Date: Thu, 7 Mar 2024 12:31:57 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, Ryan Roberts <ryan.roberts@....com>
Cc: 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 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.

-- 
Cheers,

David / dhildenb

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