Re: [PATCH v9 4/4] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

[David Hildenbrand <david@redhat.com>]

On 18.04.24 14:33, Lance Yang wrote:
> On Thu, Apr 18, 2024 at 8:03 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 18.04.24 12:57, Lance Yang wrote:
>>> This patch optimizes lazyfreeing with PTE-mapped mTHP[1]
>>> (Inspired by David Hildenbrand[2]). We aim to avoid unnecessary folio
>>> splitting if the large folio is fully mapped within the target range.
>>>
>>> If a large folio is locked or shared, or if we fail to split it, we just
>>> leave it in place and advance to the next PTE in the range. But note that
>>> the behavior is changed; previously, any failure of this sort would cause
>>> the entire operation to give up. As large folios become more common,
>>> sticking to the old way could result in wasted opportunities.
>>>
>>> On an Intel I5 CPU, lazyfreeing a 1GiB VMA backed by PTE-mapped folios of
>>> the same size results in the following runtimes for madvise(MADV_FREE) in
>>> seconds (shorter is better):
>>>
>>> Folio Size |   Old    |   New    | Change
>>> ------------------------------------------
>>>         4KiB | 0.590251 | 0.590259 |    0%
>>>        16KiB | 2.990447 | 0.185655 |  -94%
>>>        32KiB | 2.547831 | 0.104870 |  -95%
>>>        64KiB | 2.457796 | 0.052812 |  -97%
>>>       128KiB | 2.281034 | 0.032777 |  -99%
>>>       256KiB | 2.230387 | 0.017496 |  -99%
>>>       512KiB | 2.189106 | 0.010781 |  -99%
>>>      1024KiB | 2.183949 | 0.007753 |  -99%
>>>      2048KiB | 0.002799 | 0.002804 |    0%
>>>
>>> [1] https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@arm.com
>>> [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@redhat.com
>>>
>>> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
>>> Signed-off-by: Lance Yang <ioworker0@gmail.com>
>>> ---
>>>    mm/madvise.c | 85 +++++++++++++++++++++++++++-------------------------
>>>    1 file changed, 44 insertions(+), 41 deletions(-)
>>>
>>> diff --git a/mm/madvise.c b/mm/madvise.c
>>> index 4597a3568e7e..375ab3234603 100644
>>> --- a/mm/madvise.c
>>> +++ b/mm/madvise.c
>>> @@ -643,6 +643,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>>                                unsigned long end, struct mm_walk *walk)
>>>
>>>    {
>>> +     const cydp_t cydp_flags = CYDP_CLEAR_YOUNG | CYDP_CLEAR_DIRTY;
>>>        struct mmu_gather *tlb = walk->private;
>>>        struct mm_struct *mm = tlb->mm;
>>>        struct vm_area_struct *vma = walk->vma;
>>> @@ -697,44 +698,57 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>>                        continue;
>>>
>>>                /*
>>> -              * If pmd isn't transhuge but the folio is large and
>>> -              * is owned by only this process, split it and
>>> -              * deactivate all pages.
>>> +              * If we encounter a large folio, only split it if it is not
>>> +              * fully mapped within the range we are operating on. Otherwise
>>> +              * leave it as is so that it can be marked as lazyfree. If we
>>> +              * fail to split a folio, leave it in place and advance to the
>>> +              * next pte in the range.
>>>                 */
>>>                if (folio_test_large(folio)) {
>>> -                     int err;
>>> +                     bool any_young, any_dirty;
>>>
>>> -                     if (folio_likely_mapped_shared(folio))
>>> -                             break;
>>> -                     if (!folio_trylock(folio))
>>> -                             break;
>>> -                     folio_get(folio);
>>> -                     arch_leave_lazy_mmu_mode();
>>> -                     pte_unmap_unlock(start_pte, ptl);
>>> -                     start_pte = NULL;
>>> -                     err = split_folio(folio);
>>> -                     folio_unlock(folio);
>>> -                     folio_put(folio);
>>> -                     if (err)
>>> -                             break;
>>> -                     start_pte = pte =
>>> -                             pte_offset_map_lock(mm, pmd, addr, &ptl);
>>> -                     if (!start_pte)
>>> -                             break;
>>> -                     arch_enter_lazy_mmu_mode();
>>> -                     pte--;
>>> -                     addr -= PAGE_SIZE;
>>> -                     continue;
>>> +                     nr = madvise_folio_pte_batch(addr, end, folio, pte,
>>> +                                                  ptent, &any_young, NULL);
>>> +
>>> +                     if (nr < folio_nr_pages(folio)) {
>>> +                             int err;
>>> +
>>> +                             if (folio_likely_mapped_shared(folio))
>>> +                                     continue;
>>> +                             if (!folio_trylock(folio))
>>> +                                     continue;
>>> +                             folio_get(folio);
>>> +                             arch_leave_lazy_mmu_mode();
>>> +                             pte_unmap_unlock(start_pte, ptl);
>>> +                             start_pte = NULL;
>>> +                             err = split_folio(folio);
>>> +                             folio_unlock(folio);
>>> +                             folio_put(folio);
>>> +                             start_pte = pte =
>>> +                                     pte_offset_map_lock(mm, pmd, addr, &ptl);
>>
>> I'd just put it on a single line.
> 
> start_pte = pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> 
> I suddenly realized that putting it on a single line would exceed the
> 80-char limit.

Which is fine according to Documentation/process/coding-style.rst

... as long as it aids readability.

Alternatively, the following might do:

pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
start_pte = pte;

-- 
Cheers,

David / dhildenb