Re: [PATCH 4/9] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem

[Peter Xu <peterx@redhat.com>]

On Thu, Apr 08, 2021 at 04:43:22PM -0700, Axel Rasmussen wrote:
> +/*
> + * Install PTEs, to map dst_addr (within dst_vma) to page.
> + *
> + * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
> + * whether or not dst_vma is VM_SHARED. It also handles the more general
> + * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
> + * backed, or not).
> + *
> + * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
> + * shmem_mcopy_atomic_pte instead.
> + */
> +static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> +				     struct vm_area_struct *dst_vma,
> +				     unsigned long dst_addr, struct page *page,
> +				     bool newly_allocated, bool wp_copy)
> +{
> +	int ret;
> +	pte_t _dst_pte, *dst_pte;
> +	int writable;
> +	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> +	spinlock_t *ptl;
> +	struct inode *inode;
> +	pgoff_t offset, max_off;
> +
> +	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
> +	writable = dst_vma->vm_flags & VM_WRITE;
> +	/* For private, non-anon we need CoW (don't write to page cache!) */
> +	if (!vma_is_anonymous(dst_vma) && !vm_shared)
> +		writable = 0;
> +
> +	if (writable || vma_is_anonymous(dst_vma))
> +		_dst_pte = pte_mkdirty(_dst_pte);
> +	if (writable) {
> +		if (wp_copy)
> +			_dst_pte = pte_mkuffd_wp(_dst_pte);
> +		else
> +			_dst_pte = pte_mkwrite(_dst_pte);
> +	} else if (vm_shared) {
> +		/*
> +		 * Since we didn't pte_mkdirty(), mark the page dirty or it
> +		 * could be freed from under us. We could do this
> +		 * unconditionally, but doing it only if !writable is faster.
> +		 */
> +		set_page_dirty(page);
> +	}
> +
> +	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> +
> +	if (vma_is_shmem(dst_vma)) {
> +		/* The shmem MAP_PRIVATE case requires checking the i_size */

When you start to use this function in the last patch it'll be needed too even
if MAP_SHARED?

How about directly state the reason of doing this ("serialize against truncate
with the PT lock") instead of commenting about "who will need it"?

> +		inode = dst_vma->vm_file->f_inode;
> +		offset = linear_page_index(dst_vma, dst_addr);
> +		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> +		ret = -EFAULT;
> +		if (unlikely(offset >= max_off))
> +			goto out_unlock;
> +	}

[...]

> +/* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */
> +static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
> +				pmd_t *dst_pmd,
> +				struct vm_area_struct *dst_vma,
> +				unsigned long dst_addr,
> +				bool wp_copy)
> +{
> +	struct inode *inode = file_inode(dst_vma->vm_file);
> +	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
> +	struct page *page;
> +	int ret;
> +
> +	ret = shmem_getpage(inode, pgoff, &page, SGP_READ);

SGP_READ looks right, as we don't want page allocation.  However I noticed
there's very slight difference when the page was just fallocated:

	/* fallocated page? */
	if (page && !PageUptodate(page)) {
		if (sgp != SGP_READ)
			goto clear;
		unlock_page(page);
		put_page(page);
		page = NULL;
		hindex = index;
	}

I think it won't happen for your case since the page should be uptodate already
(the other thread should check and modify the page before CONTINUE), but still
raise this up, since if the page was allocated it smells better to still
install the fallocated page (do we need to clear the page and SetUptodate)?

-- 
Peter Xu