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

[Yu Zhao]

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.