Re: [PATCH v5 1/6] mm: memory: extend finish_fault() to support large folio

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/6/11 18:11, Baolin Wang wrote:
> Add large folio mapping establishment support for finish_fault() as a
> preparation, to support multi-size THP allocation of anonymous shmem pages
> in the following patches.
> 
> Keep the same behavior (per-page fault) for non-anon shmem to avoid inflating
> the RSS unintentionally, and we can discuss what size of mapping to build
> when extending mTHP to control non-anon shmem in the future.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>   mm/memory.c | 57 +++++++++++++++++++++++++++++++++++++++++++----------
>   1 file changed, 47 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index eef4e482c0c2..72775ee99ff3 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4831,9 +4831,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   {
>   	struct vm_area_struct *vma = vmf->vma;
>   	struct page *page;
> +	struct folio *folio;
>   	vm_fault_t ret;
>   	bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
>   		      !(vma->vm_flags & VM_SHARED);
> +	int type, nr_pages;
> +	unsigned long addr = vmf->address;
>   
>   	/* Did we COW the page? */
>   	if (is_cow)
> @@ -4864,24 +4867,58 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   			return VM_FAULT_OOM;
>   	}
>   
> +	folio = page_folio(page);
> +	nr_pages = folio_nr_pages(folio);
> +
> +	/*
> +	 * Using per-page fault to maintain the uffd semantics, and same
> +	 * approach also applies to non-anonymous-shmem faults to avoid
> +	 * inflating the RSS of the process.
> +	 */
> +	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
> +		nr_pages = 1;
> +	} else if (nr_pages > 1) {
> +		pgoff_t idx = folio_page_idx(folio, page);
> +		/* The page offset of vmf->address within the VMA. */
> +		pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff;
> +
						vma->vm_pgoff

> +		/*
> +		 * Fallback to per-page fault in case the folio size in page
> +		 * cache beyond the VMA limits.
> +		 */
> +		if (unlikely(vma_off < idx ||
> +			     vma_off + (nr_pages - idx) > vma_pages(vma))) {
> +			nr_pages = 1;
> +		} else {
> +			/* Now we can set mappings for the whole large folio. */
> +			addr = vmf->address - idx * PAGE_SIZE;

			addr -= idx * PAGE_SIZE;

> +			page = &folio->page;
> +		}
> +	}
> +
>   	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> -				      vmf->address, &vmf->ptl);
> +				       addr, &vmf->ptl);

no newline now,

>   	if (!vmf->pte)
>   		return VM_FAULT_NOPAGE;
>   
>   	/* Re-check under ptl */
> -	if (likely(!vmf_pte_changed(vmf))) {
> -		struct folio *folio = page_folio(page);
> -		int type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
> -
> -		set_pte_range(vmf, folio, page, 1, vmf->address);
> -		add_mm_counter(vma->vm_mm, type, 1);
> -		ret = 0;
> -	} else {
> -		update_mmu_tlb(vma, vmf->address, vmf->pte);
> +	if (nr_pages == 1 && unlikely(vmf_pte_changed(vmf))) {
> +		update_mmu_tlb(vma, addr, vmf->pte);
> +		ret = VM_FAULT_NOPAGE;
> +		goto unlock;
> +	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> +		update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
>   		ret = VM_FAULT_NOPAGE;
> +		goto unlock;
>   	}

We may add a vmf_pte_range_changed(), but separate it.

Some very small nits, up to you,

Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>

>   
> +	folio_ref_add(folio, nr_pages - 1);
> +	set_pte_range(vmf, folio, page, nr_pages, addr);
> +	type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
> +	add_mm_counter(vma->vm_mm, type, nr_pages);
> +	ret = 0;
> +
> +unlock:
>   	pte_unmap_unlock(vmf->pte, vmf->ptl);
>   	return ret;
>   }