Re: [PATCH v4 2/5] mm: LARGE_ANON_FOLIO for improved performance

From: Ryan Roberts
Date: Mon Aug 07 2023 - 13:45:55 EST

On 05/08/2023 03:50, Yin, Fengwei wrote:
> On 8/5/2023 5:58 AM, Zi Yan wrote:
>> On 4 Aug 2023, at 17:30, David Hildenbrand wrote:
>>> On 04.08.23 23:26, Yu Zhao wrote:
>>>> On Fri, Aug 4, 2023 at 3:13 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>> On 04.08.23 23:00, Yu Zhao wrote:
>>>>>> On Fri, Aug 4, 2023 at 2:23 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>>>> On 04.08.23 10:27, Ryan Roberts wrote:
>>>>>>>> On 04/08/2023 00:50, Yu Zhao wrote:
>>>>>>>>> On Thu, Aug 3, 2023 at 6:43 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>>>>>>>>> + Kirill
>>>>>>>>>> On 26/07/2023 10:51, Ryan Roberts wrote:
>>>>>>>>>>> Introduce LARGE_ANON_FOLIO feature, which allows anonymous memory to be
>>>>>>>>>>> allocated in large folios of a determined order. All pages of the large
>>>>>>>>>>> folio are pte-mapped during the same page fault, significantly reducing
>>>>>>>>>>> the number of page faults. The number of per-page operations (e.g. ref
>>>>>>>>>>> counting, rmap management lru list management) are also significantly
>>>>>>>>>>> reduced since those ops now become per-folio.
>>>>>>>>>>> The new behaviour is hidden behind the new LARGE_ANON_FOLIO Kconfig,
>>>>>>>>>>> which defaults to disabled for now; The long term aim is for this to
>>>>>>>>>>> defaut to enabled, but there are some risks around internal
>>>>>>>>>>> fragmentation that need to be better understood first.
>>>>>>>>>>> When enabled, the folio order is determined as such: For a vma, process
>>>>>>>>>>> or system that has explicitly disabled THP, we continue to allocate
>>>>>>>>>>> order-0. THP is most likely disabled to avoid any possible internal
>>>>>>>>>>> fragmentation so we honour that request.
>>>>>>>>>>> Otherwise, the return value of arch_wants_pte_order() is used. For vmas
>>>>>>>>>>> that have not explicitly opted-in to use transparent hugepages (e.g.
>>>>>>>>>>> where thp=madvise and the vma does not have MADV_HUGEPAGE), then
>>>>>>>>>>> arch_wants_pte_order() is limited to 64K (or PAGE_SIZE, whichever is
>>>>>>>>>>> bigger). This allows for a performance boost without requiring any
>>>>>>>>>>> explicit opt-in from the workload while limitting internal
>>>>>>>>>>> fragmentation.
>>>>>>>>>>> If the preferred order can't be used (e.g. because the folio would
>>>>>>>>>>> breach the bounds of the vma, or because ptes in the region are already
>>>>>>>>>>> mapped) then we fall back to a suitable lower order; first
>>>>>>>>>>> PAGE_ALLOC_COSTLY_ORDER, then order-0.
>>>>>>>>>> ...
>>>>>>>>>>> +#define ANON_FOLIO_MAX_ORDER_UNHINTED \
>>>>>>>>>>> + (ilog2(max_t(unsigned long, SZ_64K, PAGE_SIZE)) - PAGE_SHIFT)
>>>>>>>>>>> +
>>>>>>>>>>> +static int anon_folio_order(struct vm_area_struct *vma)
>>>>>>>>>>> +{
>>>>>>>>>>> + int order;
>>>>>>>>>>> +
>>>>>>>>>>> + /*
>>>>>>>>>>> + * If THP is explicitly disabled for either the vma, the process or the
>>>>>>>>>>> + * system, then this is very likely intended to limit internal
>>>>>>>>>>> + * fragmentation; in this case, don't attempt to allocate a large
>>>>>>>>>>> + * anonymous folio.
>>>>>>>>>>> + *
>>>>>>>>>>> + * Else, if the vma is eligible for thp, allocate a large folio of the
>>>>>>>>>>> + * size preferred by the arch. Or if the arch requested a very small
>>>>>>>>>>> + * size or didn't request a size, then use PAGE_ALLOC_COSTLY_ORDER,
>>>>>>>>>>> + * which still meets the arch's requirements but means we still take
>>>>>>>>>>> + * advantage of SW optimizations (e.g. fewer page faults).
>>>>>>>>>>> + *
>>>>>>>>>>> + * Finally if thp is enabled but the vma isn't eligible, take the
>>>>>>>>>>> + * arch-preferred size and limit it to ANON_FOLIO_MAX_ORDER_UNHINTED.
>>>>>>>>>>> + * This ensures workloads that have not explicitly opted-in take benefit
>>>>>>>>>>> + * while capping the potential for internal fragmentation.
>>>>>>>>>>> + */
>>>>>>>>>>> +
>>>>>>>>>>> + if ((vma->vm_flags & VM_NOHUGEPAGE) ||
