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SubjectRe: [RFC PATCH 1/2] vmsplice unmap gifted pages for recipient
* Dave Hansen (dave@sr71.net) wrote:
> On 07/25/2013 10:21 AM, Robert Jennings wrote:
> > +static void zap_buf_page(unsigned long useraddr)
> > +{
> > + struct vm_area_struct *vma;
> > +
> > + down_read(&current->mm->mmap_sem);
> > + vma = find_vma_intersection(current->mm, useraddr,
> > + useraddr + PAGE_SIZE);
> > + if (!IS_ERR_OR_NULL(vma))
> > + zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
> > + up_read(&current->mm->mmap_sem);
> > +}
> > +
> > /**
> > * splice_to_pipe - fill passed data into a pipe
> > * @pipe: pipe to fill
> > @@ -212,8 +224,16 @@ 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 */
> > + zap_buf_page(useraddr);
> > buf->flags |= PIPE_BUF_FLAG_GIFT;
> > + }
>
> There isn't quite enough context here, but is it going to do this
> zap_buf_page() very often? Seems a bit wasteful to do the up/down and
> find_vma() every trip through the loop.

The call to zap_buf_page() is in a loop where each pipe buffer is being
processed, but in that loop we have a pipe_wait() where we schedule().
So as things are structured I don't have the ability to hold mmap_sem
for multiple find_vma() calls.



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