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

[David Hildenbrand <david@redhat.com>]

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.

> 
> 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):
}

[...]

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

-- 
Cheers,

David / dhildenb