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

Ryan Roberts
2023-11-28T10:49:40Z

On 28/11/2023 09:49, Barry Song wrote:
> On Tue, Nov 28, 2023 at 10:14 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 27/11/2023 20:34, Barry Song wrote:
>>> On Tue, Nov 28, 2023 at 12:07 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> On 27/11/2023 10:28, Barry Song wrote:
>>>>> On Mon, Nov 27, 2023 at 11:11 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>
>>>>>> On 27/11/2023 09:59, Barry Song wrote:
>>>>>>> On Mon, Nov 27, 2023 at 10:35 PM Ryan Roberts <ryan.roberts@arm.com> 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.
>>>
>>> Hi Ryan,
>>>
>>> We(OPPO) started a similar project like you even before folio was imported to
>>> mainline, we have deployed the dynamic hugepage(that is how we name it)
>>> on millions of mobile phones on real products and kernels before 5.16, making
>>> a huge success on performance improvement. for example, you may
>>> find the out-of-tree 5.15 source code here
>>
>> Oh wow, thanks for reaching out and explaining this - I have to admit I feel
>> embarrassed that I clearly didn't do enough research on the prior art because I
>> wasn't aware of your work. So sorry about that.
>>
>> I sensed that you had a different model for how this should work vs what I've
>> implemented and now I understand why :). I'll review your stuff and I'm sure
>> I'll have questions. I'm sure each solution has pros and cons.
>>
>>
>>>
>>> https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/tree/oneplus/sm8550_u_14.0.0_oneplus11
>>>
>>> Our modification might not be so clean and has lots of workarounds
>>> just for the stability of products
>>>
>>> We mainly have
>>>
>>> 1. https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/mm/cont_pte_hugepage.c
>>>
>>> some CONTPTE helpers
>>>
>>> 2.https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/include/linux/mm.h
>>>
>>> some Dynamic Hugepage APIs
>>>
>>> 3. https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/mm/memory.c
>>>
>>> modified all page faults to support
>>> (1). allocation of hugepage of 64KB in do_anon_page
>>
>> My Small-Sized THP patch set is handling the equivalent of this.
>
> right, the only difference is that we did a huge-zeropage for reading
> in do_anon_page.
> mapping all large folios to CONTPTE to zero page.

FWIW, I took a slightly different approach in my original RFC for the zero page
- although I ripped it all out to simplify for the initial series. I found that
it was pretty rare for user space to read multiple consecutive pages without
ever interleving any writes, so I kept the zero page as a base page, but at CoW,
I would expand the allocation to an approprately sized THP. But for the couple
of workloads that I've gone deep with, I found that it made barely any dent on
the amount of memory that ended up contpte-mapped; the vast majority was from
write allocation in do_anonymous_page().

>
>>
>>> (2). CoW hugepage in do_wp_page
>>
>> This isn't handled yet in my patch set; the original RFC implemented it but I
>> removed it in order to strip back to the essential complexity for the initial
>> submission. DavidH has been working on a precise shared vs exclusive map
>> tracking mechanism - if that goes in, it will make CoWing large folios simpler.
>> Out of interest, what workloads benefit most from this?
>
> as a phone, Android has a design almost all processes are forked from zygote.
> thus, CoW happens quite often to all apps.

Sure. But in my analysis I concluded that most of the memory mapped in zygote is
file-backed and mostly RO so therefore doing THP CoW doesn't help much. Perhaps
there are cases where that conclusion is wrong.

>
>>
>>> (3). copy CONPTEs in copy_pte_range
>>
>> As discussed this is done as part of the contpte patch set, but its not just a
>> simple copy; the arch code will notice and set the CONT_PTE bit as needed.
>
> right, i have read all your unfold and fold stuff today, now i understand your
> approach seems quite nice!

Great - thanks!

>
>
>>
>>> (4). allocate and swap-in Hugepage as a whole in do_swap_page
>>
>> This is going to be a problem but I haven't even looked at this properly yet.
>> The advice so far has been to continue to swap-in small pages only, but improve
>> khugepaged to collapse to small-sized THP. I'll take a look at your code to
>> understand how you did this.
>
> this is also crucial to android phone as swap is always happening
> on an embedded device. if we don't support large folios in swapin,
> our large folios will never come back after it is swapped-out.
>
> and i hated the collapse solution from the first beginning as there is
> never a guarantee to succeed and its overhead is unacceptable to user UI,
> so we supported hugepage allocation in do_swap_page from the first beginning.

Understood. I agree it would be nice to preserve large folios across swap. I
think this can be layered on top of the current work though.

