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@xxxxxxxxxx>
>> Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>> 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@xxxxxxxxxx>
>
