Patchwork [v3] ext4: Prevent race while waling extent tree

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Submitter Lukas Czerner
Date Nov. 15, 2012, 4:39 p.m.
Message ID <alpine.LFD.2.00.1211151714520.27888@dhcp-1-104.brq.redhat.com>
Download mbox | patch
Permalink /patch/199343/
State Superseded
Headers show

Comments

Lukas Czerner - Nov. 15, 2012, 4:39 p.m.
On Tue, 13 Nov 2012, Zach Brown wrote:

> Date: Tue, 13 Nov 2012 10:51:04 -0800
> From: Zach Brown <zab@redhat.com>
> To: Peng Tao <bergwolf@gmail.com>
> Cc: Lukáš Czerner <lczerner@redhat.com>, linux-ext4@vger.kernel.org,
>     tytso@mit.edu, dmonakhov@openvz.org
> Subject: Re: [PATCH v3] ext4: Prevent race while waling extent tree
> 
> > The deadlock mentioned by Zach Brown can be fixed by simply switching
> > to GFP_NOFS.
> 
> That's a start, but it doesn't address the copy_to_user().  You could
> pin that memory, I suppose, but that starts to feel like more trouble
> than its worth.
> 
> - z

You're both right. The code have to be reorganized. The
ext4_ext_fiemap_cb() is ugly as hell, but luckily that will be
resolved with extent status tree patches from Zheng Liu, however the
indirection in ext4_ext_walk_space() and the callback business
is also pointless.

I have prepared a path to fix this and I am going to test this
right not. Basically it will:

1. remove the callback

2. rename functions
	ext4_ext_walk_space -> ext4_fill_fiemap_extents
	ext4_ext_fiemap_cb -> ext4_find_delayed_extent

3. put fiemap_fill_next_extent() into ext4_fill_fiemap_extents)_

4. Call ext4_find_delayed_extent() only for non existing extents

5. Use GFP_NOFS in ext4_find_delayed_extent()

5. hold the i_data_sem for:
	ext4_ext_find_extent()
	ext4_ext_next_allocated_block()
	ext4_find_delayed_extent()

6. call fiemap_fill_next_extent after releasing the i_data_sem

does it sounds ok?

There are some collisions with extent status tree patches, but it
could be easily resolved depending on which goes in first.

I am going to test the patch now, so this is entirely untested and
possibly still contains bugs, but if you're interested to see how it
looks like, here it is:



---
I should probably split that into two parts to make the changes
clearer, because removing the if (newex->ec_start == 0) in the old
ext4_ext_fiemap_cb() obfuscated it a lot.
Zach Brown - Nov. 15, 2012, 7:10 p.m.
> I have prepared a path to fix this and I am going to test this
> right not. Basically it will:
> 
> 1. remove the callback
> 
> 2. rename functions
> 	ext4_ext_walk_space -> ext4_fill_fiemap_extents
> 	ext4_ext_fiemap_cb -> ext4_find_delayed_extent
> 
> 3. put fiemap_fill_next_extent() into ext4_fill_fiemap_extents)_
> 
> 4. Call ext4_find_delayed_extent() only for non existing extents
> 
> 5. Use GFP_NOFS in ext4_find_delayed_extent()
> 
> 5. hold the i_data_sem for:
> 	ext4_ext_find_extent()
> 	ext4_ext_next_allocated_block()
> 	ext4_find_delayed_extent()
> 
> 6. call fiemap_fill_next_extent after releasing the i_data_sem
> 
> does it sounds ok?

That sounds.. good?  I'm no ext* locking expert :).

> ---
> I should probably split that into two parts to make the changes
> clearer, because removing the if (newex->ec_start == 0) in the old
> ext4_ext_fiemap_cb() obfuscated it a lot.

Yeah, definitely.  I found myself squinting looking for code changes in
all the code motion.

