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Date: Thu, 7 Mar 2024 11:54:43 +0100
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Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in
 madvise_free
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From: David Hildenbrand <david@redhat.com>
To: Ryan Roberts <ryan.roberts@arm.com>, Barry Song <21cnbao@gmail.com>
Cc: Lance Yang <ioworker0@gmail.com>, Vishal Moola <vishal.moola@gmail.com>,
 akpm@linux-foundation.org, zokeefe@google.com, shy828301@gmail.com,
 mhocko@suse.com, fengwei.yin@intel.com, xiehuan09@gmail.com,
 wangkefeng.wang@huawei.com, songmuchun@bytedance.com, peterx@redhat.com,
 minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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In-Reply-To: <8f84c7d6-982a-4933-a7a7-3f640df64991@redhat.com>
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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.
> 
> wp_page_reuse() will currently reuse a PTE part of a large folio only if
> a single PTE remains mapped (refcount == 0).

^ == 1

-- 
Cheers,

David / dhildenb