Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

["Yin, Fengwei" <fengwei.yin@intel.com>]

On 7/3/2023 9:53 PM, Ryan Roberts wrote:
> Introduce FLEXIBLE_THP feature, which allows anonymous memory to be
THP is for huge page which is 2M size. We are not huge page here. But
I don't have good name either.

> allocated in large folios of a specified order. All pages of the large
> folio are pte-mapped during the same page fault, significantly reducing
> the number of page faults. 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.
> 
> The new behaviour is hidden behind the new FLEXIBLE_THP Kconfig, which
> defaults to disabled for now; there is a long list of todos to make
> FLEXIBLE_THP robust with existing features (e.g. compaction, mlock, some
> madvise ops, etc). These items will be tackled in subsequent patches.
> 
> When enabled, the preferred folio order is as returned by
> arch_wants_pte_order(), which may be overridden by the arch as it sees
> fit. Some architectures (e.g. arm64) can coalsece TLB entries if a
> contiguous set of ptes map physically contigious, naturally aligned
> memory, so this mechanism allows the architecture to optimize as
> required.
> 
> If the preferred order can't be used (e.g. because the folio would
> breach the bounds of the vma, or because ptes in the region are already
> mapped) then we fall back to a suitable lower order.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  mm/Kconfig  |  10 ++++
>  mm/memory.c | 168 ++++++++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 165 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 7672a22647b4..1c06b2c0a24e 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -822,6 +822,16 @@ config READ_ONLY_THP_FOR_FS
>  	  support of file THPs will be developed in the next few release
>  	  cycles.
>  
> +config FLEXIBLE_THP
> +	bool "Flexible order THP"
> +	depends on TRANSPARENT_HUGEPAGE
> +	default n
> +	help
> +	  Use large (bigger than order-0) folios to back anonymous memory where
> +	  possible, even if the order of the folio is smaller than the PMD
> +	  order. This reduces the number of page faults, as well as other
> +	  per-page overheads to improve performance for many workloads.
> +
>  endif # TRANSPARENT_HUGEPAGE
>  
>  #
> diff --git a/mm/memory.c b/mm/memory.c
> index fb30f7523550..abe2ea94f3f5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3001,6 +3001,116 @@ static vm_fault_t fault_dirty_shared_page(struct vm_fault *vmf)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_FLEXIBLE_THP
> +/*
> + * Allocates, zeros and returns a folio of the requested order for use as
> + * anonymous memory.
> + */
> +static struct folio *alloc_anon_folio(struct vm_area_struct *vma,
> +				      unsigned long addr, int order)
> +{
> +	gfp_t gfp;
> +	struct folio *folio;
> +
> +	if (order == 0)
> +		return vma_alloc_zeroed_movable_folio(vma, addr);
> +
> +	gfp = vma_thp_gfp_mask(vma);
> +	folio = vma_alloc_folio(gfp, order, vma, addr, true);
> +	if (folio)
> +		clear_huge_page(&folio->page, addr, folio_nr_pages(folio));
> +
> +	return folio;
> +}
> +
> +/*
> + * Preferred folio order to allocate for anonymous memory.
> + */
> +#define max_anon_folio_order(vma)	arch_wants_pte_order(vma)
> +#else
> +#define alloc_anon_folio(vma, addr, order) \
> +				vma_alloc_zeroed_movable_folio(vma, addr)
> +#define max_anon_folio_order(vma)	0
> +#endif
> +
> +/*
> + * Returns index of first pte that is not none, or nr if all are none.
> + */
> +static inline int check_ptes_none(pte_t *pte, int nr)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr; i++) {
> +		if (!pte_none(ptep_get(pte++)))
> +			return i;
> +	}
> +
> +	return nr;
> +}
> +
> +static int calc_anon_folio_order_alloc(struct vm_fault *vmf, int order)
> +{
> +	/*
> +	 * The aim here is to determine what size of folio we should allocate
> +	 * for this fault. Factors include:
> +	 * - Order must not be higher than `order` upon entry
> +	 * - Folio must be naturally aligned within VA space
> +	 * - Folio must be fully contained inside one pmd entry
> +	 * - Folio must not breach boundaries of vma
> +	 * - Folio must not overlap any non-none ptes
> +	 *
> +	 * Additionally, we do not allow order-1 since this breaks assumptions
> +	 * elsewhere in the mm; THP pages must be at least order-2 (since they
> +	 * store state up to the 3rd struct page subpage), and these pages must
> +	 * be THP in order to correctly use pre-existing THP infrastructure such
> +	 * as folio_split().
> +	 *
> +	 * Note that the caller may or may not choose to lock the pte. If
> +	 * unlocked, the result is racy and the user must re-check any overlap
> +	 * with non-none ptes under the lock.
> +	 */
> +
> +	struct vm_area_struct *vma = vmf->vma;
> +	int nr;
> +	unsigned long addr;
> +	pte_t *pte;
> +	pte_t *first_set = NULL;
> +	int ret;
> +
> +	order = min(order, PMD_SHIFT - PAGE_SHIFT);
> +
> +	for (; order > 1; order--) {
> +		nr = 1 << order;
> +		addr = ALIGN_DOWN(vmf->address, nr << PAGE_SHIFT);
> +		pte = vmf->pte - ((vmf->address - addr) >> PAGE_SHIFT);
> +
> +		/* Check vma bounds. */
> +		if (addr < vma->vm_start ||
> +		    addr + (nr << PAGE_SHIFT) > vma->vm_end)
> +			continue;
> +
> +		/* Ptes covered by order already known to be none. */
> +		if (pte + nr <= first_set)
> +			break;
> +
> +		/* Already found set pte in range covered by order. */
> +		if (pte <= first_set)
> +			continue;
> +
> +		/* Need to check if all the ptes are none. */
> +		ret = check_ptes_none(pte, nr);
> +		if (ret == nr)
> +			break;
> +
> +		first_set = pte + ret;
> +	}
> +
> +	if (order == 1)
> +		order = 0;
> +
> +	return order;
> +}
The logic in above function should be kept is whether the order fit in vma range.

