Re: [PATCH v6 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

[David Hildenbrand <david@redhat.com>]

On 06.06.24 05:55, Lance Yang wrote:
> On Wed, Jun 5, 2024 at 10:28 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 05.06.24 16:20, Lance Yang wrote:
>>> Hi David,
>>>
>>> On Wed, Jun 5, 2024 at 8:46 PM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 21.05.24 06:02, Lance Yang wrote:
>>>>> In preparation for supporting try_to_unmap_one() to unmap PMD-mapped
>>>>> folios, start the pagewalk first, then call split_huge_pmd_address() to
>>>>> split the folio.
>>>>>
>>>>> Since TTU_SPLIT_HUGE_PMD will no longer perform immediately, we might
>>>>> encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range during
>>>>> the page walk. It’s probably necessary to mlock this THP to prevent it from
>>>>> being picked up during page reclaim.
>>>>>
>>>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>>>> Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>>> Signed-off-by: Lance Yang <ioworker0@gmail.com>
>>>>> ---
>>>>
>>>> [...] again, sorry for the late review.
>>>
>>> No worries at all, thanks for taking time to review!
>>>
>>>>
>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>> index ddffa30c79fb..08a93347f283 100644
>>>>> --- a/mm/rmap.c
>>>>> +++ b/mm/rmap.c
>>>>> @@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>>>         if (flags & TTU_SYNC)
>>>>>                 pvmw.flags = PVMW_SYNC;
>>>>>
>>>>> -     if (flags & TTU_SPLIT_HUGE_PMD)
>>>>> -             split_huge_pmd_address(vma, address, false, folio);
>>>>> -
>>>>>         /*
>>>>>          * For THP, we have to assume the worse case ie pmd for invalidation.
>>>>>          * For hugetlb, it could be much worse if we need to do pud
>>>>> @@ -1668,20 +1665,35 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>>>         mmu_notifier_invalidate_range_start(&range);
>>>>>
>>>>>         while (page_vma_mapped_walk(&pvmw)) {
>>>>> -             /* Unexpected PMD-mapped THP? */
>>>>> -             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>>>>> -
>>>>>                 /*
>>>>>                  * If the folio is in an mlock()d vma, we must not swap it out.
>>>>>                  */
>>>>>                 if (!(flags & TTU_IGNORE_MLOCK) &&
>>>>>                     (vma->vm_flags & VM_LOCKED)) {
>>>>>                         /* Restore the mlock which got missed */
>>>>> -                     if (!folio_test_large(folio))
>>>>> +                     if (!folio_test_large(folio) ||
>>>>> +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
>>>>>                                 mlock_vma_folio(folio, vma);
>>>>
>>>> Can you elaborate why you think this would be required? If we would have
>>>> performed the  split_huge_pmd_address() beforehand, we would still be
>>>> left with a large folio, no?
>>>
>>> Yep, there would still be a large folio, but it wouldn't be PMD-mapped.
>>>
>>> After Weifeng's series[1], the kernel supports mlock for PTE-mapped large
>>> folio, but there are a few scenarios where we don't mlock a large folio, such
>>> as when it crosses a VM_LOCKed VMA boundary.
>>>
>>>    -                     if (!folio_test_large(folio))
>>>    +                     if (!folio_test_large(folio) ||
>>>    +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
>>>
>>> And this check is just future-proofing and likely unnecessary. If encountering a
>>> PMD-mapped THP missing the mlock for some reason, we can mlock this
>>> THP to prevent it from being picked up during page reclaim, since it is fully
>>> mapped and doesn't cross the VMA boundary, IIUC.
>>>
>>> What do you think?
>>> I would appreciate any suggestions regarding this check ;)
>>
>> Reading this patch only, I wonder if this change makes sense in the
>> context here.
> 
> Allow me to try explaining it again ;)
> 
>>
>> Before this patch, we would have PTE-mapped the PMD-mapped THP before
>> reaching this call and skipped it due to "!folio_test_large(folio)".
> 
> Yes, there is only a PTE-mapped THP when doing the "!folio_test_large(folio)"
> check, as we will first conditionally split the PMD via
> split_huge_pmd_address().
> 
>>
>> After this patch, we either
> 
> Things will change. We'll first do the "!folio_test_large(folio)" check, then
> conditionally split the PMD via split_huge_pmd_address().
> 
>>
>> a) PTE-remap the THP after this check, but retry and end-up here again,
>> whereby we would skip it due to "!folio_test_large(folio)".
> 
> Hmm...
> 
> IIUC, we will skip it after this check, stop the page walk, and not
> PTE-remap the THP.
> 
>>
>> b) Discard the PMD-mapped THP due to lazyfree directly. Can that
>> co-exist with mlock and what would be the problem here with mlock?
> 
> Before discarding a PMD-mapped THP as a whole, as patch #3 did,
> we also perform the "!folio_test_large(folio)" check. If the THP coexists
> with mlock, we will skip it, stop the page walk, and not discard it. IIUC.

But "!folio_test_large(folio)" would *skip* the THP and not consider it 
regarding mlock.

I'm probably missing something and should try current mm/mm-unstable 
with MADV_FREE + mlock() on a PMD-mapped THP.

> 
>>
>>
>> So if the check is required in this patch, we really have to understand
>> why. If not, we should better drop it from this patch.
> 
> I added the "!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD))" check
> in this patch just to future-proof mlock for a PMD-mapped THP missing
> the mlock, to prevent it from being picked up during page reclaim.
> 
> But is this really required? It seems like nothing should really be broken
> without this check.
> 
> Perhaps, we should drop it from this patch until we fully understand the
> reason for it. Could you get me some suggestions?

We should drop it from this patch, agreed. We might need it 
("!pvmw.pte") in patch #3, but I still have to understand if there 
really would be a problem.

-- 
Cheers,

David / dhildenb