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

Barry Song
2023-12-06T10:26:26Z

On Wed, Dec 6, 2023 at 11:16 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 05/12/2023 20:16, Barry Song wrote:
> > On Tue, Dec 5, 2023 at 11:48 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>
> >> On 05/12/2023 01:24, Barry Song wrote:
> >>> 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?
> >>
> >> I'm currently following the same allocation pattern as is done for PMD-sized
> >> THP. In earlier versions of this patch I was trying to be smarter and use the
> >> __GFP_ZERO/__GFP_ZEROTAGS as you suggest, but I was advised to keep it simple
> >> and follow the existing pattern.
> >>
> >> I have a vague recollection __GFP_ZERO is not preferred for large folios because
> >> of some issue with virtually indexed caches? (Matthew: did I see you mention
> >> that in some other context?)
> >>
> >> That said, I wasn't aware that Android ships with
> >> CONFIG_INIT_ON_ALLOC_DEFAULT_ON (I thought it was only used as a debug option),
> >> so I can see the potential for some overhead reduction here.
> >>
> >> Options:
> >>
> >> 1) leave it as is and accept the duplicated clearing
> >> 2) Pass __GFP_ZERO and remove clear_huge_page()
> >> 3) define __GFP_SKIP_ZERO even when kasan is not enabled and pass it down so
> >> clear_huge_page() is the only clear
> >> 4) make clear_huge_page() conditional on !want_init_on_alloc()
> >>
> >> I prefer option 4. What do you think?
> >
> > either 1 and 4 is ok to me if we will finally remove this duplicated
> > clear_huge_page on top.
> > 4 is even better as it can at least temporarily resolve the problem.
>
> I'm going to stick with option 1 for this series. Then we can fix it uniformly
> here and for PMD-sized THP in a separate patch (possibly with the approach
> suggested in 4).

Ok. Thanks. there is no one fixing PMD-sized THP, probably because PMD-sized
THP is shutdown immediately after Android boots :-)

>
> >
> > in Android gki_defconfig,
> > https://android.googlesource.com/kernel/common/+/refs/heads/android14-6.1-lts/arch/arm64/configs/gki_defconfig
> >
> > Android always has the below,
> > CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y
> >
> > here is some explanation for the reason,
> > https://source.android.com/docs/security/test/memory-safety/zero-initialized-memory
> >
> >>
> >> As an aside, I've also noticed that clear_huge_page() should take vmf->address
> >> so that it clears the faulting page last to keep the cache hot. If we decide on
> >> an option that keeps clear_huge_page(), I'll also make that change.
>
> I'll make this change for the next version.
>
> >>
> >> Thanks,
> >> Ryan
> >>

Thanks
Barry