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

[Ryan Roberts]

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?