>>>>>>>>>>> + test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags) ||
>>>>>>>>>>> + !hugepage_flags_enabled())
>>>>>>>>>>> + order = 0;
>>>>>>>>>>> + else {
>>>>>>>>>>> + order = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER);
>>>>>>>>>>> +
>>>>>>>>>>> + if (!hugepage_vma_check(vma, vma->vm_flags, false, true, true))
>>>>>>>>>>> + order = min(order, ANON_FOLIO_MAX_ORDER_UNHINTED);
>>>>>>>>>>> + }
>>>>>>>>>>> +
>>>>>>>>>>> + return order;
>>>>>>>>>>> +}
>>>>>>>>>> Hi All,
>>>>>>>>>> I'm writing up the conclusions that we arrived at during discussion in the THP
>>>>>>>>>> meeting yesterday, regarding linkage with exiting THP ABIs. It would be great if
>>>>>>>>>> I can get explicit "agree" or disagree + rationale from at least David, Yu and
>>>>>>>>>> Kirill.
>>>>>>>>>> In summary; I think we are converging on the approach that is already coded, but
>>>>>>>>>> I'd like confirmation.
>>>>>>>>>> The THP situation today
>>>>>>>>>> -----------------------
>>>>>>>>>> - At system level: THP can be set to "never", "madvise" or "always"
>>>>>>>>>> - At process level: THP can be "never" or "defer to system setting"
>>>>>>>>>> - At VMA level: no-hint, MADV_HUGEPAGE, MADV_NOHUGEPAGE
>>>>>>>>>> That gives us this table to describe how a page fault is handled, according to
>>>>>>>>>> process state (columns) and vma flags (rows):
>>>>>>>>>> | never | madvise | always
>>>>>>>>>> ----------------|-----------|-----------|-----------
>>>>>>>>>> no hint | S | S | THP>S
>>>>>>>>>> MADV_HUGEPAGE | S | THP>S | THP>S
>>>>>>>>>> MADV_NOHUGEPAGE | S | S | S
>>>>>>>>>> Legend:
>>>>>>>>>> S allocate single page (PTE-mapped)
>>>>>>>>>> LAF allocate lage anon folio (PTE-mapped)
>>>>>>>>>> THP allocate THP-sized folio (PMD-mapped)
>>>>>>>>>>> fallback (usually because vma size/alignment insufficient for folio)
>>>>>>>>>> Principles for Large Anon Folios (LAF)
>>>>>>>>>> --------------------------------------
>>>>>>>>>> David tells us there are use cases today (e.g. qemu live migration) which use
>>>>>>>>>> MADV_NOHUGEPAGE to mean "don't fill any PTEs that are not explicitly faulted"
>>>>>>>>>> and these use cases will break (i.e. functionally incorrect) if this request is
>>>>>>>>>> not honoured.
>>>>>>>>> I don't remember David saying this. I think he was referring to UFFD,
>>>>>>>>> not MADV_NOHUGEPAGE, when discussing what we need to absolutely
>>>>>>>>> respect.
>>>>>>>> My understanding was that MADV_NOHUGEPAGE was being applied to regions *before*
>>>>>>>> UFFD was being registered, and the app relied on MADV_NOHUGEPAGE to not back any
>>>>>>>> unfaulted pages. It's not completely clear to me how not honouring
>>>>>>>> MADV_NOHUGEPAGE would break things though. David?
>>>>>>> Sorry, I'm still lagging behind on some threads.
>>>>>>> Imagine the following for VM postcopy live migration:
>>>>>>> (1) Set MADV_NOHUGEPAGE on guest memory and discard all memory (e.g.,
>>>>>>> MADV_DONTNEED), to start with a clean slate.
>>>>>>> (2) Migrates some pages during precopy from the source and stores them
>>>>>>> into guest memory on the destination. Some of the memory locations
>>>>>>> will have pages populated.
>>>>>>> (3) At some point, decide to enable postcopy: enable userfaultfd on
>>>>>>> guest memory.
>>>>>>> (4) Discard *selected* pages again that have been dirtied in the
>>>>>>> meantime on the source. These are pages that have been migrated
>>>>>>> previously.
>>>>>>> (5) Start running the VM on the destination.
>>>>>>> (6) Anything that's not populated will trigger userfaultfd missing
>>>>>>> faults. Then, you can request them from the source and place them.
>>>>>>> Assume you would populate more than required during 2), you can end up
>>>>>>> not getting userfaultfd faults during 4) and corrupt your guest state.
>>>>>>> It works if during (2) you migrated all guest memory, or if during 4)
>>>>>>> you zap everything that still needs migr
>>>>>> I see what you mean now. Thanks.
>>>>>> Yes, in this case we have to interpret MADV_NOHUGEPAGE as nothing >4KB.