>
>>
>>>
>>> 4. https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/mm/vmscan.c
>>> https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/oneplus/sm8550_u_14.0.0_oneplus11/mm/rmap.c
>>>
>>> reclaim hugepage as a whole and LRU optimization for 64KB dynamic hugepage.
>>
>> I think this is all naturally handled by the folio code that exists in modern
>> kernels?
>
> We had a CONTPTE hugepage pool, if the pool is very limited, we let LRU
> reclaim large folios to the pool. as phones are running lots of apps
> and drivers, and the memory is very limited, after a couple of hours,
> it will become very hard to allocate large folios in the original buddy. thus,
> large folios totally disappeared after running the phone for some time
> if we didn't have the pool.
>
>>
>>>
>>> So we are 100% interested in your patchset and hope it can find a way
>>> to land on the
>>> mainline, thus decreasing all the cost we have to maintain out-of-tree
>>> code from a
>>> kernel to another kernel version which we have done on a couple of
>>> kernel versions
>>> before 5.16. Firmly, we are 100% supportive of large anon folios
>>> things you are leading.
>>
>> That's great to hear! Of course Reviewed-By's and Tested-By's will all help move
>> it closer :). If you had any ability to do any A/B performance testing, it would
>> be very interesting to see how this stacks up against your solution - if there
>> are gaps it would be good to know where and develop a plan to plug the gap.
>>
>
> sure.
>
>>>
>>> A big pain was we found lots of races especially on CONTPTE unfolding
>>> and especially a part
>>> of basepages ran away from the 16 CONPTEs group since userspace is
>>> always working
>>> on basepages, having no idea of small-THP. We ran our code on millions of
>>> real phones, and now we have got them fixed (or maybe "can't reproduce"),
>>> no outstanding issue.
>>
>> I'm going to be brave and say that my solution shouldn't suffer from these
>> problems; but of course the proof is only in the testing. I did a lot of work
>> with our architecture group and micro architects to determine exactly what is
>> and isn't safe; We even tightened the Arm ARM spec very subtlely to allow the
>> optimization in patch 13 (see the commit log for details). Of course this has
>> all been checked with partners and we are confident that all existing
>> implementations conform to the modified wording.
>
> cool. I like your try_unfold/fold code. it seems your code is setting/dropping
> CONT automatically based on ALIGHMENT, Page number etc. Alternatively,
> our code is always stupidly checking some conditions before setting and dropping
> CONT everywhere.
>
>>
>>>
>>> Particularly for the rmap issue we are discussing, our out-of-tree is
>>> using the entire_map for
>>> CONTPTE in the way I sent to you. But I guess we can learn from you to decouple
>>> CONTPTE from mm-core.
>>>
>>> We are doing this in mm/memory.c
>>>
>>> copy_present_cont_pte(struct vm_area_struct *dst_vma, struct
>>> vm_area_struct *src_vma,
>>> pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
>>> struct page **prealloc)
>>> {
>>> struct mm_struct *src_mm = src_vma->vm_mm;
>>> unsigned long vm_flags = src_vma->vm_flags;
>>> pte_t pte = *src_pte;
>>> struct page *page;
>>>
>>> page = vm_normal_page(src_vma, addr, pte);
>>> ...
>>>
>>> get_page(page);
>>> page_dup_rmap(page, true); // an entire dup_rmap as you can
>>> see.............
>>> rss[mm_counter(page)] += HPAGE_CONT_PTE_NR;
>>> }
>>>
>>> and we have a split in mm/cont_pte_hugepage.c to handle partially unmap,
>>>
>>> static void __split_huge_cont_pte_locked(struct vm_area_struct *vma, pte_t *pte,
>>> unsigned long haddr, bool freeze)
>>> {
>>> ...
>>> if (compound_mapcount(head) > 1 && !TestSetPageDoubleMap(head)) {
>>> for (i = 0; i < HPAGE_CONT_PTE_NR; i++)
>>> atomic_inc(&head[i]._mapcount);
>>> atomic_long_inc(&cont_pte_double_map_count);
>>> }
>>>
>>>
>>> if (atomic_add_negative(-1, compound_mapcount_ptr(head))) {
>>> ...
>>> }
>>>
>>> I am not selling our solution any more, but just showing you some differences we
>>> have :-)
>>
>> OK, I understand what you were saying now. I'm currently struggling to see how
>> this could fit into my model. Do you have any workloads and numbers on perf
>> improvement of using entire_mapcount?
>
> TBH, I don't have any data on this as from the first beginning, we were using
> entire_map. So I have no comparison at all.
>
>>
>>>
>>>>
>>>>>
>>>>> 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.
>>>
>>> Basically, userspace heap library has a PAGESIZE setting and allows users
>>> to allocate/free all kinds of small objects such as 16,32,64,128,256,512 etc.
>>> The default size is for sure equal to the basepage SIZE. once some objects are
>>> freed by free() and libc get a free "page", userspace heap libraries might free
>>> the PAGESIZE page to kernel by things like MADV_DONTNEED, then zap_pte_range().
>>> it is quite similar with kernel slab.
>>>
>>> so imagine we have small-THP now, but userspace libraries have *NO*
>>> idea at all, so it can frequently cause unfolding.
>>>
>>>>
>>>> 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?
>>>
>>> right. If this is happening on the majority of small-THP folios, we
>>> don't have performance
>>> improvement, and probably regression instead. This is really true on
>>> real workloads!!
>>>
>>> So that is why we really love a per-VMA hint to enable small-THP but
>>> obviously you
>>> have already supported it now by
>>> mm: thp: Introduce per-size thp sysfs interface
>>> https://lore.kernel.org/linux-mm/20231122162950.3854897-4-ryan.roberts@arm.com/
>>>
>>> we can use MADVISE rather than ALWAYS and set fixed size like 64KB, so userspace
>>> can set the VMA flag when it is quite sure this VMA is working with
>>> the alignment
>>> of 64KB?
>>
>> Yes, that all exists in the series today. We have also discussed the possibility
>> of adding a new madvise_process() call that would take the set of THP sizes that
>> should be considered. Then you can set different VMAs to use different sizes;
>> the plan was to layer that on top if/when a workload was identified. Sounds like
>> you might be able to help there?
>
> i'm not quite sure as on phones, we are using fixed-size CONTPTE. so we ask
> for either 64KB or 4KB. If we think one VMA is all good to use CONTPTE, we
> set a flag in this VMA and try to allocate 64KB.

When you say "we set a flag" do you mean user space? Or is there some heuristic
in the kernel?

>
> But I will try to understand this requirement to madvise THPs size on a specific
> VMA.
>
>>
>>>
>>>>
>>>> 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