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

[Lance Yang]

Hey Barry, Ryan, David,

Thanks a lot for taking the time to explain and provide suggestions!
I really appreciate your time!

IIUC, here's what we need to do for v3:

If folio_likely_mapped_shared() is true, or if we cannot acquire
the folio lock, we simply skip the batched PTEs. Then, we compare
the number of batched PTEs against folio_mapcount(). Finally,
batch-update the access and dirty only.

I'm not sure if I've understood correctly, could you please confirm?

Thanks,
Lance

On Thu, Mar 7, 2024 at 7:17 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 07.03.24 12:13, Ryan Roberts 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@xxxxxxx> wrote:
> >>>>>>
> >>>>>> On 07/03/2024 08:10, Barry Song wrote:
> >>>>>>> On Thu, Mar 7, 2024 at 9:00 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>> Hey Barry,
> >>>>>>>>
> >>>>>>>> Thanks for taking time to review!
> >>>>>>>>
> >>>>>>>> On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@gmailcom> wrote:
> >>>>>>>>>
> >>>>>>>>> On Thu, Mar 7, 2024 at 7:15 PM Lance Yang <ioworker0@xxxxxxxxx> 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@xxxxxxxxxx/
> >>>>>>>>
> >>>>>>>> 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.
> >>>
> >>> wp_page_reuse() will currently reuse a PTE part of a large folio only if
> >>> a single PTE remains mapped (refcount == 0).
> >>
> >> ^ == 1
> >
> > 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.
>
> We likely want to detect a folio batch the "usual" way (as relaxed as
> possible), then do all the checks (#pte == folio_mapcount() under folio
> lock), and finally batch-update the access and dirty only.
>
> --
> Cheers,
>
> David / dhildenb
>