I'm glad we have agreement on this.

In some threads Yu has been talking about this series in the short term, vs long
term roadmap; so to be clear, I interpret this as meaning we must consider that
MADV_NOHUGEPAGE means nothing bigger than order-0 both in the context of this
series and for the long term - that's behavior that user space depends upon.

I think we should also apply the same logic to system/process THP mode =
"never", even if the vma does not have MADV_NOHUGEPAGE. If the user has
explicitly set "never" on the system or process, that means "nothing bigger than
order-0". Shout if you disagree.

>>>>> Note that it's still even unclear to me why we want to *not* call these
>>>>> things THP. It would certainly make everything less confusing if we call
>>>>> them THP, but with additional attributes.

I think I've stated in the past that I don't have a strong opinion on what we
call them. But I do think you make a convincing argument for calling them after
THP. Regardless, I'd rather agree on a name up front, before this initial series
goes in - it's always better to be consistent across all the commit messages and
comments to make things more grepable.

The only concrete objection I remember hearing to a name with "THP" in the title
was that there are stats (meminfo, vmstats, etc) that count THPs and this
becomes confusing if those counters now only mean a subset of THPs. But that
feels like a small issue in the scheme of things.

>>>>> I think that is one of the first things we should figure out because it
>>>>> also indirectly tells us what all these toggles mean and how/if we
>>>>> should redefine them (and if they even apply).
>>>>> Currently THP == PMD size
>>>>> In 2016, Hugh already envisioned PUD/PGD THP (see 49920d28781d ("mm:
>>>>> make transparent hugepage size public")) when he explicitly exposed
>>>>> "hpage_pmd_size". Not "hpage_size".
>>>>> For hugetlb on arm64 we already support various sizes that are < PMD
>>>>> size and *not* call them differently. It's a huge(tlb) page. Sometimes
>>>>> we refer to them as cont-PTE hugetlb pages.
>>>>> So, nowadays we do have "PMD-sized THP", someday we might have
>>>>> "PUD-sized THP". Can't we come up with a name to describe sub-PMD THP?

I think one subtle difference is that these sub-PMD THPs, likely won't always
have a single size.

>>>>> Is it really of value if we invent a new term for them? Yes, I was not
>>>>> enjoying "Flexible THP".

How about "variable-order THP"? Or "SW THP" vs "HW THP"?