- z
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Theodore Ts'o - Nov. 19, 2012, 3:24 a.m.
On Thu, Nov 15, 2012 at 05:39:06PM +0100, Lukáš Czerner wrote:
> You're both right. The code have to be reorganized. The
> ext4_ext_fiemap_cb() is ugly as hell, but luckily that will be
> resolved with extent status tree patches from Zheng Liu, however the
> indirection in ext4_ext_walk_space() and the callback business
> is also pointless.

Hi Lukas,

Is your patch going to be dependent on whether the extent status tree
patches are applied?

If so, you might want to develop your patch versus the ext4 git tree's
master branch, which includes the extent status patches currently
queued for the next merge window.

Thanks!!

					- Ted
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Lukas Czerner - Nov. 19, 2012, 11:11 a.m.
On Sun, 18 Nov 2012, Theodore Ts'o wrote:

> Date: Sun, 18 Nov 2012 22:24:30 -0500
> From: Theodore Ts'o <tytso@mit.edu>
> To: Lukáš Czerner <lczerner@redhat.com>
> Cc: Zach Brown <zab@redhat.com>, Peng Tao <bergwolf@gmail.com>,
>     linux-ext4@vger.kernel.org, dmonakhov@openvz.org
> Subject: Re: [PATCH v3] ext4: Prevent race while waling extent tree
> 
> On Thu, Nov 15, 2012 at 05:39:06PM +0100, Lukáš Czerner wrote:
> > You're both right. The code have to be reorganized. The
> > ext4_ext_fiemap_cb() is ugly as hell, but luckily that will be
> > resolved with extent status tree patches from Zheng Liu, however the
> > indirection in ext4_ext_walk_space() and the callback business
> > is also pointless.
> 
> Hi Lukas,
> 
> Is your patch going to be dependent on whether the extent status tree
> patches are applied?
> 
> If so, you might want to develop your patch versus the ext4 git tree's
> master branch, which includes the extent status patches currently
> queued for the next merge window.
> 
> Thanks!!
> 
> 					- Ted

Yes, there is a small conflict. I'll base it on top of the ext4 git
tree.

Thanks!
-Lukas

Patch

diff --git a/fs/ext4/ext4_extents.h b/fs/ext4/ext4_extents.h
index cb1b2c9..1718ff1 100644
--- a/fs/ext4/ext4_extents.h
+++ b/fs/ext4/ext4_extents.h
@@ -144,20 +144,6 @@  struct ext4_ext_path {
  */
 
 /*
- * to be called by ext4_ext_walk_space()
- * negative retcode - error
- * positive retcode - signal for ext4_ext_walk_space(), see below
- * callback must return valid extent (passed or newly created)
- */
-typedef int (*ext_prepare_callback)(struct inode *, ext4_lblk_t,
-					struct ext4_ext_cache *,
-					struct ext4_extent *, void *);
-
-#define EXT_CONTINUE   0
-#define EXT_BREAK      1
-#define EXT_REPEAT     2
-
-/*
  * Maximum number of logical blocks in a file; ext4_extent's ee_block is
  * __le32.
  */
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 7011ac9..5c56194 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -109,6 +109,9 @@  static int ext4_split_extent_at(handle_t *handle,
 			     int split_flag,
 			     int flags);
 
+static int ext4_find_delayed_extent(struct inode *inode,
+				    struct ext4_ext_cache *newex);
+
 static int ext4_ext_truncate_extend_restart(handle_t *handle,
 					    struct inode *inode,
 					    int needed)
@@ -1959,27 +1962,35 @@  cleanup:
 	return err;
 }
 
