Re: [PATCH v6 5/9] mm: thp: Extend THP to allocate anonymous large folios

[Daniel Gomez <da.gomez@samsung.com>]

On Fri, Sep 29, 2023 at 12:44:16PM +0100, Ryan Roberts wrote:

Hi Ryan,
> Introduce the logic to allow THP to be configured (through the new
> anon_orders interface we just added) to allocate large folios to back
> anonymous memory, which are smaller than PMD-size (for example order-2,
> order-3, order-4, etc).
>
> These THPs continue 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 because the anon_orders
> defaults to only enabling PMD-order, but can be enabled at runtime by
> writing to anon_orders (see documentation in previous commit). The long
> term aim is to default anon_orders 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>
> ---
>  Documentation/admin-guide/mm/transhuge.rst |   9 +-
>  include/linux/huge_mm.h                    |   6 +-
>  mm/memory.c                                | 108 +++++++++++++++++++--
>  3 files changed, 111 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 9f954e73a4ca..732c3b2f4ba8 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -353,7 +353,9 @@ anonymous transparent huge pages, it is necessary to read
>  ``/proc/PID/smaps`` and count the AnonHugePages and AnonHugePteMap
>  fields for each mapping. Note that in both cases, AnonHugePages refers
>  only to PMD-mapped THPs. AnonHugePteMap refers to THPs that are mapped
> -using PTEs.
> +using PTEs. This includes all THPs whose order is smaller than
> +PMD-order, as well as any PMD-order THPs that happen to be PTE-mapped
> +for other reasons.
>
>  The number of file transparent huge pages mapped to userspace is available
>  by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``.
> @@ -367,6 +369,11 @@ frequently will incur overhead.
>  There are a number of counters in ``/proc/vmstat`` that may be used to
>  monitor how successfully the system is providing huge pages for use.
>
> +.. note::
> +   Currently the below counters only record events relating to
> +   PMD-order THPs. Events relating to smaller order THPs are not
> +   included.
> +
>  thp_fault_alloc
>  	is incremented every time a huge page is successfully
>  	allocated to handle a page fault.
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2e7c338229a6..c4860476a1f5 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 b5b82fc8e164..92ed9c782dc9 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4059,6 +4059,87 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	return ret;
>  }
>
> +static bool vmf_pte_range_changed(struct vm_fault *vmf, int nr_pages)
> +{
> +	int i;
> +
> +	if (nr_pages == 1)
> +		return vmf_pte_changed(vmf);
> +
> +	for (i = 0; i < nr_pages; i++) {
> +		if (!pte_none(ptep_get_lockless(vmf->pte + i)))
> +			return true;
> +	}
> +
> +	return false;
> +}
> +
> +#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 int 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 = hugepage_vma_check(vma, vma->vm_flags, false, true, true,
> +				    BIT(PMD_ORDER) - 1);
> +	orders = transhuge_vma_suitable(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 (!vmf_pte_range_changed(vmf, 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);

I was checking your series and noticed about the hugepage flag. I think
you've changed it from v1 -> v2 from being false to true when orders >=2
but I'm not sure about the reasoning. Is this because of your statement
in the cover letter [1]?

[1] cover letter snippet:

"to implement variable order, large folios for anonymous memory.
(previously called ..., but now exposed as an extension to THP;
"small-order THP")"

Thanks,
Daniel

> +		if (folio) {
> +			clear_huge_page(&folio->page, addr, 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
> +
>  /*
>   * We enter with non-exclusive mmap_lock (to exclude vma changes,
>   * but allow concurrent faults), and pte mapped but not yet locked.
> @@ -4066,6 +4147,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>   */
>  static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  {
> +	int i;
> +	int nr_pages = 1;
> +	unsigned long addr = vmf->address;
>  	bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct folio *folio;
> @@ -4110,10 +4194,15 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	/* Allocate our own private page. */
>  	if (unlikely(anon_vma_prepare(vma)))
>  		goto oom;
> -	folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
> +	folio = alloc_anon_folio(vmf);
> +	if (IS_ERR(folio))
> +		return 0;
>  	if (!folio)
>  		goto oom;
>
> +	nr_pages = folio_nr_pages(folio);
> +	addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
> +
>  	if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
>  		goto oom_free_page;
>  	folio_throttle_swaprate(folio, GFP_KERNEL);
> @@ -4130,12 +4219,12 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	if (vma->vm_flags & VM_WRITE)
>  		entry = pte_mkwrite(pte_mkdirty(entry), vma);
>
> -	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)
>  		goto release;
> -	if (vmf_pte_changed(vmf)) {
> -		update_mmu_tlb(vma, vmf->address, vmf->pte);
> +	if (vmf_pte_range_changed(vmf, nr_pages)) {
> +		for (i = 0; i < nr_pages; i++)
> +			update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
>  		goto release;
>  	}
>
> @@ -4150,16 +4239,17 @@ 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, nr_pages - 1);
> +	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> +	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, nr_pages);
>
>  	/* No need to invalidate - it was non-present before */
> -	update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> +	update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
>  unlock:
>  	if (vmf->pte)
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
> --
> 2.25.1
>