>>>>> Once we figured that out, we should figure out if MADV_HUGEPAGE meant
>>>>> "only PMD-sized THP" or anything else?
>>>>> Also, we can then figure out if MADV_NOHUGEPAGE meant "only PMD-sized
>>>>> THP" or anything else?

Based on the existing user space expectation that MADV_NOHUGEPAGE means "nothing
bigger than order-0" I'm not sure how we could ever decide MADV_NOHUGEPAGE means
anything different? This feels set in stone to me.

>>>>> The simplest approach to me would be "they imply any THP, and once we
>>>>> need more tunables we might add some", similar to what Kirill also raised.


>>>>> Again, it's all unclear to me at this point and I'm happy to hear
>>>>> opinions, because I really don't know.
>>>> I agree these points require more discussion. But I don't think we
>>>> need to conclude them now, unless they cause correctness issues like
>>>> ignoring MADV_NOHUGEPAGE would. My concern is that if we decide to go
>>>> with "they imply any THP" and *expose this to userspace now*, we might
>>>> regret later.
>>> If we don't think they are THP, probably MADV_NOHUGEPAGE should not apply and we should be ready to find other ways to deal with the mess we eventually create. If we want to go down that path, sure.
>>> If they are THP, to me there is not really a question if MADV_NOHUGEPAGE applies to them or not. Unless we want to build a confusing piece of software ;)
>> I think it is good to call them THP, since they are transparent huge (>order-0) pages.
>> But the concern is that before we have a reasonable management policy for order>0 &&
>> order<9 THPs, mixing them with existing order-9 THP might give user unexpected
>> performance outcome. Unless we are sure they will always performance improvement,
>> we might repeat the old THP path, namely users begin to disable THP by default
>> to avoid unexpected performance hiccup. That is the reason Yu wants to separate
>> LAF from THP at the moment.

(for the purposes of this; LAF="sub-PMD THP", THP="PMD-size THP", we treat them
both as forms of THP)...

How about this for a strawman:

When introducing LAF we can either use an opt-in or an opt-out model. The opt-in
model would require a new ABI from day 1 (something I think there is concensus
that we do not want to do) and would prevent apps from automatically getting
benefit. So I don't like that model.

If going with the opt-out model, we already have an opt-out mechanism
(thp="never" and MADV_NOHUGEPAGE) that we can piggyback. But that mechanism
doesn't give us all the control we would like for benchmarking/characterizing
the interactions between LAF/THP for different workloads. Ideally we need a way
to enable THP while keeping LAF disabled and enable LAF while keeping THP disabled.

Can we do this with debugfs? I think controls in there can come and go without
too much concern about back-compat?

Perhaps 2 controls:

enable/disable LAF independently of THP

applies to both LAF and THP
when max_order < PMD-order, THP acts like thp="never"
puts a ceiling on folio order allocated by LAF

This gives:

laf_enable=1, laf_max_order=PMD-order (LAF+THP):

| never | madvise | always
no hint | S | LAF>S | THP>LAF>S

laf_enable=0, laf_max_order=PMD-order (THP only):

| never | madvise | always
no hint | S | S | THP>S

laf_enable=1, laf_max_order=(PMD-order - 1) (LAF only):

| never | madvise | always
no hint | S | LAF>S | LAF>S

This would allow us to get something into the kernel that would allow people to
more broadly characterize different workloads under THP, LAF, THP+LAF, which
would give us a better understanding of if/how we want to design ABIs for the
long term.

>> Maybe call it THP (experimental) for now and merge it to THP when we have a stable
>> policy. For knobs, we might add "any-order" to the existing "never", "madvise"
>> and another interface to specify max hinted order (enforcing <9) for "any-order".
>> Later, we can allow users to specify any max hinted order, including 9. Just an
>> idea.
> I suspect that all the config knobs (enable/disable mixing mode, define "any-order"
> or "specific-order") will be exist long term. Because there are always new workloads
> need be tuned against these configs.
> Regards
> Yin, Fengwei
>> --
>> Best Regards,
>> Yan, Zi