-static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
-			       ext4_lblk_t num, ext_prepare_callback func,
-			       void *cbdata)
+static int ext4_fill_fiemap_extents(struct inode *inode,
+				    ext4_lblk_t block, ext4_lblk_t num,
+				    struct fiemap_extent_info *fieinfo)
 {
 	struct ext4_ext_path *path = NULL;
-	struct ext4_ext_cache cbex;
+	struct ext4_ext_cache newex;
 	struct ext4_extent *ex;
 	ext4_lblk_t next, start = 0, end = 0;
 	ext4_lblk_t last = block + num;
-	int depth, exists, err = 0;
+	int exists, depth = 0, err = 0;
+	unsigned int flags = 0;
+	unsigned char blksize_bits = inode->i_sb->s_blocksize_bits;
 
-	BUG_ON(func == NULL);
 	BUG_ON(inode == NULL);
 
 	while (block < last && block != EXT_MAX_BLOCKS) {
 		num = last - block;
 		/* find extent for this block */
 		down_read(&EXT4_I(inode)->i_data_sem);
+
+		if (path && ext_depth(inode) != depth) {
+			/* depth was changed. we have to realloc path */
+			kfree(path);
+			path = NULL;
+		}
+
 		path = ext4_ext_find_extent(inode, block, path);
-		up_read(&EXT4_I(inode)->i_data_sem);
 		if (IS_ERR(path)) {
+			up_read(&EXT4_I(inode)->i_data_sem);
 			err = PTR_ERR(path);
 			path = NULL;
 			break;
@@ -1987,12 +1998,14 @@  static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
 
 		depth = ext_depth(inode);
 		if (unlikely(path[depth].p_hdr == NULL)) {
+			up_read(&EXT4_I(inode)->i_data_sem);
 			EXT4_ERROR_INODE(inode, "path[%d].p_hdr == NULL", depth);
 			err = -EIO;
 			break;
 		}
 		ex = path[depth].p_ext;
 		next = ext4_ext_next_allocated_block(path);
+		ext4_ext_drop_refs(path);
 
 		exists = 0;
 		if (!ex) {
@@ -2030,40 +2043,49 @@  static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
 		BUG_ON(end <= start);
 
 		if (!exists) {
-			cbex.ec_block = start;
-			cbex.ec_len = end - start;
-			cbex.ec_start = 0;
+			newex.ec_block = start;
+			newex.ec_len = end - start;
+			newex.ec_start = 0;
+			err = ext4_find_delayed_extent(inode, &newex);
 		} else {
-			cbex.ec_block = le32_to_cpu(ex->ee_block);
-			cbex.ec_len = ext4_ext_get_actual_len(ex);
-			cbex.ec_start = ext4_ext_pblock(ex);
+			newex.ec_block = le32_to_cpu(ex->ee_block);
+			newex.ec_len = ext4_ext_get_actual_len(ex);
+			newex.ec_start = ext4_ext_pblock(ex);
+			if (ext4_ext_is_uninitialized(ex))
+				flags |= FIEMAP_EXTENT_UNWRITTEN;
 		}
+		up_read(&EXT4_I(inode)->i_data_sem);
 
-		if (unlikely(cbex.ec_len == 0)) {
-			EXT4_ERROR_INODE(inode, "cbex.ec_len == 0");
+		if (unlikely(newex.ec_len == 0)) {
+			EXT4_ERROR_INODE(inode, "newex.ec_len == 0");
 			err = -EIO;
 			break;
 		}
-		err = func(inode, next, &cbex, ex, cbdata);
-		ext4_ext_drop_refs(path);
-
 		if (err < 0)
 			break;
-
-		if (err == EXT_REPEAT)
-			continue;
-		else if (err == EXT_BREAK) {
-			err = 0;
-			break;
+		if (err == 1) {
+			exists = 1;
+			flags |= FIEMAP_EXTENT_DELALLOC;
 		}
 
-		if (ext_depth(inode) != depth) {
-			/* depth was changed. we have to realloc path */
-			kfree(path);
-			path = NULL;
+		if (next == EXT_MAX_BLOCKS)
+			flags |= FIEMAP_EXTENT_LAST;
+
+		if (exists) {
+			err = fiemap_fill_next_extent(fieinfo,
+				(__u64)newex.ec_block << blksize_bits,
+				(__u64)newex.ec_start << blksize_bits,
+				(__u64)newex.ec_len << blksize_bits,
+				flags);
+			if (err < 0)
+				break;
+			if (err == 1) {
+				err = 0;
+				break;
+			}
 		}
 
