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

From: Ryan Roberts
Date: Tue Aug 08 2023 - 15:22:57 EST

On 08/08/2023 00:21, Yu Zhao wrote:
> On Mon, Aug 7, 2023 at 1:07 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> On 07/08/2023 06:24, Yu Zhao wrote:
>>> On Wed, Jul 26, 2023 at 3:52 AM Ryan Roberts <ryan.roberts@xxxxxxx> 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.
>>>>
>>>> arch_wants_pte_order() can be overridden by the architecture if desired.
>>>> Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
>>>> set of ptes map physically contigious, naturally aligned memory, so this
>>>> mechanism allows the architecture to optimize as required.
>>>>
>>>> Here we add the default implementation of arch_wants_pte_order(), used
>>>> when the architecture does not define it, which returns -1, implying
>>>> that the HW has no preference. In this case, mm will choose it's own
>>>> default order.
>>>>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>> ---
>>>> include/linux/pgtable.h | 13 ++++
>>>> mm/Kconfig | 10 +++
>>>> mm/memory.c | 166 ++++++++++++++++++++++++++++++++++++----
>>>> 3 files changed, 172 insertions(+), 17 deletions(-)
>>>>
>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>> index 5063b482e34f..2a1d83775837 100644
>>>> --- a/include/linux/pgtable.h
>>>> +++ b/include/linux/pgtable.h
>>>> @@ -313,6 +313,19 @@ static inline bool arch_has_hw_pte_young(void)
>>>> }
>>>> #endif
>>>>
>>>> +#ifndef arch_wants_pte_order
>>>> +/*
>>>> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
>>>> + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
>>>> + * to be at least order-2. Negative value implies that the HW has no preference
>>>> + * and mm will choose it's own default order.
>>>> + */
>>>> +static inline int arch_wants_pte_order(void)
>>>> +{
>>>> + return -1;
>>>> +}
>>>> +#endif
>>>> +
>>>> #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>>>> static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>>>> unsigned long address,
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index 09130434e30d..fa61ea160447 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -1238,4 +1238,14 @@ config LOCK_MM_AND_FIND_VMA
>>>>
>>>> source "mm/damon/Kconfig"
>>>>
>>>> +config LARGE_ANON_FOLIO
>>>> + bool "Allocate large folios for anonymous memory"
>>>> + depends on TRANSPARENT_HUGEPAGE
>>>> + default n
>>>> + help
>>>> + Use large (bigger than order-0) folios to back anonymous memory where
>>>> + possible, even for pte-mapped memory. This reduces the number of page
>>>> + faults, as well as other per-page overheads to improve performance for
>>>> + many workloads.
>>>> +
>>>> endmenu
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 01f39e8144ef..64c3f242c49a 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -4050,6 +4050,127 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>> return ret;
>>>> }
>>>>
>>>> +static bool vmf_pte_range_changed(struct vm_fault *vmf, int nr_pages)
>>>> +{
>>>> + int i;
>>>> +
>>>> + if (nr_pages == 1)
>>>> + return vmf_pte_changed(vmf);
>>>> +
>>>> + for (i = 0; i < nr_pages; i++) {
>>>> + if (!pte_none(ptep_get_lockless(vmf->pte + i)))
>>>> + return true;
>>>> + }
>>>> +
>>>> + return false;
>>>> +}
>>>> +
>>>> +#ifdef CONFIG_LARGE_ANON_FOLIO
>>>> +#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;
>>>> +}
>>>> +
>>>> +static int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
>>>> +{
>>>> + int i;
>>>> + gfp_t gfp;
>>>> + pte_t *pte;
>>>> + unsigned long addr;
>>>> + struct vm_area_struct *vma = vmf->vma;
>>>> + int prefer = anon_folio_order(vma);
>>>> + int orders[] = {
>>>> + prefer,
>>>> + prefer > PAGE_ALLOC_COSTLY_ORDER ? PAGE_ALLOC_COSTLY_ORDER : 0,
>>>> + 0,
>>>> + };
>>>> +
>>>> + *folio = NULL;
>>>> +
>>>> + if (vmf_orig_pte_uffd_wp(vmf))
>>>> + goto fallback;
>>>
>>> Per the discussion, we need to check hugepage_vma_check() for
>>> correctness of VM LM. I'd just check it here and fall back to order 0
>>> if that helper returns false.
>>
>> I'm not sure if either you haven't noticed the logic in anon_folio_order()
>> above, or whether you are making this suggestion because you disagree with the
>> subtle difference in my logic?
>
> The latter, or more generally the policy you described earlier.
>
>> My logic is deliberately not calling hugepage_vma_check() because that would
>> return false for the thp=madvise,mmap=unhinted case, whereas the policy I'm
>> implementing wants to apply LAF in that case.
>>
>>
>> My intended policy:
>>
>> | never | madvise | always
>> ----------------|-----------|-----------|-----------
>> no hint | S | LAF>S | THP>LAF>S
>> MADV_HUGEPAGE | S | THP>LAF>S | THP>LAF>S
>> MADV_NOHUGEPAGE | S | S | S
>>
>>
>> What your suggestion would give:
>>
>> | never | madvise | always
>> ----------------|-----------|-----------|-----------
>> no hint | S | S | THP>LAF>S
>> MADV_HUGEPAGE | S | THP>LAF>S | THP>LAF>S
>> MADV_NOHUGEPAGE | S | S | S
>
> This is not what I'm suggesting.
>
> Let me reiterate [1]:
> My impression is we only agreed on one thing: at the current stage, we
> should respect things we absolutely have to. We didn't agree on what
> "never" means ("never 2MB" or "never >4KB"), and we didn't touch on
> how "always" should behave at all.
>
> And [2]:
> (Thanks to David, now I agree that) we have to interpret MADV_NOHUGEPAGE
> as nothing >4KB.
>
> My final take [3]:
> 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.

