Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()

[Ryan Roberts]

On 27/11/2023 10:28, Barry Song wrote:
> On Mon, Nov 27, 2023 at 11:11 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> On 27/11/2023 09:59, Barry Song wrote:
>>> On Mon, Nov 27, 2023 at 10:35 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>>>
>>>> On 27/11/2023 08:42, Barry Song wrote:
>>>>>>> +           for (i = 0; i < nr; i++, page++) {
>>>>>>> +                   if (anon) {
>>>>>>> +                           /*
>>>>>>> +                            * If this page may have been pinned by the
>>>>>>> +                            * parent process, copy the page immediately for
>>>>>>> +                            * the child so that we'll always guarantee the
>>>>>>> +                            * pinned page won't be randomly replaced in the
>>>>>>> +                            * future.
>>>>>>> +                            */
>>>>>>> +                           if (unlikely(page_try_dup_anon_rmap(
>>>>>>> +                                           page, false, src_vma))) {
>>>>>>> +                                   if (i != 0)
>>>>>>> +                                           break;
>>>>>>> +                                   /* Page may be pinned, we have to copy. */
>>>>>>> +                                   return copy_present_page(
>>>>>>> +                                           dst_vma, src_vma, dst_pte,
>>>>>>> +                                           src_pte, addr, rss, prealloc,
>>>>>>> +                                           page);
>>>>>>> +                           }
>>>>>>> +                           rss[MM_ANONPAGES]++;
>>>>>>> +                           VM_BUG_ON(PageAnonExclusive(page));
>>>>>>> +                   } else {
>>>>>>> +                           page_dup_file_rmap(page, false);
>>>>>>> +                           rss[mm_counter_file(page)]++;
>>>>>>> +                   }
>>>>>>>             }
>>>>>>> -           rss[MM_ANONPAGES]++;
>>>>>>> -   } else if (page) {
>>>>>>> -           folio_get(folio);
>>>>>>> -           page_dup_file_rmap(page, false);
>>>>>>> -           rss[mm_counter_file(page)]++;
>>>>>>> +
>>>>>>> +           nr = i;
>>>>>>> +           folio_ref_add(folio, nr);
>>>>>>
>>>>>> You're changing the order of mapcount vs. refcount increment. Don't.
>>>>>> Make sure your refcount >= mapcount.
>>>>>>
>>>>>> You can do that easily by doing the folio_ref_add(folio, nr) first and
>>>>>> then decrementing in case of error accordingly. Errors due to pinned
>>>>>> pages are the corner case.
>>>>>>
>>>>>> I'll note that it will make a lot of sense to have batch variants of
>>>>>> page_try_dup_anon_rmap() and page_dup_file_rmap().
>>>>>>
>>>>>
>>>>> i still don't understand why it is not a entire map+1, but an increment
>>>>> in each basepage.
>>>>
>>>> Because we are PTE-mapping the folio, we have to account each individual page.
>>>> If we accounted the entire folio, where would we unaccount it? Each page can be
>>>> unmapped individually (e.g. munmap() part of the folio) so need to account each
>>>> page. When PMD mapping, the whole thing is either mapped or unmapped, and its
>>>> atomic, so we can account the entire thing.
>>>
>>> Hi Ryan,
>>>
>>> There is no problem. for example, a large folio is entirely mapped in
>>> process A with CONPTE,
>>> and only page2 is mapped in process B.
>>> then we will have
>>>
>>> entire_map = 0
>>> page0.map = -1
>>> page1.map = -1
>>> page2.map = 0
>>> page3.map = -1
>>> ....
>>>
>>>>
>>>>>
>>>>> as long as it is a CONTPTE large folio, there is no much difference with
>>>>> PMD-mapped large folio. it has all the chance to be DoubleMap and need
>>>>> split.
>>>>>
>>>>> When A and B share a CONTPTE large folio, we do madvise(DONTNEED) or any
>>>>> similar things on a part of the large folio in process A,
>>>>>
>>>>> this large folio will have partially mapped subpage in A (all CONTPE bits
>>>>> in all subpages need to be removed though we only unmap a part of the
>>>>> large folioas HW requires consistent CONTPTEs); and it has entire map in
>>>>> process B(all PTEs are still CONPTES in process B).
>>>>>
>>>>> isn't it more sensible for this large folios to have entire_map = 0(for
>>>>> process B), and subpages which are still mapped in process A has map_count
>>>>> =0? (start from -1).
>>>>>
>>>>>> Especially, the batch variant of page_try_dup_anon_rmap() would only
>>>>>> check once if the folio maybe pinned, and in that case, you can simply
>>>>>> drop all references again. So you either have all or no ptes to process,
>>>>>> which makes that code easier.
>>>>
>>>> I'm afraid this doesn't make sense to me. Perhaps I've misunderstood. But
>>>> fundamentally you can only use entire_mapcount if its only possible to map and
>>>> unmap the whole folio atomically.
>>>
>>>
>>>
>>> My point is that CONTPEs should either all-set in all 16 PTEs or all are dropped
>>> in 16 PTEs. if all PTEs have CONT, it is entirely mapped; otherwise,
>>> it is partially
>>> mapped. if a large folio is mapped in one processes with all CONTPTEs
>>> and meanwhile in another process with partial mapping(w/o CONTPTE), it is
>>> DoubleMapped.
>>
>> There are 2 problems with your proposal, as I see it;
>>
>> 1) the core-mm is not enlightened for CONTPTE mappings. As far as it is
>> concerned, its just mapping a bunch of PTEs. So it has no hook to inc/dec
>> entire_mapcount. The arch code is opportunistically and *transparently* managing
>> the CONT_PTE bit.
>>
>> 2) There is nothing to say a folio isn't *bigger* than the contpte block; it may
>> be 128K and be mapped with 2 contpte blocks. Or even a PTE-mapped THP (2M) and
>> be mapped with 32 contpte blocks. So you can't say it is entirely mapped
>> unless/until ALL of those blocks are set up. And then of course each block could
>> be unmapped unatomically.
>>
>> For the PMD case there are actually 2 properties that allow using the
>> entire_mapcount optimization; It's atomically mapped/unmapped through the PMD
>> and we know that the folio is exactly PMD sized (since it must be at least PMD
>> sized to be able to map it with the PMD, and we don't allocate THPs any bigger
>> than PMD size). So one PMD map or unmap operation corresponds to exactly one
>> *entire* map or unmap. That is not true when we are PTE mapping.
> 
> well. Thanks for clarification. based on the above description, i agree the
> current code might make more sense by always using mapcount in subpage.
> 
> I gave my proposals as  I thought we were always CONTPTE size for small-THP
> then we could drop the loop to iterate 16 times rmap. if we do it
> entirely, we only
> need to do dup rmap once for all 16 PTEs by increasing entire_map.