-		block = cbex.ec_block + cbex.ec_len;
+		block = newex.ec_block + newex.ec_len;
 	}
 
 	if (path) {
@@ -4574,204 +4596,179 @@  int ext4_convert_unwritten_extents(struct inode *inode, loff_t offset,
 /*
  * Callback function called for each extent to gather FIEMAP information.
  */
-static int ext4_ext_fiemap_cb(struct inode *inode, ext4_lblk_t next,
-		       struct ext4_ext_cache *newex, struct ext4_extent *ex,
-		       void *data)
+static int ext4_find_delayed_extent(struct inode *inode,
+				    struct ext4_ext_cache *newex)
 {
-	__u64	logical;
-	__u64	physical;
-	__u64	length;
-	__u32	flags = 0;
 	int		ret = 0;
-	struct fiemap_extent_info *fieinfo = data;
-	unsigned char blksize_bits;
+	unsigned int flags = 0;
+	ext4_lblk_t	end = 0;
+	pgoff_t		last_offset;
+	pgoff_t		offset;
+	pgoff_t		index;
+	pgoff_t		start_index = 0;
+	struct page	**pages = NULL;
+	struct buffer_head *bh = NULL;
+	struct buffer_head *head = NULL;
+	unsigned int nr_pages = PAGE_SIZE / sizeof(struct page *);
+	unsigned char blksize_bits = inode->i_sb->s_blocksize_bits;
 
-	blksize_bits = inode->i_sb->s_blocksize_bits;
-	logical = (__u64)newex->ec_block << blksize_bits;
+	/*
+	 * No extent in extent-tree contains block @newex->ec_start,
+	 * then the block may stay in 1)a hole or 2)delayed-extent.
+	 *
+	 * Holes or delayed-extents are processed as follows.
+	 * 1. lookup dirty pages with specified range in pagecache.
+	 *    If no page is got, then there is no delayed-extent and
+	 *    return with EXT_CONTINUE.
+	 * 2. find the 1st mapped buffer,
+	 * 3. check if the mapped buffer is both in the request range
+	 *    and a delayed buffer. If not, there is no delayed-extent,
+	 *    then return.
+	 * 4. a delayed-extent is found, the extent will be collected.
+	 */
 
-	if (newex->ec_start == 0) {
-		/*
-		 * No extent in extent-tree contains block @newex->ec_start,
-		 * then the block may stay in 1)a hole or 2)delayed-extent.
-		 *
-		 * Holes or delayed-extents are processed as follows.
-		 * 1. lookup dirty pages with specified range in pagecache.
-		 *    If no page is got, then there is no delayed-extent and
-		 *    return with EXT_CONTINUE.
-		 * 2. find the 1st mapped buffer,
-		 * 3. check if the mapped buffer is both in the request range
-		 *    and a delayed buffer. If not, there is no delayed-extent,
-		 *    then return.
-		 * 4. a delayed-extent is found, the extent will be collected.
-		 */
-		ext4_lblk_t	end = 0;
-		pgoff_t		last_offset;
-		pgoff_t		offset;
-		pgoff_t		index;
-		pgoff_t		start_index = 0;
-		struct page	**pages = NULL;
-		struct buffer_head *bh = NULL;
-		struct buffer_head *head = NULL;
-		unsigned int nr_pages = PAGE_SIZE / sizeof(struct page *);
-
-		pages = kmalloc(PAGE_SIZE, GFP_KERNEL);
-		if (pages == NULL)
-			return -ENOMEM;
+	pages = kmalloc(PAGE_SIZE, GFP_NOFS);
+	if (pages == NULL)
+		return -ENOMEM;
 