check_ptes_none() is not accurate here because no page table lock hold and concurrent
fault could happen. So may just drop the check here? Check_ptes_none() is done after
take the page table lock.

We pick the arch prefered order or order 0 now.

> +
>  /*
>   * Handle write page faults for pages that can be reused in the current vma
>   *
> @@ -3073,7 +3183,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  		goto oom;
>  
>  	if (is_zero_pfn(pte_pfn(vmf->orig_pte))) {
> -		new_folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
> +		new_folio = alloc_anon_folio(vma, vmf->address, 0);
>  		if (!new_folio)
>  			goto oom;
>  	} else {
> @@ -4040,6 +4150,9 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	struct folio *folio;
>  	vm_fault_t ret = 0;
>  	pte_t entry;
> +	int order;
> +	int pgcount;
> +	unsigned long addr;
>  
>  	/* File mapping without ->vm_ops ? */
>  	if (vma->vm_flags & VM_SHARED)
> @@ -4081,24 +4194,51 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  			pte_unmap_unlock(vmf->pte, vmf->ptl);
>  			return handle_userfault(vmf, VM_UFFD_MISSING);
>  		}
> -		goto setpte;
> +		if (uffd_wp)
> +			entry = pte_mkuffd_wp(entry);
> +		set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
> +
> +		/* No need to invalidate - it was non-present before */
> +		update_mmu_cache(vma, vmf->address, vmf->pte);
> +		goto unlock;
> +	}
> +
> +	/*
> +	 * If allocating a large folio, determine the biggest suitable order for
> +	 * the VMA (e.g. it must not exceed the VMA's bounds, it must not
> +	 * overlap with any populated PTEs, etc). We are not under the ptl here
> +	 * so we will need to re-check that we are not overlapping any populated
> +	 * PTEs once we have the lock.
> +	 */
> +	order = uffd_wp ? 0 : max_anon_folio_order(vma);
> +	if (order > 0) {
> +		vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
> +		order = calc_anon_folio_order_alloc(vmf, order);
> +		pte_unmap(vmf->pte);
>  	}
>  
> -	/* Allocate our own private page. */
> +	/* Allocate our own private folio. */
>  	if (unlikely(anon_vma_prepare(vma)))
>  		goto oom;
> -	folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
> +	folio = alloc_anon_folio(vma, vmf->address, order);
> +	if (!folio && order > 0) {
> +		order = 0;
> +		folio = alloc_anon_folio(vma, vmf->address, order);
> +	}
>  	if (!folio)
>  		goto oom;
>  
> +	pgcount = 1 << order;
> +	addr = ALIGN_DOWN(vmf->address, pgcount << PAGE_SHIFT);
> +
>  	if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
>  		goto oom_free_page;
>  	folio_throttle_swaprate(folio, GFP_KERNEL);
>  
>  	/*
>  	 * The memory barrier inside __folio_mark_uptodate makes sure that
> -	 * preceding stores to the page contents become visible before
> -	 * the set_pte_at() write.
> +	 * preceding stores to the folio contents become visible before
> +	 * the set_ptes() write.
>  	 */
>  	__folio_mark_uptodate(folio);
>  
> @@ -4107,11 +4247,12 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	if (vma->vm_flags & VM_WRITE)
>  		entry = pte_mkwrite(pte_mkdirty(entry));
>  
> -	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_changed(vmf)) {
>  		update_mmu_tlb(vma, vmf->address, vmf->pte);
>  		goto release;
> +	} else if (order > 0 && check_ptes_none(vmf->pte, pgcount) != pgcount) {
This could be the case that we allocated order 4 page and find a neighbor PTE is
filled by concurrent fault. Should we put current folio and fallback to order 0
and try again immedately (goto order 0 allocation instead of return from this
function which will go through some page fault path again)?


Regards
Yin, Fengwei

> +		goto release;
>  	}
>  
>  	ret = check_stable_address_space(vma->vm_mm);
> @@ -4125,16 +4266,17 @@ 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, pgcount - 1);
> +	add_mm_counter(vma->vm_mm, MM_ANONPAGES, pgcount);
> +	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, pgcount);
>  
>  	/* No need to invalidate - it was non-present before */
> -	update_mmu_cache(vma, vmf->address, vmf->pte);
> +	update_mmu_cache_range(vma, addr, vmf->pte, pgcount);
>  unlock:
>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
>  	return ret;