Thanks, I've read all of these comments previously, and appreciate the time you
have put into the feedback. I'm not sure I fully agree with your point that we
don't need to conclude on a policy now; I certainly don't think we need the
whole thing in place on day 1, but I do think that whatever we put in should
strive to be a strict subset of where we think we are going. For example, if we
put something in with one policy (i.e. "never" only means "never 2MB") then find
a problem and have to change that to be more conservative, are we risking perf
regressions for any LAF users that started using it on day 1?

>
> But I should have been clear about the parameters to
> hugepage_vma_check(): enforce_sysfs=false.

So hugepage_vma_check(..., smaps=false, in_pf=true, enforce_sysfs=false) would
give us:

| prctl/fw | sysfs | sysfs | sysfs
| disable | never | madvise | always
----------------|-----------|-----------|-----------|-----------
no hint | S | LAF>S | LAF>S | THP>LAF>S
MADV_HUGEPAGE | S | LAF>S | THP>LAF>S | THP>LAF>S
MADV_NOHUGEPAGE | S | S | S | S

Where "prctl/fw disable" trumps the sysfs setting.

I can certainly see the benefit of this approach; it gives us a way to enable
LAF while disabling THP (thp=never). It doesn't give us a way to enable THP
without enabling LAF though (unless you recompile with LAF disabled). Does
anyone see a problem with this?



>
> [1] https://lore.kernel.org/linux-mm/CAOUHufYQTcOdKU=1mPq-fdLV7a66sHx1=EJpPpMVogciCNKO9A@xxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/CAOUHufafd4GNna2GKdSyQdW6CLVh0gxhNgeOc6t+ZOphwgw7tw@xxxxxxxxxxxxxx/
> [3] https://lore.kernel.org/linux-mm/CAOUHufYQTcOdKU=1mPq-fdLV7a66sHx1=EJpPpMVogciCNKO9A@xxxxxxxxxxxxxx/