-		offset = logical >> PAGE_SHIFT;
+	offset = ((__u64)newex->ec_block << PAGE_SHIFT) >>
+			blksize_bits;
 repeat:
-		last_offset = offset;
-		head = NULL;
-		ret = find_get_pages_tag(inode->i_mapping, &offset,
-					PAGECACHE_TAG_DIRTY, nr_pages, pages);
-
-		if (!(flags & FIEMAP_EXTENT_DELALLOC)) {
-			/* First time, try to find a mapped buffer. */
-			if (ret == 0) {
+	last_offset = offset;
+	head = NULL;
+	ret = find_get_pages_tag(inode->i_mapping, &offset,
+				PAGECACHE_TAG_DIRTY, nr_pages, pages);
+
+	if (!(flags & FIEMAP_EXTENT_DELALLOC)) {
+		/* First time, try to find a mapped buffer. */
+		if (ret == 0) {
 out:
-				for (index = 0; index < ret; index++)
-					page_cache_release(pages[index]);
-				/* just a hole. */
-				kfree(pages);
-				return EXT_CONTINUE;
-			}
-			index = 0;
+			for (index = 0; index < ret; index++)
+				page_cache_release(pages[index]);
+			/* just a hole. */
+			kfree(pages);
+			return 0;
+		}
+		index = 0;
 
 next_page:
-			/* Try to find the 1st mapped buffer. */
-			end = ((__u64)pages[index]->index << PAGE_SHIFT) >>
-				  blksize_bits;
-			if (!page_has_buffers(pages[index]))
-				goto out;
-			head = page_buffers(pages[index]);
-			if (!head)
-				goto out;
+		/* Try to find the 1st mapped buffer. */
+		end = ((__u64)pages[index]->index << PAGE_SHIFT) >>
+			  blksize_bits;
+		if (!page_has_buffers(pages[index]))
+			goto out;
+		head = page_buffers(pages[index]);
+		if (!head)
+			goto out;
 
-			index++;
-			bh = head;
-			do {
-				if (end >= newex->ec_block +
-					newex->ec_len)
-					/* The buffer is out of
-					 * the request range.
-					 */
-					goto out;
+		index++;
+		bh = head;
+		do {
+			if (end >= newex->ec_block +
+				newex->ec_len)
+				/* The buffer is out of
+				 * the request range.
+				 */
+				goto out;
 
-				if (buffer_mapped(bh) &&
-				    end >= newex->ec_block) {
-					start_index = index - 1;
-					/* get the 1st mapped buffer. */
-					goto found_mapped_buffer;
-				}
+			if (buffer_mapped(bh) &&
+			    end >= newex->ec_block) {
+				start_index = index - 1;
+				/* get the 1st mapped buffer. */
+				goto found_mapped_buffer;
+			}
 
-				bh = bh->b_this_page;
-				end++;
-			} while (bh != head);
+			bh = bh->b_this_page;
+			end++;
+		} while (bh != head);
 
-			/* No mapped buffer in the range found in this page,
-			 * We need to look up next page.
+		/* No mapped buffer in the range found in this page,
+		 * We need to look up next page.
+		 */
+		if (index >= ret) {
+			/* There is no page left, but we need to limit
+			 * newex->ec_len.
 			 */
-			if (index >= ret) {
-				/* There is no page left, but we need to limit
-				 * newex->ec_len.
-				 */
-				newex->ec_len = end - newex->ec_block;
-				goto out;
-			}
-			goto next_page;
-		} else {
-			/*Find contiguous delayed buffers. */
-			if (ret > 0 && pages[0]->index == last_offset)
-				head = page_buffers(pages[0]);
-			bh = head;
-			index = 1;
-			start_index = 0;
+			newex->ec_len = end - newex->ec_block;
+			goto out;
 		}
+		goto next_page;
+	} else {
+		/*Find contiguous delayed buffers. */
+		if (ret > 0 && pages[0]->index == last_offset)
+			head = page_buffers(pages[0]);
+		bh = head;
+		index = 1;
+		start_index = 0;
+	}
 
