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

[Ryan Roberts]

On 04/07/2023 04:45, Yin, Fengwei wrote:
> 
> 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.

Is that really true? On arm64 with 16K pages, huge pages are 32M and with 64K
base page, they are 512M. So huge pages already have a variable size. And they
sometimes get PTE-mapped. So can't we just think of this as an extension of the
THP feature?

> 
>> 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@xxxxxxx>
>> ---
>>  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.

I agree it is just an estimate given the lock is not held; the comment at the
top says the same. But I don't think we can wait until after the lock is taken
to measure this. We can't hold the lock while allocating the folio and we need a
guess at what to allocate. If we don't guess here, we will allocate the biggest,
then take the lock, see that it doesn't fit, and exit. Then the system will
re-fault and we will follow the exact same path - ending up in live 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)?

That's how it worked in v1, but I had review comments from Yang Shi asking me to
re-fault instead. This approach is certainly cleaner from a code point of view.
And I expect races of that nature will be rare.

> 
> 
> 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;