Re: [PATCH v8 04/10] mm: thp: Support allocation of anonymous multi-size THP

Barry Song
2023-12-05T01:25:14Z

On Tue, Dec 5, 2023 at 9:15 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Mon, Dec 4, 2023 at 6:21 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >
> > Introduce the logic to allow THP to be configured (through the new sysfs
> > interface we just added) to allocate large folios to back anonymous
> > memory, which are larger than the base page size but smaller than
> > PMD-size. We call this new THP extension "multi-size THP" (mTHP).
> >
> > mTHP continues to be PTE-mapped, but in many cases can still provide
> > similar benefits to traditional PMD-sized THP: Page faults are
> > significantly reduced (by a factor of e.g. 4, 8, 16, etc. depending on
> > the configured order), but latency spikes are much less prominent
> > because the size of each page isn't as huge as the PMD-sized variant and
> > there is less memory to clear in each page fault. 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.
> >
> > Some architectures also employ TLB compression mechanisms to squeeze
> > more entries in when a set of PTEs are virtually and physically
> > contiguous and approporiately aligned. In this case, TLB misses will
> > occur less often.
> >
> > The new behaviour is disabled by default, but can be enabled at runtime
> > by writing to /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled
> > (see documentation in previous commit). The long term aim is to change
> > the default to include suitable lower orders, but there are some risks
> > around internal fragmentation that need to be better understood first.
> >
> > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> > ---
> > include/linux/huge_mm.h | 6 ++-
> > mm/memory.c | 106 ++++++++++++++++++++++++++++++++++++----
> > 2 files changed, 101 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index bd0eadd3befb..91a53b9835a4 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -68,9 +68,11 @@ extern struct kobj_attribute shmem_enabled_attr;
> > #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
> >
> > /*
> > - * Mask of all large folio orders supported for anonymous THP.
> > + * Mask of all large folio orders supported for anonymous THP; all orders up to
> > + * and including PMD_ORDER, except order-0 (which is not "huge") and order-1
> > + * (which is a limitation of the THP implementation).
> > */
> > -#define THP_ORDERS_ALL_ANON BIT(PMD_ORDER)
> > +#define THP_ORDERS_ALL_ANON ((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1)))
> >
> > /*
> > * Mask of all large folio orders supported for file THP.
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 3ceeb0f45bf5..bf7e93813018 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4125,6 +4125,84 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > return ret;
> > }
> >
> > +static bool pte_range_none(pte_t *pte, int nr_pages)
> > +{
> > + int i;
> > +
> > + for (i = 0; i < nr_pages; i++) {
> > + if (!pte_none(ptep_get_lockless(pte + i)))
> > + return false;
> > + }
> > +
> > + return true;
> > +}
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> > +{
> > + gfp_t gfp;
> > + pte_t *pte;
> > + unsigned long addr;
> > + struct folio *folio;
> > + struct vm_area_struct *vma = vmf->vma;
> > + unsigned long orders;
> > + int order;
> > +
> > + /*
> > + * If uffd is active for the vma we need per-page fault fidelity to
> > + * maintain the uffd semantics.
> > + */
> > + if (userfaultfd_armed(vma))
> > + goto fallback;
> > +
> > + /*
> > + * Get a list of all the (large) orders below PMD_ORDER that are enabled
> > + * for this vma. Then filter out the orders that can't be allocated over
> > + * the faulting address and still be fully contained in the vma.
> > + */
> > + orders = thp_vma_allowable_orders(vma, vma->vm_flags, false, true, true,
> > + BIT(PMD_ORDER) - 1);
> > + orders = thp_vma_suitable_orders(vma, vmf->address, orders);
> > +
> > + if (!orders)
> > + goto fallback;
> > +
> > + pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
> > + if (!pte)
> > + return ERR_PTR(-EAGAIN);
> > +
> > + order = first_order(orders);
> > + while (orders) {
> > + addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> > + vmf->pte = pte + pte_index(addr);
> > + if (pte_range_none(vmf->pte, 1 << order))
> > + break;
> > + order = next_order(&orders, order);
> > + }
> > +
> > + vmf->pte = NULL;
> > + pte_unmap(pte);
> > +
> > + gfp = vma_thp_gfp_mask(vma);
> > +
> > + while (orders) {
> > + addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> > + folio = vma_alloc_folio(gfp, order, vma, addr, true);
> > + if (folio) {
> > + clear_huge_page(&folio->page, addr, 1 << order);
>
> Minor.
>
> Do we have to constantly clear a huge page? Is it possible to let
> post_alloc_hook()
> finish this job by using __GFP_ZERO/__GFP_ZEROTAGS as
> vma_alloc_zeroed_movable_folio() is doing?
>
> struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
> unsigned long vaddr)
> {
> gfp_t flags = GFP_HIGHUSER_MOVABLE | __GFP_ZERO;
>
> /*
> * If the page is mapped with PROT_MTE, initialise the tags at the
> * point of allocation and page zeroing as this is usually faster than
> * separate DC ZVA and STGM.
> */
> if (vma->vm_flags & VM_MTE)
> flags |= __GFP_ZEROTAGS;
>
> return vma_alloc_folio(flags, 0, vma, vaddr, false);
> }

I am asking this because Android and some other kernels might always set
CONFIG_INIT_ON_ALLOC_DEFAULT_ON, that means one more explicit
clear_page is doing a duplicated job.

when the below is true, post_alloc_hook has cleared huge_page before
vma_alloc_folio() returns the folio,

static inline bool want_init_on_alloc(gfp_t flags)
{
if (static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
&init_on_alloc))
return true;
return flags & __GFP_ZERO;
}

>
> > + return folio;
> > + }
> > + order = next_order(&orders, order);
> > + }
> > +
> > +fallback:
> > + return vma_alloc_zeroed_movable_folio(vma, vmf->address);
> > +}
> > +#else
> > +#define alloc_anon_folio(vmf) \
> > + vma_alloc_zeroed_movable_folio((vmf)->vma, (vmf)->address)
> > +#endif
> > +
> > /*
> > * We enter with non-exclusive mmap_lock (to exclude vma changes,
> > * but allow concurrent faults), and pte mapped but not yet locked.
> > @@ -4132,6 +4210,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > */
> > static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> > {
> > + int i;
> > + 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;
> > @@ -4176,10 +4257,15 @@ 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);
> > + folio = alloc_anon_folio(vmf);
> > + if (IS_ERR(folio))
> > + 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);
> > @@ -4196,12 +4282,13 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> > if (vma->vm_flags & VM_WRITE)
> > entry = pte_mkwrite(pte_mkdirty(entry), vma);
> >
> > - 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 ((nr_pages == 1 && vmf_pte_changed(vmf)) ||
> > + (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages))) {
> > + for (i = 0; i < nr_pages; i++)
> > + update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
> > goto release;
> > }
> >
> > @@ -4216,16 +4303,17 @@ 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);
> > setpte:
> > if (uffd_wp)
> > entry = pte_mkuffd_wp(entry);
> > - set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
> > + set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
> >
> > /* No need to invalidate - it was non-present before */
> > - update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> > + update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
> > unlock:
> > if (vmf->pte)
> > pte_unmap_unlock(vmf->pte, vmf->ptl);
> > --
> > 2.25.1
> >