Re: [PATCH] cleanup block based fiemap

[Josef Bacik <josef@redhat.com>]

On Fri, Apr 23, 2010 at 08:27:02AM -0700, Linus Torvalds wrote:
> 
> 
> On Fri, 23 Apr 2010, Josef Bacik wrote:
> > 
> > __generic_block_fiemap is called by generic_block_fiemap which takes the
> > i_mutex.  The only reason we have __generic_block_fiemap is because gfs2 needs
> > to do its own locking magic before we go calling get_block.  The idea is that
> > the file size doesn't change while we're doing this.
> 
> Ok, maybe just a comment then..
> 
> > As for reading the size several times, I can read it once and store it in a
> > local variable if you prefer, but theres no way to know if len is smaller than
> > the size or not, which is why I'm constantly doing i_size_read().  If thats what
> > you would prefer I can do that, just let me know.
> 
> .. or a comment _and_ a "read size once into a variable". Not a big deal, 
> it just wasn't entirely clear to me and I hadn't checked the callchain.
> 
> > > Issue #2: "blk_to_logical(inode, 0)"? WTF? Since when has shifting zero 
> > > ever resulted in anything interesting or relevant? There's at least two of 
> > > those things.
> > > 
> > 
> > Umm, yeah I'm sorry?  I have no idea why I did that.  I think its because I was
> > getting the logical offset of the first block + size, which is just stupid
> > because the logical offset is 0, so all I can say is I'm sorry that me a year
> > ago was alot dumber than me now :).  Thanks,
> 
> Ok, fix that, and I think the patch will be a clear improvement.
> 

Great, here is the updated patch.  Thanks for the review!

Josef

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 6c75110..7faefb4 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -228,14 +228,23 @@ static int ioctl_fiemap(struct file *filp, unsigned long arg)
 
 #ifdef CONFIG_BLOCK
 
-#define blk_to_logical(inode, blk) (blk << (inode)->i_blkbits)
-#define logical_to_blk(inode, offset) (offset >> (inode)->i_blkbits);
+static inline sector_t logical_to_blk(struct inode *inode, loff_t offset)
+{
+	return (offset >> inode->i_blkbits);
+}
+
+static inline loff_t blk_to_logical(struct inode *inode, sector_t blk)
+{
+	return (blk << inode->i_blkbits);
+}
 
 /**
  * __generic_block_fiemap - FIEMAP for block based inodes (no locking)
- * @inode - the inode to map
- * @arg - the pointer to userspace where we copy everything to
- * @get_block - the fs's get_block function
+ * @inode: the inode to map
+ * @fieinfo: the fiemap info struct that will be passed back to userspace
+ * @start: where to start mapping in the inode
+ * @len: how much space to map
+ * @get_block: the fs's get_block function
  *
  * This does FIEMAP for block based inodes.  Basically it will just loop
  * through get_block until we hit the number of extents we want to map, or we
@@ -250,58 +259,63 @@ static int ioctl_fiemap(struct file *filp, unsigned long arg)
  */
 
 int __generic_block_fiemap(struct inode *inode,
-			   struct fiemap_extent_info *fieinfo, u64 start,
-			   u64 len, get_block_t *get_block)
+			   struct fiemap_extent_info *fieinfo, loff_t start,
+			   loff_t len, get_block_t *get_block)
 {
-	struct buffer_head tmp;
-	unsigned long long start_blk;
-	long long length = 0, map_len = 0;
+	struct buffer_head map_bh;
+	sector_t start_blk, last_blk;
+	loff_t isize = i_size_read(inode);
 	u64 logical = 0, phys = 0, size = 0;
 	u32 flags = FIEMAP_EXTENT_MERGED;
-	int ret = 0, past_eof = 0, whole_file = 0;
+	bool past_eof = false, whole_file = false;
+	int ret = 0;
 
-	if ((ret = fiemap_check_flags(fieinfo, FIEMAP_FLAG_SYNC)))
+	ret = fiemap_check_flags(fieinfo, FIEMAP_FLAG_SYNC);
+	if (ret)
 		return ret;
 
-	start_blk = logical_to_blk(inode, start);
-
-	length = (long long)min_t(u64, len, i_size_read(inode));
-	if (length < len)
-		whole_file = 1;
+	/*
+	 * Either the i_mutex or other appropriate locking needs to be held
+	 * since we expect isize to not change at all through the duration of
+	 * this call.
+	 */
+	if (len >= isize) {
+		whole_file = true;
+		len = isize;
+	}
 
