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

[Ryan Roberts]

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@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
> 
> 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.

> This can
> make a part of PTE writable while the others are read-only.
> 
>> 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.
> 
> nice to know ptep_test_and_clear_young() won't unfold and fold CONT-PTE.
> I probably have missed this part of your CONT-PTE series as I was quite busy
> with others :-)
> 
>>
>> 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.
>>
>>>>
>>>> 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