Re: [RFC PATCH 09/12] khugepaged: Introduce vma_collapse_anon_folio()

[David Hildenbrand <david@redhat.com>]

On 17.12.24 11:07, Dev Jain wrote:
> 
> On 16/12/24 10:36 pm, David Hildenbrand wrote:
>> On 16.12.24 17:51, Dev Jain wrote:
>>> In contrast to PMD-collapse, we do not need to operate on two levels
>>> of pagetable
>>> simultaneously. Therefore, downgrade the mmap lock from write to read
>>> mode. Still
>>> take the anon_vma lock in exclusive mode so as to not waste time in
>>> the rmap path,
>>> which is anyways going to fail since the PTEs are going to be
>>> changed. Under the PTL,
>>> copy page contents, clear the PTEs, remove folio pins, and (try to)
>>> unmap the
>>> old folios. Set the PTEs to the new folio using the set_ptes() API.
>>>
>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>> ---
>>> Note: I have been trying hard to get rid of the locks in here: we
>>> still are
>>> taking the PTL around the page copying; dropping the PTL and taking
>>> it after
>>> the copying should lead to a deadlock, for example:
>>> khugepaged                        madvise(MADV_COLD)
>>> folio_lock()                        lock(ptl)
>>> lock(ptl)                        folio_lock()
>>>
>>> We can create a locked folio list, altogether drop both the locks,
>>> take the PTL,
>>> do everything which __collapse_huge_page_isolate() does *except* the
>>> isolation and
>>> again try locking folios, but then it will reduce efficiency of
>>> khugepaged
>>> and almost looks like a forced solution :)
>>> Please note the following discussion if anyone is interested:
>>> https://lore.kernel.org/all/66bb7496-a445-4ad7-8e56-4f2863465c54@arm.com/
>>>
>>> (Apologies for not CCing the mailing list from the start)
>>>
>>>    mm/khugepaged.c | 108 ++++++++++++++++++++++++++++++++++++++----------
>>>    1 file changed, 87 insertions(+), 21 deletions(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 88beebef773e..8040b130e677 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -714,24 +714,28 @@ static void
>>> __collapse_huge_page_copy_succeeded(pte_t *pte,
>>>                            struct vm_area_struct *vma,
>>>                            unsigned long address,
>>>                            spinlock_t *ptl,
>>> -                        struct list_head *compound_pagelist)
>>> +                        struct list_head *compound_pagelist, int order)
>>>    {
>>>        struct folio *src, *tmp;
>>>        pte_t *_pte;
>>>        pte_t pteval;
>>>    -    for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
>>> +    for (_pte = pte; _pte < pte + (1UL << order);
>>>             _pte++, address += PAGE_SIZE) {
>>>            pteval = ptep_get(_pte);
>>>            if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>>>                add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>>>                if (is_zero_pfn(pte_pfn(pteval))) {
>>> -                /*
>>> -                 * ptl mostly unnecessary.
>>> -                 */
>>> -                spin_lock(ptl);
>>> -                ptep_clear(vma->vm_mm, address, _pte);
>>> -                spin_unlock(ptl);
>>> +                if (order == HPAGE_PMD_ORDER) {
>>> +                    /*
>>> +                    * ptl mostly unnecessary.
>>> +                    */
>>> +                    spin_lock(ptl);
>>> +                    ptep_clear(vma->vm_mm, address, _pte);
>>> +                    spin_unlock(ptl);
>>> +                } else {
>>> +                    ptep_clear(vma->vm_mm, address, _pte);
>>> +                }
>>>                    ksm_might_unmap_zero_page(vma->vm_mm, pteval);
>>>                }
>>>            } else {
>>> @@ -740,15 +744,20 @@ static void
>>> __collapse_huge_page_copy_succeeded(pte_t *pte,
>>>                src = page_folio(src_page);
>>>                if (!folio_test_large(src))
>>>                    release_pte_folio(src);
>>> -            /*
>>> -             * ptl mostly unnecessary, but preempt has to
>>> -             * be disabled to update the per-cpu stats
>>> -             * inside folio_remove_rmap_pte().
>>> -             */
>>> -            spin_lock(ptl);
>>> -            ptep_clear(vma->vm_mm, address, _pte);
>>> -            folio_remove_rmap_pte(src, src_page, vma);
>>> -            spin_unlock(ptl);
>>> +            if (order == HPAGE_PMD_ORDER) {
>>> +                /*
>>> +                * ptl mostly unnecessary, but preempt has to
>>> +                * be disabled to update the per-cpu stats
>>> +                * inside folio_remove_rmap_pte().
>>> +                */
>>> +                spin_lock(ptl);
>>> +                ptep_clear(vma->vm_mm, address, _pte);
>>
>>
>>
>>
>>> + folio_remove_rmap_pte(src, src_page, vma);
>>> +                spin_unlock(ptl);
>>> +            } else {
>>> +                ptep_clear(vma->vm_mm, address, _pte);
>>> +                folio_remove_rmap_pte(src, src_page, vma);
>>> +            }
>>
>> As I've talked to Nico about this code recently ... :)
>>
>> Are you clearing the PTE after the copy succeeded? If so, where is the
>> TLB flush?
>>
>> How do you sync against concurrent write acess + GUP-fast?
>>
>>
>> The sequence really must be: (1) clear PTE/PMD + flush TLB (2) check
>> if there are unexpected page references (e.g., GUP) if so back off (3)
>> copy page content (4) set updated PTE/PMD.
> 
> Thanks...we need to ensure GUP-fast does not write when we are copying
> contents, so (2) will ensure that GUP-fast will see the cleared PTE and
> back-off.

Yes, and of course, that also the CPU cannot concurrently still modify 
the page content while/after you copy the page content, but before you 
unmap+flush.

>>
>> To Nico, I suggested doing it simple initially, and still clear the
>> high-level PMD entry + flush under mmap write lock, then re-map the
>> PTE table after modifying the page table. It's not as efficient, but
>> "harder to get wrong".
>>
>> Maybe that's already happening, but I stumbled over this clearing
>> logic in __collapse_huge_page_copy_succeeded(), so I'm curious.
> 
> No, I am not even touching the PMD. I guess the sequence you described
> should work? I just need to reverse the copying and PTE clearing order
> to implement this sequence.

That would work, but you really have to hold the PTL for the whole 
period: from when you temporarily clear PTEs +_ flush the TLB, when you 
copy, until you re-insert the updated ones.

When having to back-off (restore original PTEs), or for copying, you'll 
likely need access to the original PTEs, which were already cleared. So 
likely you need a temporary copy of the original PTEs somehow.

That's why temporarily clearing the PMD und mmap write lock is easier to 
implement, at the cost of requiring the mmap lock in write mode like PMD 
collapse.

-- 
Cheers,

David / dhildenb