PTE this is
> currently ondergoing protection changes. So we'd avoid the pte_none()
> temporarily, if possible.
> 
> Without that, TTU_SYNC feels like the right thing to do.
>