Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free

[David Hildenbrand <david@redhat.com>]

On 08.03.24 14:05, Ryan Roberts wrote:
> On 07/03/2024 18:54, Barry Song wrote:
>> On Fri, Mar 8, 2024 at 12:31 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>
>>> On 07/03/2024 12:01, Barry Song wrote:
>>>> On Thu, Mar 7, 2024 at 7:45 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>> On 07.03.24 12:42, Ryan Roberts wrote:
>>>>>> On 07/03/2024 11:31, David Hildenbrand wrote:
>>>>>>> On 07.03.24 12:26, Barry Song wrote:
>>>>>>>> On Thu, Mar 7, 2024 at 7:13 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>>>>
>>>>>>>>> On 07/03/2024 10:54, David Hildenbrand wrote:
>>>>>>>>>> On 07.03.24 11:54, David Hildenbrand wrote:
>>>>>>>>>>> On 07.03.24 11:50, Ryan Roberts wrote:
>>>>>>>>>>>> On 07/03/2024 09:33, Barry Song wrote:
>>>>>>>>>>>>> On Thu, Mar 7, 2024 at 10:07 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On 07/03/2024 08:10, Barry Song wrote:
>>>>>>>>>>>>>>> On Thu, Mar 7, 2024 at 9:00 PM Lance Yang <ioworker0@gmail.com> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Hey Barry,
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Thanks for taking time to review!
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@gmail.com> wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On Thu, Mar 7, 2024 at 7:15 PM Lance Yang <ioworker0@gmail.com> wrote:
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> [...]
>>>>>>>>>>>>>>>>>> +static inline bool can_mark_large_folio_lazyfree(unsigned long addr,
>>>>>>>>>>>>>>>>>> +                                                struct folio *folio,
>>>>>>>>>>>>>>>>>> pte_t *start_pte)
>>>>>>>>>>>>>>>>>> +{
>>>>>>>>>>>>>>>>>> +       int nr_pages = folio_nr_pages(folio);
>>>>>>>>>>>>>>>>>> +       fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>> +       for (int i = 0; i < nr_pages; i++)
>>>>>>>>>>>>>>>>>> +               if (page_mapcount(folio_page(folio, i)) != 1)
>>>>>>>>>>>>>>>>>> +                       return false;
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> we have moved to folio_estimated_sharers though it is not precise, so
>>>>>>>>>>>>>>>>> we don't do
>>>>>>>>>>>>>>>>> this check with lots of loops and depending on the subpage's mapcount.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> If we don't check the subpage’s mapcount, and there is a cow folio
>>>>>>>>>>>>>>>> associated
>>>>>>>>>>>>>>>> with this folio and the cow folio has smaller size than this folio,
>>>>>>>>>>>>>>>> should we still
>>>>>>>>>>>>>>>> mark this folio as lazyfree?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I agree, this is true. However, we've somehow accepted the fact that
>>>>>>>>>>>>>>> folio_likely_mapped_shared
>>>>>>>>>>>>>>> can result in false negatives or false positives to balance the
>>>>>>>>>>>>>>> overhead.  So I really don't know :-)
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Maybe David and Vishal can give some comments here.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> BTW, do we need to rebase our work against David's changes[1]?
>>>>>>>>>>>>>>>>> [1]
>>>>>>>>>>>>>>>>> https://lore.kernel.org/linux-mm/20240227201548.857831-1-david@redhat.com/
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Yes, we should rebase our work against David’s changes.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>> +       return nr_pages == folio_pte_batch(folio, addr, start_pte,
>>>>>>>>>>>>>>>>>> +                                        ptep_get(start_pte), nr_pages,
>>>>>>>>>>>>>>>>>> flags, NULL);
>>>>>>>>>>>>>>>>>> +}
>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>>       static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>>>>>>>>>>>>>>>>>                                      unsigned long end, struct mm_walk
>>>>>>>>>>>>>>>>>> *walk)
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> @@ -676,11 +690,45 @@ static int madvise_free_pte_range(pmd_t *pmd,
>>>>>>>>>>>>>>>>>> unsigned long addr,
>>>>>>>>>>>>>>>>>>                       */
>>>>>>>>>>>>>>>>>>                      if (folio_test_large(folio)) {
>>>>>>>>>>>>>>>>>>                              int err;