Well its always good to have the discussion - so thanks for the ideas. I think
there is a bigger question lurking here; should we be exposing the concept of
contpte mappings to the core-mm rather than burying it in the arm64 arch code?
I'm confident that would be a huge amount of effort and the end result would be
similar performace to what this approach gives. One potential benefit of letting
core-mm control it is that it would also give control to core-mm over the
granularity of access/dirty reporting (my approach implicitly ties it to the
folio). Having sub-folio access tracking _could_ potentially help with future
work to make THP size selection automatic, but we are not there yet, and I think
there are other (simpler) ways to achieve the same thing. So my view is that
_not_ exposing it to core-mm is the right way for now.

> 
> BTW, I have concerns that a variable small-THP size will really work
> as userspace
> is probably friendly to only one fixed size. for example, userspace
> heap management
> might be optimized to a size for freeing memory to the kernel. it is
> very difficult
> for the heap to adapt to various sizes at the same time. frequent unmap/free
> size not equal with, and particularly smaller than small-THP size will
> defeat all
> efforts to use small-THP.

I'll admit to not knowing a huge amount about user space allocators. But I will
say that as currently defined, the small-sized THP interface to user space
allows a sysadmin to specifically enable the set of sizes that they want; so a
single size can be enabled. I'm diliberately punting that decision away from the
kernel for now.

FWIW, My experience with the Speedometer/JavaScript use case is that performance
is a little bit better when enabling 64+32+16K vs just 64K THP.

Functionally, it will not matter if the allocator is not enlightened for the THP
size; it can continue to free, and if a partial folio is unmapped it is put on
the deferred split list, then under memory pressure it is split and the unused
pages are reclaimed. I guess this is the bit you are concerned about having a
performance impact?

Regardless, it would be good to move this conversation to the small-sized THP
patch series since this is all independent of contpte mappings.

> 
>>
>>>
>>> Since we always hold ptl to set or drop CONTPTE bits, set/drop is
>>> still atomic in a
>>> spinlock area.
>>>
>>>>
>>>>>>
>>>>>> But that can be added on top, and I'll happily do that.
>>>>>>
>>>>>> --
>>>>>> Cheers,
>>>>>>
>>>>>> David / dhildenb
>>>>>
>>>
> 
> Thanks
> Barry