linux-kernel - Re: [PATCH 2/2] vmsplice: Add limited zero copy to vmsplice

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Date:	Thu, 17 Oct 2013 13:23:10 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Robert C Jennings <rcj@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Cc:	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave@...1.net>,
	Matt Helsley <matt.helsley@...il.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Michael Roth <mdroth@...ux.vnet.ibm.com>,
	Lei Li <lilei@...ux.vnet.ibm.com>,
	Leonardo Garcia <lagarcia@...ux.vnet.ibm.com>
Subject: Re: [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@...il.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@...il.com>
> Signed-off-by: Robert C Jennings <rcj@...ux.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
> 

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