Re: [PATCH v9 04/10] mm: thp: Support allocation of anonymous multi-size THP

[Ryan Roberts <ryan.roberts@arm.com>]

On 12/12/2023 15:02, David Hildenbrand wrote:
> On 07.12.23 17:12, Ryan Roberts 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.
> 
> I'll note that with always-pte-mapped-thp it will be much easier to support
> incremental page clearing (e.g., zero only parts of the folio and map the
> remainder in a pro-non-like fashion whereby we'll zero on the next page fault).
> With a PMD-sized thp, you have to eventually place/rip out page tables to
> achieve that.

But then you lose the benefits of reduced number of page faults; reducing page
faults gives a big speed up for workloads with lots of short lived processes
like compiling.

But yes, I agree this could be an interesting future optimization for some
workloads.

> 
>>
>> 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.
>>
>> Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> Tested-by: John Hubbard <jhubbard@nvidia.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>   include/linux/huge_mm.h |   6 ++-
>>   mm/memory.c             | 111 ++++++++++++++++++++++++++++++++++++----
>>   2 files changed, 106 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 609c153bae57..fa7a38a30fc6 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)
> 
> [...]
> 
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>> +{
>> +    struct vm_area_struct *vma = vmf->vma;
>> +    unsigned long orders;
>> +    struct folio *folio;
>> +    unsigned long addr;
>> +    pte_t *pte;
>> +    gfp_t gfp;
>> +    int order;
>> +
>> +    /*
>> +     * If uffd is active for the vma we need per-page fault fidelity to
>> +     * maintain the uffd semantics.
>> +     */
>> +    if (unlikely(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);
>> +
>> +    /*
>> +     * Find the highest order where the aligned range is completely
>> +     * pte_none(). Note that all remaining orders will be completely
>> +     * pte_none().
>> +     */
>> +    order = highest_order(orders);
>> +    while (orders) {
>> +        addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
>> +        if (pte_range_none(pte + pte_index(addr), 1 << order))
>> +            break;
>> +        order = next_order(&orders, order);
>> +    }
>> +
>> +    pte_unmap(pte);
>> +
>> +    /* Try allocating the highest of the remaining orders. */
>> +    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, vmf->address, 1 << order);
>> +            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
> 
> A neater alternative might be
> 
> static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> {
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>     /* magic */
> fallback:
> #endif
>     return vma_alloc_zeroed_movable_folio((vmf)->vma, (vmf)->address):
> }

I guess beauty lies in the eye of the beholder... I don't find it much neater
personally :). But happy to make the change if you insist; what's the process
now that its in mm-unstable? Just send a patch to Andrew for squashing?

> 
> [...]
> 
> Acked-by: David Hildenbrand <david@redhat.com>
>