>>>>>>>>>>>>>>>>>> +                       unsigned long next_addr, align;
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> -                       if (folio_estimated_sharers(folio) != 1)
>>>>>>>>>>>>>>>>>> -                               break;
>>>>>>>>>>>>>>>>>> -                       if (!folio_trylock(folio))
>>>>>>>>>>>>>>>>>> -                               break;
>>>>>>>>>>>>>>>>>> +                       if (folio_estimated_sharers(folio) != 1 ||
>>>>>>>>>>>>>>>>>> +                           !folio_trylock(folio))
>>>>>>>>>>>>>>>>>> +                               goto skip_large_folio;
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I don't think we can skip all the PTEs for nr_pages, as some of them
>>>>>>>>>>>>>>>>> might be
>>>>>>>>>>>>>>>>> pointing to other folios.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> for example, for a large folio with 16PTEs, you do MADV_DONTNEED(15-16),
>>>>>>>>>>>>>>>>> and write the memory of PTE15 and PTE16, you get page faults, thus PTE15
>>>>>>>>>>>>>>>>> and PTE16 will point to two different small folios. We can only skip
>>>>>>>>>>>>>>>>> when we
>>>>>>>>>>>>>>>>> are sure nr_pages == folio_pte_batch() is sure.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Agreed. Thanks for pointing that out.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>> +                       align = folio_nr_pages(folio) * PAGE_SIZE;
>>>>>>>>>>>>>>>>>> +                       next_addr = ALIGN_DOWN(addr + align, align);
>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>> +                       /*
>>>>>>>>>>>>>>>>>> +                        * If we mark only the subpages as lazyfree, or
>>>>>>>>>>>>>>>>>> +                        * cannot mark the entire large folio as
>>>>>>>>>>>>>>>>>> lazyfree,
>>>>>>>>>>>>>>>>>> +                        * then just split it.
>>>>>>>>>>>>>>>>>> +                        */
>>>>>>>>>>>>>>>>>> +                       if (next_addr > end || next_addr - addr !=
>>>>>>>>>>>>>>>>>> align ||
>>>>>>>>>>>>>>>>>> +                           !can_mark_large_folio_lazyfree(addr, folio,
>>>>>>>>>>>>>>>>>> pte))
>>>>>>>>>>>>>>>>>> +                               goto split_large_folio;
>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>> +                       /*
>>>>>>>>>>>>>>>>>> +                        * Avoid unnecessary folio splitting if the
>>>>>>>>>>>>>>>>>> large
>>>>>>>>>>>>>>>>>> +                        * folio is entirely within the given range.
>>>>>>>>>>>>>>>>>> +                        */
>>>>>>>>>>>>>>>>>> +                       folio_clear_dirty(folio);
>>>>>>>>>>>>>>>>>> +                       folio_unlock(folio);
>>>>>>>>>>>>>>>>>> +                       for (; addr != next_addr; pte++, addr +=
>>>>>>>>>>>>>>>>>> PAGE_SIZE) {
>>>>>>>>>>>>>>>>>> +                               ptent = ptep_get(pte);
>>>>>>>>>>>>>>>>>> +                               if (pte_young(ptent) ||
>>>>>>>>>>>>>>>>>> pte_dirty(ptent)) {
>>>>>>>>>>>>>>>>>> +                                       ptent =
>>>>>>>>>>>>>>>>>> ptep_get_and_clear_full(
>>>>>>>>>>>>>>>>>> +                                               mm, addr, pte,
>>>>>>>>>>>>>>>>>> tlb->fullmm);
>>>>>>>>>>>>>>>>>> +                                       ptent = pte_mkold(ptent);
>>>>>>>>>>>>>>>>>> +                                       ptent = pte_mkclean(ptent);
>>>>>>>>>>>>>>>>>> +                                       set_pte_at(mm, addr, pte,
>>>>>>>>>>>>>>>>>> ptent);
>>>>>>>>>>>>>>>>>> +                                       tlb_remove_tlb_entry(tlb, pte,
>>>>>>>>>>>>>>>>>> addr);
>>>>>>>>>>>>>>>>>> +                               }
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Can we do this in batches? for a CONT-PTE mapped large folio, you are
>>>>>>>>>>>>>>>>> unfolding
>>>>>>>>>>>>>>>>> and folding again. It seems quite expensive.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I'm not convinced we should be doing this in batches. We want the initial
>>>>>>>>>>>>>> folio_pte_batch() to be as loose as possible regarding permissions so
>>>>>>>>>>>>>> that we
>>>>>>>>>>>>>> reduce our chances of splitting folios to the min. (e.g. ignore SW bits
