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

[Yin Fengwei]

On 7/4/23 22:20, Ryan Roberts wrote:
> 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?
My understanding is the THP has several fixed size on different arch.
The 32K or 16K which could be picked here are not THP size.

> 
>>
>>> 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.
It will not if we try order0 immediately. But see my comments to the refault.

> 
>>
>> 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.
I must miss that discussion in v1. My bad. I should jump in that discussion.
So I will drop my comment here even I still think we should avoid refault.
I don't want the comment back and forth.


Regards
Yin, Fengwei

> 
>>
>>
>> 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;
>