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

[Barry Song <21cnbao@gmail.com>]

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
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.

[1] https://github.com/OnePlusOSS/android_kernel_oneplus_sm8650/blob/oneplus/sm8650_u_14.0.0_oneplus12/mm/memory.c#L3977
[2] https://github.com/OnePlusOSS/android_kernel_oneplus_sm8650/blob/oneplus/sm8650_u_14.0.0_oneplus12/mm/memory.c#L3812

>
> Ahh yes. That's what I meant. I got the behacviour vagulely right though.
>
> Anyway, regardless, I'm not sure we want to batch here. Or if we do, we want to
> batch function that will only clear access and dirty.
>

Thanks
Barry