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

[Ryan Roberts]

On 07/03/2024 09:07, Ryan Roberts 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@xxxxxxxxx> 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
> that then do this bit as a batch, you will end up smeering all the ptes with
> whatever properties were set on the first pte, which probably isn't right.
> 
> I've done a similar conversion for madvise_cold_or_pageout_pte_range() as part
> of my swap-out series v4 (hoping to post imminently, but still working out a
> latent bug that it triggers). I use ptep_test_and_clear_young() in that, which
> arm64 can apply per-pte but avoid doing a contpte unfold/fold. I know you have
> to clear dirty here too, but I think this pattern is preferable.
> 
> FYI, my swap-out series also halfway-batches madvise_free_pte_range() so that I
> can batch free_swap_and_cache() for the swap entry case. Ideally the work you
> are doing here would be rebased on top of that and plug-in to the approach
> implemented there. (subject to others' views of course).
> 
> I'll cc you when I post it.

I just sent out the swap-out series v4, as I presed the button I realized I
forgot to cc you - sorry about that! It's at [1]. Patch 2 and 6 are the
interesting ones from this PoV.

[1] https://lore.kernel.org/linux-mm/20240311150058.1122862-1-ryan.roberts@xxxxxxx/


> 
>>>
>>> Thanks for your suggestion. I'll do this in batches in v3.
>>>
>>> Thanks again for your time!
>>>
>>> Best,
>>> Lance
>>>
>>>>
>>>>> +                       }
>>>>> +                       folio_mark_lazyfree(folio);
>>>>> +                       goto next_folio;
>>>>> +
>>>>> +split_large_folio:
>>>>>                         folio_get(folio);
>>>>>                         arch_leave_lazy_mmu_mode();
>>>>>                         pte_unmap_unlock(start_pte, ptl);
>>>>> @@ -688,13 +736,28 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>>>>                         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();
>>>>> +
>>>>> +                       /*
>>>>> +                        * If the large folio is locked or cannot be split,
>>>>> +                        * we just skip it.
>>>>> +                        */
>>>>> +                       if (err) {
>>>>> +skip_large_folio:
>>>>> +                               if (next_addr >= end)
>>>>> +                                       break;
>>>>> +                               pte += (next_addr - addr) / PAGE_SIZE;
>>>>> +                               addr = next_addr;
>>>>> +                       }
>>>>> +
>>>>> +                       if (!start_pte) {
>>>>> +                               start_pte = pte = pte_offset_map_lock(
>>>>> +                                       mm, pmd, addr, &ptl);
>>>>> +                               if (!start_pte)
>>>>> +                                       break;
>>>>> +                               arch_enter_lazy_mmu_mode();
>>>>> +                       }
>>>>> +
>>>>> +next_folio:
>>>>>                         pte--;
>>>>>                         addr -= PAGE_SIZE;
>>>>>                         continue;
>>>>> --
>>>>> 2.33.1
>>>>>
>>
>> Thanks
>> Barry
>