lkml.org 
[lkml]   [2013]   [Oct]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/2] vmsplice: Add limited zero copy to vmsplice
On 10/07/2013 10:21 PM, Robert C Jennings wrote:
> From: Matt Helsley <matt.helsley@gmail.com>
>
> It is sometimes useful to move anonymous pages over a pipe rather than
> save/swap them. Check the SPLICE_F_GIFT and SPLICE_F_MOVE flags to see
> if userspace would like to move such pages. This differs from plain
> SPLICE_F_GIFT in that the memory written to the pipe will no longer
> have the same contents as the original -- it effectively faults in new,
> empty anonymous pages.
>
> On the read side the page written to the pipe will be copied unless
> SPLICE_F_MOVE is used. Otherwise copying will be performed and the page
> will be reclaimed. Note that so long as there is a mapping to the page
> copies will be done instead because rmap will have upped the map count for
> each anonymous mapping; this can happen do to fork(), for example. This
> is necessary because moving the page will usually change the anonymous
> page's nonlinear index and that can only be done if it's unmapped.

You might want to update comments of vmsplice_to_user() and
SYSCALL_DEFINE4(vmsplice) as they both explain how it's done only via
copying.

Vlastimil

> Signed-off-by: Matt Helsley <matt.helsley@gmail.com>
> Signed-off-by: Robert C Jennings <rcj@linux.vnet.ibm.com>
> ---
> fs/splice.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 63 insertions(+)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index a62d61e..9d2ed128 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -32,6 +32,10 @@
> #include <linux/gfp.h>
> #include <linux/socket.h>
> #include <linux/compat.h>
> +#include <linux/page-flags.h>
> +#include <linux/hugetlb.h>
> +#include <linux/ksm.h>
> +#include <linux/swapops.h>
> #include "internal.h"
>
> /*
> @@ -1562,6 +1566,65 @@ static int pipe_to_user(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
> char *src;
> int ret;
>
> + if (!buf->offset && (buf->len == PAGE_SIZE) &&
> + (buf->flags & PIPE_BUF_FLAG_GIFT) && (sd->flags & SPLICE_F_MOVE)) {
> + struct page *page = buf->page;
> + struct mm_struct *mm;
> + struct vm_area_struct *vma;
> + spinlock_t *ptl;
> + pte_t *ptep, pte;
> + unsigned long useraddr;
> +
> + if (!PageAnon(page))
> + goto copy;
> + if (PageCompound(page))
> + goto copy;
> + if (PageHuge(page) || PageTransHuge(page))
> + goto copy;
> + if (page_mapped(page))
> + goto copy;
> + useraddr = (unsigned long)sd->u.userptr;
> + mm = current->mm;
> +
> + ret = -EAGAIN;
> + down_read(&mm->mmap_sem);
> + vma = find_vma_intersection(mm, useraddr, useraddr + PAGE_SIZE);
> + if (IS_ERR_OR_NULL(vma))
> + goto up_copy;
> + if (!vma->anon_vma) {
> + ret = anon_vma_prepare(vma);
> + if (ret)
> + goto up_copy;
> + }
> + zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
> + ret = lock_page_killable(page);
> + if (ret)
> + goto up_copy;
> + ptep = get_locked_pte(mm, useraddr, &ptl);
> + if (!ptep)
> + goto unlock_up_copy;
> + pte = *ptep;
> + if (pte_present(pte))
> + goto unlock_up_copy;
> + get_page(page);
> + page_add_anon_rmap(page, vma, useraddr);
> + pte = mk_pte(page, vma->vm_page_prot);
> + set_pte_at(mm, useraddr, ptep, pte);
> + update_mmu_cache(vma, useraddr, ptep);
> + pte_unmap_unlock(ptep, ptl);
> + ret = 0;
> +unlock_up_copy:
> + unlock_page(page);
> +up_copy:
> + up_read(&mm->mmap_sem);
> + if (!ret) {
> + ret = sd->len;
> + goto out;
> + }
> + /* else ret < 0 and we should fallback to copying */
> + VM_BUG_ON(ret > 0);
> + }
> +copy:
> /*
> * See if we can use the atomic maps, by prefaulting in the
> * pages and doing an atomic copy
>



\
 
 \ /
  Last update: 2013-10-17 13:41    [W:0.136 / U:2.144 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site