-	map_len = length;
+	start_blk = logical_to_blk(inode, start);
+	last_blk = logical_to_blk(inode, start + len - 1);
 
 	do {
 		/*
 		 * we set b_size to the total size we want so it will map as
 		 * many contiguous blocks as possible at once
 		 */
-		memset(&tmp, 0, sizeof(struct buffer_head));
-		tmp.b_size = map_len;
+		memset(&map_bh, 0, sizeof(struct buffer_head));
+		map_bh.b_size = len;
 
-		ret = get_block(inode, start_blk, &tmp, 0);
+		ret = get_block(inode, start_blk, &map_bh, 0);
 		if (ret)
 			break;
 
 		/* HOLE */
-		if (!buffer_mapped(&tmp)) {
-			length -= blk_to_logical(inode, 1);
+		if (!buffer_mapped(&map_bh)) {
 			start_blk++;
 
 			/*
-			 * we want to handle the case where there is an
+			 * We want to handle the case where there is an
 			 * allocated block at the front of the file, and then
 			 * nothing but holes up to the end of the file properly,
 			 * to make sure that extent at the front gets properly
 			 * marked with FIEMAP_EXTENT_LAST
 			 */
 			if (!past_eof &&
-			    blk_to_logical(inode, start_blk) >=
-			    blk_to_logical(inode, 0)+i_size_read(inode))
+			    blk_to_logical(inode, start_blk) >= isize)
 				past_eof = 1;
 
 			/*
-			 * first hole after going past the EOF, this is our
+			 * First hole after going past the EOF, this is our
 			 * last extent
 			 */
 			if (past_eof && size) {
@@ -309,15 +323,18 @@ int __generic_block_fiemap(struct inode *inode,
 				ret = fiemap_fill_next_extent(fieinfo, logical,
 							      phys, size,
 							      flags);
-				break;
+			} else if (size) {
+				ret = fiemap_fill_next_extent(fieinfo, logical,
+							      phys, size, flags);
+				size = 0;
 			}
 
 			/* if we have holes up to/past EOF then we're done */
-			if (length <= 0 || past_eof)
+			if (start_blk > last_blk || past_eof || ret)
 				break;
 		} else {
 			/*
-			 * we have gone over the length of what we wanted to
+			 * We have gone over the length of what we wanted to
 			 * map, and it wasn't the entire file, so add the extent
 			 * we got last time and exit.
 			 *
@@ -331,7 +348,7 @@ int __generic_block_fiemap(struct inode *inode,
 			 * are good to go, just add the extent to the fieinfo
 			 * and break
 			 */
-			if (length <= 0 && !whole_file) {
+			if (start_blk > last_blk && !whole_file) {
 				ret = fiemap_fill_next_extent(fieinfo, logical,
 							      phys, size,
 							      flags);
@@ -351,11 +368,10 @@ int __generic_block_fiemap(struct inode *inode,
 			}
 
 			logical = blk_to_logical(inode, start_blk);
-			phys = blk_to_logical(inode, tmp.b_blocknr);
-			size = tmp.b_size;
+			phys = blk_to_logical(inode, map_bh.b_blocknr);
+			size = map_bh.b_size;
 			flags = FIEMAP_EXTENT_MERGED;
 
-			length -= tmp.b_size;
 			start_blk += logical_to_blk(inode, size);
 
 			/*
@@ -363,15 +379,13 @@ int __generic_block_fiemap(struct inode *inode,
 			 * soon as we find a hole that the last extent we found
 			 * is marked with FIEMAP_EXTENT_LAST
 			 */
-			if (!past_eof &&
-			    logical+size >=
-			    blk_to_logical(inode, 0)+i_size_read(inode))
-				past_eof = 1;
+			if (!past_eof && logical + size >= isize)
+				past_eof = true;
 		}
 		cond_resched();
 	} while (1);
 
-	/* if ret is 1 then we just hit the end of the extent array */
+	/* If ret is 1 then we just hit the end of the extent array */
 	if (ret == 1)
 		ret = 0;
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 39d57bc..44f35ae 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2315,8 +2315,9 @@ extern int vfs_fstatat(int , char __user *, struct kstat *, int);
 extern int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
 		    unsigned long arg);
 extern int __generic_block_fiemap(struct inode *inode,
-				  struct fiemap_extent_info *fieinfo, u64 start,
-				  u64 len, get_block_t *get_block);
+				  struct fiemap_extent_info *fieinfo,
+				  loff_t start, loff_t len,
+				  get_block_t *get_block);
 extern int generic_block_fiemap(struct inode *inode,
 				struct fiemap_extent_info *fieinfo, u64 start,
 				u64 len, get_block_t *get_block);