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SubjectRe: [PATCH 1/2] vmsplice: unmap gifted pages for recipient
On 10/07/2013 10:21 PM, Robert C Jennings wrote:
> Introduce use of the unused SPLICE_F_MOVE flag for vmsplice to zap
> pages.
>
> When vmsplice is called with flags (SPLICE_F_GIFT | SPLICE_F_MOVE) the
> writer's gift'ed pages would be zapped. This patch supports further work
> to move vmsplice'd pages rather than copying them. That patch has the
> restriction that the page must not be mapped by the source for the move,
> otherwise it will fall back to copying the page.
>
> Signed-off-by: Matt Helsley <matt.helsley@gmail.com>
> Signed-off-by: Robert C Jennings <rcj@linux.vnet.ibm.com>
> ---
> Since the RFC went out I have coalesced the zap_page_range() call to
> operate on VMAs rather than calling this for each page. For a 256MB
> vmsplice this reduced the write side 50% from the RFC.
> ---
> fs/splice.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++-
> include/linux/splice.h | 1 +
> 2 files changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index 3b7ee65..a62d61e 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -188,12 +188,17 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
> {
> unsigned int spd_pages = spd->nr_pages;
> int ret, do_wakeup, page_nr;
> + struct vm_area_struct *vma;
> + unsigned long user_start, user_end;
>
> ret = 0;
> do_wakeup = 0;
> page_nr = 0;
> + vma = NULL;
> + user_start = user_end = 0;
>
> pipe_lock(pipe);
> + down_read(&current->mm->mmap_sem);

Seems like you could take the mmap_sem only when GIFT and MOVE is set.
Maybe it won't help that much for performance but at least serve as
documenting the reason it's needed?

Vlastimil

> for (;;) {
> if (!pipe->readers) {
> @@ -212,8 +217,44 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
> buf->len = spd->partial[page_nr].len;
> buf->private = spd->partial[page_nr].private;
> buf->ops = spd->ops;
> - if (spd->flags & SPLICE_F_GIFT)
> + if (spd->flags & SPLICE_F_GIFT) {
> + unsigned long useraddr =
> + spd->partial[page_nr].useraddr;
> +
> + if ((spd->flags & SPLICE_F_MOVE) &&
> + !buf->offset &&
> + (buf->len == PAGE_SIZE)) {
> + /* Can move page aligned buf, gather
> + * requests to make a single
> + * zap_page_range() call per VMA
> + */
> + if (vma && (useraddr == user_end) &&
> + ((useraddr + PAGE_SIZE) <=
> + vma->vm_end)) {
> + /* same vma, no holes */
> + user_end += PAGE_SIZE;
> + } else {
> + if (vma)
> + zap_page_range(vma,
> + user_start,
> + (user_end -
> + user_start),
> + NULL);
> + vma = find_vma_intersection(
> + current->mm,
> + useraddr,
> + (useraddr +
> + PAGE_SIZE));
> + if (!IS_ERR_OR_NULL(vma)) {
> + user_start = useraddr;
> + user_end = (useraddr +
> + PAGE_SIZE);
> + } else
> + vma = NULL;
> + }
> + }
> buf->flags |= PIPE_BUF_FLAG_GIFT;
> + }
>
> pipe->nrbufs++;
> page_nr++;
> @@ -255,6 +296,10 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
> pipe->waiting_writers--;
> }
>
> + if (vma)
> + zap_page_range(vma, user_start, (user_end - user_start), NULL);
> +
> + up_read(&current->mm->mmap_sem);
> pipe_unlock(pipe);
>
> if (do_wakeup)
> @@ -485,6 +530,7 @@ fill_it:
>
> spd.partial[page_nr].offset = loff;
> spd.partial[page_nr].len = this_len;
> + spd.partial[page_nr].useraddr = index << PAGE_CACHE_SHIFT;
> len -= this_len;
> loff = 0;
> spd.nr_pages++;
> @@ -656,6 +702,7 @@ ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
> this_len = min_t(size_t, vec[i].iov_len, res);
> spd.partial[i].offset = 0;
> spd.partial[i].len = this_len;
> + spd.partial[i].useraddr = (unsigned long)vec[i].iov_base;
> if (!this_len) {
> __free_page(spd.pages[i]);
> spd.pages[i] = NULL;
> @@ -1475,6 +1522,8 @@ static int get_iovec_page_array(const struct iovec __user *iov,
>
> partial[buffers].offset = off;
> partial[buffers].len = plen;
> + partial[buffers].useraddr = (unsigned long)base;
> + base = (void*)((unsigned long)base + PAGE_SIZE);
>
> off = 0;
> len -= plen;
> diff --git a/include/linux/splice.h b/include/linux/splice.h
> index 74575cb..56661e3 100644
> --- a/include/linux/splice.h
> +++ b/include/linux/splice.h
> @@ -44,6 +44,7 @@ struct partial_page {
> unsigned int offset;
> unsigned int len;
> unsigned long private;
> + unsigned long useraddr;
> };
>
> /*
>



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