Re: [PATCH] Add block device speciffic splice write method

[Jens Axboe]

On Mon, Oct 20 2008, Jens Axboe wrote:
> On Sun, Oct 19 2008, Dmitri Monakhov wrote:
> > Block device write procedure is different from regular file:
> >  - Actual write performed without i_mutex.
> >  - It has no metadata, so generic_osync_inode(O_SYNCMETEDATA) can not livelock.
> >  - We do not have to worry about S_ISUID/S_ISGID bits.
> 
> I already did an O_DIRECT part of block device splicing [1], I'll fold
> this into the splice branch and double check with some testing.
> 
> [1] http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=fbb724a0484aba938024d41ca1dd86337d2550c9;hp=08c7910b275a4c580ad646ae8654439c8dfae4c5

The below is what I merged. Note that I changed the naming and made the
function look a lot more like the other splice helpers, so it's more
apparent how it differs. Let me know if I can add you Signed-off-by to
this one (preferably after you test it as well :-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 4d154dc..083198a 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1288,7 +1288,7 @@ new_bio:
  * Splice to file opened with O_DIRECT. Bypass caching completely and
  * just go direct-to-bio
  */
-static ssize_t __block_splice_write(struct pipe_inode_info *pipe,
+static ssize_t __block_splice_direct_write(struct pipe_inode_info *pipe,
 				    struct file *out, loff_t *ppos, size_t len,
 				    unsigned int flags)
 {
@@ -1318,6 +1318,9 @@ static ssize_t __block_splice_write(struct pipe_inode_info *pipe,
 	if (bsd.bio)
 		submit_bio(WRITE, bsd.bio);
 
+	if (ret > 0)
+		*ppos += ret;
+
 	return ret;
 }
 
@@ -1327,12 +1330,11 @@ static ssize_t block_splice_write(struct pipe_inode_info *pipe,
 {
 	ssize_t ret;
 
-	if (out->f_flags & O_DIRECT) {
-		ret = __block_splice_write(pipe, out, ppos, len, flags);
-		if (ret > 0)
-			*ppos += ret;
-	} else
-		ret = generic_file_splice_write(pipe, out, ppos, len, flags);
+	if (out->f_flags & O_DIRECT)
+		ret = __block_splice_direct_write(pipe, out, ppos, len, flags);
+	else
+		ret = generic_file_splice_write_file_nolock(pipe, out, ppos,
+								len, flags);
 
 	return ret;
 }
diff --git a/fs/splice.c b/fs/splice.c
index 4108264..eb1e1ac 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -788,6 +788,59 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
 }
 
 /**
+ * generic_file_splice_write_file_nolock - splice data from a pipe to a file
+ * @pipe:	pipe info
+ * @out:	file to write to
+ * @ppos:	position in @out
+ * @len:	number of bytes to splice
+ * @flags:	splice modifier flags
+ *
+ * Description:
+ *    Will either move or copy pages (determined by @flags options) from
+ *    the given pipe inode to the given block device.
+ *    Note: this is like @generic_file_splice_write, except that we
+ *    don't bother locking the output file. Useful for splicing directly
+ *    to a block device.
+ */
+ssize_t generic_file_splice_write_file_nolock(struct pipe_inode_info *pipe,
+					      struct file *out, loff_t *ppos,
+					      size_t len, unsigned int flags)
+{
+	struct address_space *mapping = out->f_mapping;
+	struct inode *inode = mapping->host;
+	struct splice_desc sd = {
+		.total_len = len,
+		.flags = flags,
+		.pos = *ppos,
+		.u.file = out,
+	};
+	ssize_t ret;
+
+	mutex_lock(&pipe->inode->i_mutex);
+	ret = __splice_from_pipe(pipe, &sd, pipe_to_file);
+	mutex_unlock(&pipe->inode->i_mutex);
+
+	if (ret > 0) {
+		unsigned long nr_pages;
+
+		*ppos += ret;
+		nr_pages = (ret + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
+
+		if (unlikely((out->f_flags & O_SYNC) || IS_SYNC(inode))) {
+			int er;
+
+			er = sync_page_range_nolock(inode, mapping, *ppos, ret);
+			if (er)
+				ret = er;
+		}
+		balance_dirty_pages_ratelimited_nr(mapping, nr_pages);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(generic_file_splice_write_file_nolock);
+
+/**
  * generic_file_splice_write_nolock - generic_file_splice_write without mutexes
  * @pipe:	pipe info
  * @out:	file to write to
diff --git a/include/linux/fs.h b/include/linux/fs.h
index a6a625b..5c9b880 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1957,6 +1957,8 @@ extern ssize_t generic_file_splice_write(struct pipe_inode_info *,
 		struct file *, loff_t *, size_t, unsigned int);
 extern ssize_t generic_file_splice_write_nolock(struct pipe_inode_info *,
 		struct file *, loff_t *, size_t, unsigned int);
+extern ssize_t generic_file_splice_write_file_nolock(struct pipe_inode_info *,
+		struct file *, loff_t *, size_t, unsigned int);
 extern ssize_t generic_splice_sendpage(struct pipe_inode_info *pipe,
 		struct file *out, loff_t *, size_t len, unsigned int flags);
 extern long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,

-- 
Jens Axboe

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