[PATCH v4 2/5] mm: LARGE_ANON_FOLIO for improved performance
Yu Zhao
yuzhao at google.com
Wed Jul 26 21:31:12 PDT 2023
On Wed, Jul 26, 2023 at 10:41 AM Yu Zhao <yuzhao at google.com> wrote:
>
> On Wed, Jul 26, 2023 at 3:52 AM Ryan Roberts <ryan.roberts at arm.com> 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 at arm.com>
> > ---
> > 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.
> > + */
>
> What empirical evidence is SZ_64K based on?
> What workloads would benefit from it?
> How much would they benefit from it?
> Would they benefit more or less from different values?
> How much internal fragmentation would it cause?
> What cost function was used to arrive at the conclusion that its
> benefits outweigh its costs?
>
> > + 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);
> > + }
I'm a bit surprised to see the above: why can we overload existing
ABIs? I don't think we can. Assuming we could, you would have to
update Documentation/admin-guide/mm/transhuge.rst in the same
patchset, and the man page for madvise() in a separate patch.
Most importantly, existing userspace programs that don't work well
with THPs won't be able to use (try) large folios either -- this is a
big no no.
> > +
> > + return order;
> > +}
> > +
> > +static int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
>
> static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>
> and use ERR_PTR() and its friends.
>
> > +{
> > + 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;
> > +
> > + for (i = 0; orders[i]; i++) {
> > + addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
> > + if (addr >= vma->vm_start &&
> > + addr + (PAGE_SIZE << orders[i]) <= vma->vm_end)
> > + break;
> > + }
> > +
> > + if (!orders[i])
> > + goto fallback;
> > +
> > + pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
> > + if (!pte)
> > + return -EAGAIN;
> > +
> > + for (; orders[i]; i++) {
> > + addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
> > + vmf->pte = pte + pte_index(addr);
> > + if (!vmf_pte_range_changed(vmf, 1 << orders[i]))
> > + break;
> > + }
> > +
> > + vmf->pte = NULL;
> > + pte_unmap(pte);
> > +
> > + gfp = vma_thp_gfp_mask(vma);
> > +
> > + for (; orders[i]; i++) {
> > + addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
> > + *folio = vma_alloc_folio(gfp, orders[i], vma, addr, true);
> > + if (*folio) {
> > + clear_huge_page(&(*folio)->page, addr, 1 << orders[i]);
> > + return 0;
> > + }
> > + }
> > +
> > +fallback:
> > + *folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
> > + return *folio ? 0 : -ENOMEM;
> > +}
> > +#else
> > +static inline int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
> > +{
> > + *folio = vma_alloc_zeroed_movable_folio(vmf->vma, vmf->address);
> > + return *folio ? 0 : -ENOMEM;
> > +}
> > +#endif
> > +
> > /*
> > * We enter with non-exclusive mmap_lock (to exclude vma changes,
> > * but allow concurrent faults), and pte mapped but not yet locked.
> > @@ -4057,6 +4178,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > */
> > static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> > {
> > + int i = 0;
> > + int nr_pages = 1;
> > + unsigned long addr = vmf->address;
> > bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
> > struct vm_area_struct *vma = vmf->vma;
> > struct folio *folio;
> > @@ -4101,10 +4225,15 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> > /* Allocate our own private page. */
> > if (unlikely(anon_vma_prepare(vma)))
> > goto oom;
> > - folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
> > + ret = alloc_anon_folio(vmf, &folio);
> > + if (unlikely(ret == -EAGAIN))
> > + return 0;
> > if (!folio)
> > goto oom;
> >
> > + nr_pages = folio_nr_pages(folio);
> > + addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
> > +
> > if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
> > goto oom_free_page;
> > folio_throttle_swaprate(folio, GFP_KERNEL);
> > @@ -4116,17 +4245,12 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> > */
> > __folio_mark_uptodate(folio);
> >
> > - entry = mk_pte(&folio->page, vma->vm_page_prot);
> > - entry = pte_sw_mkyoung(entry);
> > - if (vma->vm_flags & VM_WRITE)
> > - entry = pte_mkwrite(pte_mkdirty(entry));
> > -
> > - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> > - &vmf->ptl);
> > + vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
> > if (!vmf->pte)
> > goto release;
> > - if (vmf_pte_changed(vmf)) {
> > - update_mmu_tlb(vma, vmf->address, vmf->pte);
> > + if (vmf_pte_range_changed(vmf, nr_pages)) {
> > + for (i = 0; i < nr_pages; i++)
> > + update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
> > goto release;
> > }
> >
> > @@ -4141,16 +4265,24 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> > return handle_userfault(vmf, VM_UFFD_MISSING);
> > }
> >
> > - inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> > - folio_add_new_anon_rmap(folio, vma, vmf->address);
> > + folio_ref_add(folio, nr_pages - 1);
> > + add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> > + folio_add_new_anon_rmap(folio, vma, addr);
> > folio_add_lru_vma(folio, vma);
> > +
> > + for (i = 0; i < nr_pages; i++) {
> > + entry = mk_pte(folio_page(folio, i), vma->vm_page_prot);
> > + entry = pte_sw_mkyoung(entry);
> > + if (vma->vm_flags & VM_WRITE)
> > + entry = pte_mkwrite(pte_mkdirty(entry));
> > setpte:
> > - if (uffd_wp)
> > - entry = pte_mkuffd_wp(entry);
> > - set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
> > + if (uffd_wp)
> > + entry = pte_mkuffd_wp(entry);
> > + set_pte_at(vma->vm_mm, addr + PAGE_SIZE * i, vmf->pte + i, entry);
> >
> > - /* No need to invalidate - it was non-present before */
> > - update_mmu_cache(vma, vmf->address, vmf->pte);
> > + /* No need to invalidate - it was non-present before */
> > + update_mmu_cache(vma, addr + PAGE_SIZE * i, vmf->pte + i);
> > + }
> > unlock:
> > if (vmf->pte)
> > pte_unmap_unlock(vmf->pte, vmf->ptl);
>
> The rest looks good to me.
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