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Date: Thu, 7 Mar 2024 11:54:22 +0100
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Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in
 madvise_free
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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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From: David Hildenbrand <david@redhat.com>
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Organization: Red Hat
In-Reply-To: <501c9f77-1459-467a-8619-78e86b46d300@arm.com>
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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).

-- 
Cheers,

David / dhildenb