>>>>>>>>>>>>>> like
>>>>>>>>>>>>>> soft dirty, etc). I think it might be possible that some PTEs are RO and
>>>>>>>>>>>>>> other
>>>>>>>>>>>>>> RW too (e.g. due to cow - although with the current cow impl, probably not.
>>>>>>>>>>>>>> But
>>>>>>>>>>>>>> its fragile to assume that). Anyway, if we do an initial batch that ignores
>>>>>>>>>>>>>> all
>>>>>>>>>>>>>
>>>>>>>>>>>>> You are correct. I believe this scenario could indeed occur. For instance,
>>>>>>>>>>>>> if process A forks process B and then unmaps itself, leaving B as the
>>>>>>>>>>>>> sole process owning the large folio.  The current wp_page_reuse() function
>>>>>>>>>>>>> will reuse PTE one by one while the specific subpage is written.
>>>>>>>>>>>>
>>>>>>>>>>>> Hmm - I thought it would only reuse if the total mapcount for the folio
>>>>>>>>>>>> was 1.
>>>>>>>>>>>> And since it is a large folio with each page mapped once in proc B, I thought
>>>>>>>>>>>> every subpage write would cause a copy except the last one? I haven't
>>>>>>>>>>>> looked at
>>>>>>>>>>>> the code for a while. But I had it in my head that this is an area we need to
>>>>>>>>>>>> improve for mTHP.
>>>>>>>>
>>>>>>>> So sad I am wrong again 😢
>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> wp_page_reuse() will currently reuse a PTE part of a large folio only if
>>>>>>>>>>> a single PTE remains mapped (refcount == 0).
>>>>>>>>>>
>>>>>>>>>> ^ == 1
>>>>>>>>
>>>>>>>> seems this needs improvement. it is a waste the last subpage can
>>>>>>>
>>>>>>> My take that is WIP:
>>>>>>>
>>>>>>> https://lore.kernel.org/all/20231124132626.235350-1-david@redhat.com/T/#u
>>>>>>>
>>>>>>>> reuse the whole large folio. i was doing it in a quite different way,
>>>>>>>> if the large folio had only one subpage left, i would do copy and
>>>>>>>> released the large folio[1]. and if i could reuse the whole large folio
>>>>>>>> with CONT-PTE, i would reuse the whole large folio[2]. in mainline,
>>>>>>>> we don't have this cont-pte luxury exposed to mm, so i guess we can
>>>>>>>> not do [2] easily, but [1] seems to be an optimization.
>>>>>>>
>>>>>>> Yeah, I had essentially the same idea: just free up the large folio if most of
>>>>>>> the stuff is unmapped. But that's rather a corner-case optimization, so I did
>>>>>>> not proceed with that.
>>>>>>>
>>>>>>
>>>>>> I'm not sure it's a corner case, really? - process forks, then both parent and
>>>>>> child and write to all pages in what was previously a fully & contiguously
>>>>>> mapped large folio?
>>>>>
>>>>> Well, with 2 MiB my assumption was that while it can happen, it's rather
>>>>> rare. With smaller THP it might get more likely, agreed.
>>>>>
>>>>>>
>>>>>> Reggardless, why is it an optimization to do the copy for the last subpage and
>>>>>> syncrhonously free the large folio? It's already partially mapped so is on the
>>>>>> deferred split list and can be split if memory is tight.
>>>>
>>>> we don't want reclamation overhead later. and we want memories immediately
>>>> available to others.
>>>
>>> But by that logic, you also don't want to leave the large folio partially mapped
>>> all the way until the last subpage is CoWed. Surely you would want to reclaim it
>>> when you reach partial map status?
>>
>> To some extent, I agree. But then we will have two many copies. The last
>> subpage is small, and a safe place to copy instead.
>>
>> We actually had to tune userspace to decrease partial map as too much
>> partial map both unfolded CONT-PTE and wasted too much memory. if a
>> vma had too much partial map, we disabled mTHP on this VMA.
> 
> I actually had a whacky idea around introducing selectable page size ABI
> per-process that might help here. I know Android is doing work to make the
> system 16K page compatible. You could run most of the system processes with 16K
> ABI on top of 4K kernel. Then those processes don't even have the ability to
> madvise/munmap/mprotect/mremap anything less than 16K alignment so that acts as
> an anti-fragmentation mechanism while allowing non-16K capable processes to run
> side-by-side. Just a passing thought...

It sounds interesting, but and also like a lot of work.

-- 
Cheers,

David / dhildenb