Message-ID: <CAGsJ_4wGe9SdMvojw_2XchEttrbww3RttoOENoF-O4bLWUd_rw@mail.gmail.com>
Date: Fri, 8 Mar 2024 02:54:49 +0800
From: Barry Song <21cnbao@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: David Hildenbrand <david@...hat.com>, Lance Yang <ioworker0@...il.com>, 
	Vishal Moola <vishal.moola@...il.com>, akpm@...ux-foundation.org, zokeefe@...gle.com, 
	shy828301@...il.com, mhocko@...e.com, fengwei.yin@...el.com, 
	xiehuan09@...il.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com, 
	peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free

On Fri, Mar 8, 2024 at 12:31 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 07/03/2024 12:01, Barry Song wrote:
> > On Thu, Mar 7, 2024 at 7:45 PM David Hildenbrand <david@...hat.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@....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@....com> wrote:
> >>>>>>>>>>>
> >>>>>>>>>>> On 07/03/2024 08:10, Barry Song wrote:
> >>>>>>>>>>>> On Thu, Mar 7, 2024 at 9:00 PM Lance Yang <ioworker0@...il.com> wrote:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Hey Barry,
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Thanks for taking time to review!
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@...il.com> wrote:
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> On Thu, Mar 7, 2024 at 7:15 PM Lance Yang <ioworker0@...il.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.

>
> > reclamation will always cause latency and affect User
> > experience. split_folio is not cheap :-)
>
> But neither is memcpy(4K) I'd imagine. But I get your point.

In a real product scenario, we need to consider the success rate of
allocating large folios.
Currently, it's only 7%, as reported here[1], with no method to keep
large folios intact in a
buddy system.

Yu's TAO[2] chose to release the large folio entirely after copying
the mapped parts
onto smaller folios in vmscan,

[1] https://lore.kernel.org/linux-mm/20240305083743.24950-1-21cnbao@gmail.com/
[2] https://lore.kernel.org/linux-mm/20240229183436.4110845-1-yuzhao@googlecom/

>
> > if the number of this kind of
> > large folios
> > is huge, the waste can be huge for some while.
> >
> > it is not a corner case for large folio swap-in. while someone writes
> > one subpage, I swap-in a large folio, wp_reuse will immediately
> > be called. This can cause waste quite often. One outcome of this
> > discussion is that I realize I should investigate this issue immediately
> > in the swap-in series as my off-tree code has optimized reuse but
> > mainline hasn't.
> >
> >>
> >> At least for 2 MiB THP, it might make sense to make that large folio
> >> available immediately again, even without memory pressure. Even
> >> compaction would not compact it.
> >
> > It is also true for 64KiB. as we want other processes to allocate
> > 64KiB successfully as much as possible, and reduce the rate of
> > falling back to small folios. by releasing 64KiB directly to buddy
> > rather than splitting and returning 15*4KiB in shrinker, we reduce
> > buddy fragmentation too.
> >
> >>
> >> --
> >> Cheers,
> >>
> >> David / dhildenb

Thanks
Barry

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