Re: patch: improve generic_file_buffered_write() (2nd try 1/2)

[Bernd Schubert <bs@q-leap.de>]

Hello Randy,

thanks for your review.

On Wednesday 05 September 2007 17:35:29 Randy Dunlap wrote:
> On Wed, 5 Sep 2007 15:45:36 +0200 Bernd Schubert wrote:
> > Hi,
>
> meta-comments:
> > filemap.c |  144
> > +++++++++++++++++++++++++++++++++++++++++---------------------
> >  1 file changed, 96 insertions(+), 48 deletions(-)
>
> Use "diffstat -p 1 -w 70" per Documentation/SubmittingPatches.

Thanks, never would have thought this is documented.

[...]

> Use proper kernel-doc notation, per
> Documentation/kernel-doc-nano-HOWTO.txt.

Ouch, I really should have read these files before.
<joking> 
Now I know why there are so few functions commented. Nobody wants to read the 
documentation.
</joking>

>
> This comment block should be:
>
> /**
>  * generic_file_buffered_write - handle an iov
>  * @iocb:	file operations
>  * @iov:	vector of data to write
>  * @nr_segs:	number of iov segments
>  * @pos:	position in the file
>  * @ppos:	position in the file after this function
>  * @count:	number of bytes to write
>  * @written:	offset in iov->base (data to skip on write)
>  *
>  * This function will do 3 main tasks for each iov:
>  * - prepare a write
>  * - copy the data from iov into a new page
>  * - commit this page

Thanks, done.

I also removed the FIXMEs and created a second patch.

Signed-off-by: Bernd Schubert <bs@q-leap.de>
Signed-off-by: Goswin von Brederlow <goswin@vonbrederlow.de>

 mm/filemap.c |  142 +++++++++++++++++++++++++++++++++----------------
 1 file changed, 96 insertions(+), 46 deletions(-)

Index: linux-2.6.20.3/mm/filemap.c
===================================================================
--- linux-2.6.20.3.orig/mm/filemap.c	2007-09-05 14:04:18.000000000 +0200
+++ linux-2.6.20.3/mm/filemap.c	2007-09-05 18:50:26.000000000 +0200
@@ -2057,6 +2057,21 @@
 }
 EXPORT_SYMBOL(generic_file_direct_write);
 
+/**
+ * generic_file_buffered_write - handle iov'ectors
+ * @iob:	file operations
+ * @iov:	vector of data to write
+ * @nr_segs:	number of iov segments
+ * @pos:	position in the file
+ * @ppos:	position in the file after this function
+ * @count:	number of bytes to write
+ * written:	offset in iov->base (data to skip on write)
+ *
+ * This function will do 3 main tasks for each iov:
+ * - prepare a write
+ * - copy the data from iov into a new page
+ * - commit this page
+ */
 ssize_t
 generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 		unsigned long nr_segs, loff_t pos, loff_t *ppos,
@@ -2074,6 +2089,11 @@
 	const struct iovec *cur_iov = iov; /* current iovec */
 	size_t		iov_base = 0;	   /* offset in the current iovec */
 	char __user	*buf;
+	unsigned long	data_start = (pos & (PAGE_CACHE_SIZE -1)); /* Within page */
+	loff_t		wpos = pos; /* the position in the file we will return */
+
+	/* position in file as index of pages */
+	unsigned long	index = pos >> PAGE_CACHE_SHIFT;
 
 	pagevec_init(&lru_pvec, 0);
 
@@ -2087,9 +2107,15 @@
 		buf = cur_iov->iov_base + iov_base;
 	}
 
+	page = __grab_cache_page(mapping, index, &cached_page, &lru_pvec);
+	if (!page) {
+		status = -ENOMEM;
+		goto out;
+	}
+
 	do {
-		unsigned long index;
 		unsigned long offset;
+		unsigned long data_end; /* end of data within the page */
 		size_t copied;
 
 		offset = (pos & (PAGE_CACHE_SIZE -1)); /* Within page */
@@ -2106,6 +2132,8 @@
 		 */
 		bytes = min(bytes, cur_iov->iov_len - iov_base);
 
+		data_end = offset + bytes;
+
 		/*
 		 * Bring in the user page that we will copy from _first_.
 		 * Otherwise there's a nasty deadlock on copying from the
@@ -2114,34 +2142,30 @@
 		 */
 		fault_in_pages_readable(buf, bytes);
 
-		page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
-		if (!page) {
-			status = -ENOMEM;
-			break;
-		}
-
 		if (unlikely(bytes == 0)) {
 			status = 0;
 			copied = 0;
 			goto zero_length_segment;
 		}
 
