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Date: Thu, 7 Mar 2024 12:17:25 +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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In-Reply-To: <d24f8553-33f2-4ae7-a06d-badaf9462d84@arm.com>
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On 07.03.24 12:13, Ryan Roberts 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@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
> 
> Ahh yes. That's what I meant. I got the behacviour vagulely right though.
> 
> Anyway, regardless, I'm not sure we want to batch here. Or if we do, we want to
> batch function that will only clear access and dirty.

We likely want to detect a folio batch the "usual" way (as relaxed as 
possible), then do all the checks (#pte == folio_mapcount() under folio 
lock), and finally batch-update the access and dirty only.

-- 
Cheers,

David / dhildenb