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/