-		status = a_ops->prepare_write(file, page, offset, offset+bytes);
-		if (unlikely(status)) {
-			loff_t isize = i_size_read(inode);
-
-			if (status != AOP_TRUNCATED_PAGE)
-				unlock_page(page);
-			page_cache_release(page);
-			if (status == AOP_TRUNCATED_PAGE)
-				continue;
+		if (data_end == PAGE_CACHE_SIZE || count == bytes) {
 			/*
-			 * prepare_write() may have instantiated a few blocks
-			 * outside i_size.  Trim these off again.
+			 * Only prepare a write if its an entire page or if
+			 * we don't have more data
 			 */
-			if (pos + bytes > isize)
-				vmtruncate(inode, isize);
-			break;
+			status = a_ops->prepare_write(file, page, data_start, data_end);
+			if (unlikely(status)) {
+				loff_t isize = i_size_read(inode);
+
+				if (status == AOP_TRUNCATED_PAGE)
+					continue;
+				/*
+				* prepare_write() may have instantiated a few blocks
+				* outside i_size.  Trim these off again.
+				*/
+				if (pos + bytes > isize)
+					vmtruncate(inode, isize);
+			}
 		}
 		if (likely(nr_segs == 1))
 			copied = filemap_copy_from_user(page, offset,
@@ -2149,60 +2173,86 @@
 		else
 			copied = filemap_copy_from_user_iovec(page, offset,
 						cur_iov, iov_base, bytes);
-		flush_dcache_page(page);
-		status = a_ops->commit_write(file, page, offset, offset+bytes);
-		if (status == AOP_TRUNCATED_PAGE) {
+
+		if (data_end == PAGE_CACHE_SIZE || count == bytes) {
+			/*
+			 * Same as above, always try fill pages up to
+			 * PAGE_CACHE_SIZE if possible (sufficient data)
+			 */
+			flush_dcache_page(page);
+			status = a_ops->commit_write(file, page,
+						     data_start, data_end);
+			if (status == AOP_TRUNCATED_PAGE) {
+				continue;
+			}
+			unlock_page(page);
+			mark_page_accessed(page);
 			page_cache_release(page);
-			continue;
+			balance_dirty_pages_ratelimited(mapping);
+			cond_resched();
+			if (bytes < count) { /* still more iov data to write */
+				page = __grab_cache_page(mapping, index + 1,
+							 &cached_page, &lru_pvec);
+				if (!page) {
+					status = -ENOMEM;
+					goto out;
+				}
+			} else {
+				page = NULL;
+			}
+			written += data_end - data_start;
+			wpos    += data_end - data_start;
+			data_start = 0; /* correct would be data_end % PAGE_SIZE
+			                 * but if this would be != 0, we don't
+			                 * have more data
+			                 */
 		}
 zero_length_segment:
 		if (likely(copied >= 0)) {
-			if (!status)
-				status = copied;
-
-			if (status >= 0) {
-				written += status;
-				count -= status;
-				pos += status;
-				buf += status;
+			if (!status) {
+				count -= copied;
+				pos += copied;
+				buf += copied;
 				if (unlikely(nr_segs > 1)) {
 					filemap_set_next_iovec(&cur_iov,
-							&iov_base, status);
+							&iov_base, copied);
 					if (count)
 						buf = cur_iov->iov_base +
 							iov_base;
 				} else {
-					iov_base += status;
+					iov_base += copied;
 				}
 			}
 		}
 		if (unlikely(copied != bytes))
-			if (status >= 0)
+			if (!status)
 				status = -EFAULT;
-		unlock_page(page);
-		mark_page_accessed(page);
-		page_cache_release(page);
-		if (status < 0)
+		if (status)
 			break;
-		balance_dirty_pages_ratelimited(mapping);
-		cond_resched();
 	} while (count);
-	*ppos = pos;
+
+out:
+	*ppos = wpos;
 
 	if (cached_page)
 		page_cache_release(cached_page);
 
+	if (page) {
+		unlock_page(page);
+		page_cache_release(page);
+	}
+
 	/*
 	 * For now, when the user asks for O_SYNC, we'll actually give O_DSYNC
 	 */
-	if (likely(status >= 0)) {
+	if (likely(!status)) {
 		if (unlikely((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
 			if (!a_ops->writepage || !is_sync_kiocb(iocb))
 				status = generic_osync_inode(inode, mapping,
 						OSYNC_METADATA|OSYNC_DATA);
 		}
-  	}
-	
+	}
+
 	/*
 	 * If we get here for O_DIRECT writes then we must have fallen through
 	 * to buffered writes (block instantiation inside i_size).  So we sync



-- 
Bernd Schubert
Q-Leap Networks GmbH
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