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Date: Fri, 8 Mar 2024 13:48:59 +0000
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Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in
 madvise_free
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To: David Hildenbrand <david@redhat.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: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <2962269e-fb74-4da3-b7b7-b75a5c436e0e@redhat.com>
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On 08/03/2024 13:27, David Hildenbrand wrote:
> On 08.03.24 14:05, Ryan Roberts wrote:
>> On 07/03/2024 18:54, Barry Song wrote:
>>> On Fri, Mar 8, 2024 at 12:31 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> On 07/03/2024 12:01, Barry Song wrote:
>>>>> On Thu, Mar 7, 2024 at 7:45 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>>
>>>>>> On 07.03.24 12:42, Ryan Roberts wrote:
>>>>>>> On 07/03/2024 11:31, David Hildenbrand wrote:
>>>>>>>> On 07.03.24 12:26, Barry Song wrote:
>>>>>>>>> On Thu, Mar 7, 2024 at 7:13 PM Ryan Roberts <ryan.roberts@arm.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@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.
>>>>>>>>>
>>>>>>>>> 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.
>>>>>>>>
>>>>>>>
>>>>>>> I'm not sure it's a corner case, really? - process forks, then both
>>>>>>> parent and
>>>>>>> child and write to all pages in what was previously a fully & contiguously
>>>>>>> mapped large folio?
>>>>>>
>>>>>> Well, with 2 MiB my assumption was that while it can happen, it's rather
>>>>>> rare. With smaller THP it might get more likely, agreed.
>>>>>>
>>>>>>>
>>>>>>> Reggardless, why is it an optimization to do the copy for the last
>>>>>>> subpage and
>>>>>>> syncrhonously free the large folio? It's already partially mapped so is
>>>>>>> on the
>>>>>>> deferred split list and can be split if memory is tight.
>>>>>
>>>>> we don't want reclamation overhead later. and we want memories immediately
>>>>> available to others.
>>>>
>>>> But by that logic, you also don't want to leave the large folio partially
>>>> mapped
>>>> all the way until the last subpage is CoWed. Surely you would want to
>>>> reclaim it
>>>> when you reach partial map status?
>>>
>>> To some extent, I agree. But then we will have two many copies. The last
>>> subpage is small, and a safe place to copy instead.
>>>
>>> We actually had to tune userspace to decrease partial map as too much
>>> partial map both unfolded CONT-PTE and wasted too much memory. if a
>>> vma had too much partial map, we disabled mTHP on this VMA.
>>
>> I actually had a whacky idea around introducing selectable page size ABI
>> per-process that might help here. I know Android is doing work to make the
>> system 16K page compatible. You could run most of the system processes with 16K
>> ABI on top of 4K kernel. Then those processes don't even have the ability to
>> madvise/munmap/mprotect/mremap anything less than 16K alignment so that acts as
>> an anti-fragmentation mechanism while allowing non-16K capable processes to run
>> side-by-side. Just a passing thought...
> 
> It sounds interesting, but and also like a lot of work.

I have working patches. But I was originally doing it in the context of also
using the 16K (or 64K) page table structure for those processes. But that was
too hard because there are lots of edge cases (page cache, drivers, current CoW
impl, etc) where 4K mapping is needed. So abandoned. The user space ABI part was
the easy bit. I think Google have also mentioned something similar (at
plumbers?) that they were doing to allow emulation of 16K ABI on x86.