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

[Yu Zhao]

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

Also that "Flexible THP" Kconfig is just a placeholder, from my POV.
It should be removed after we nail down the runtime ABI, which again
IMO, isn't now.