 found_mapped_buffer:
-		if (bh != NULL && buffer_delay(bh)) {
-			/* 1st or contiguous delayed buffer found. */
-			if (!(flags & FIEMAP_EXTENT_DELALLOC)) {
-				/*
-				 * 1st delayed buffer found, record
-				 * the start of extent.
-				 */
-				flags |= FIEMAP_EXTENT_DELALLOC;
-				newex->ec_block = end;
-				logical = (__u64)end << blksize_bits;
+	if (bh != NULL && buffer_delay(bh)) {
+		/* 1st or contiguous delayed buffer found. */
+		if (!(flags & FIEMAP_EXTENT_DELALLOC)) {
+			/*
+			 * 1st delayed buffer found, record
+			 * the start of extent.
+			 */
+			flags |= FIEMAP_EXTENT_DELALLOC;
+			newex->ec_block = end;
+		}
+		/* Find contiguous delayed buffers. */
+		do {
+			if (!buffer_delay(bh))
+				goto found_delayed_extent;
+			bh = bh->b_this_page;
+			end++;
+		} while (bh != head);
+
+		for (; index < ret; index++) {
+			if (!page_has_buffers(pages[index])) {
+				bh = NULL;
+				break;
+			}
+			head = page_buffers(pages[index]);
+			if (!head) {
+				bh = NULL;
+				break;
+			}
+
+			if (pages[index]->index !=
+			    pages[start_index]->index + index
+			    - start_index) {
+				/* Blocks are not contiguous. */
+				bh = NULL;
+				break;
 			}
-			/* Find contiguous delayed buffers. */
+			bh = head;
 			do {
 				if (!buffer_delay(bh))
+					/* Delayed-extent ends. */
 					goto found_delayed_extent;
 				bh = bh->b_this_page;
 				end++;
 			} while (bh != head);
-
-			for (; index < ret; index++) {
-				if (!page_has_buffers(pages[index])) {
-					bh = NULL;
-					break;
-				}
-				head = page_buffers(pages[index]);
-				if (!head) {
-					bh = NULL;
-					break;
-				}
-
-				if (pages[index]->index !=
-				    pages[start_index]->index + index
-				    - start_index) {
-					/* Blocks are not contiguous. */
-					bh = NULL;
-					break;
-				}
-				bh = head;
-				do {
-					if (!buffer_delay(bh))
-						/* Delayed-extent ends. */
-						goto found_delayed_extent;
-					bh = bh->b_this_page;
-					end++;
-				} while (bh != head);
-			}
-		} else if (!(flags & FIEMAP_EXTENT_DELALLOC))
-			/* a hole found. */
-			goto out;
-
-found_delayed_extent:
-		newex->ec_len = min(end - newex->ec_block,
-						(ext4_lblk_t)EXT_INIT_MAX_LEN);
-		if (ret == nr_pages && bh != NULL &&
-			newex->ec_len < EXT_INIT_MAX_LEN &&
-			buffer_delay(bh)) {
-			/* Have not collected an extent and continue. */
-			for (index = 0; index < ret; index++)
-				page_cache_release(pages[index]);
-			goto repeat;
 		}
+	} else if (!(flags & FIEMAP_EXTENT_DELALLOC))
+		/* a hole found. */
+		goto out;
 
+found_delayed_extent:
+	newex->ec_len = min(end - newex->ec_block,
+					(ext4_lblk_t)EXT_INIT_MAX_LEN);
+	if (ret == nr_pages && bh != NULL &&
+		newex->ec_len < EXT_INIT_MAX_LEN &&
+		buffer_delay(bh)) {
+		/* Have not collected an extent and continue. */
 		for (index = 0; index < ret; index++)
 			page_cache_release(pages[index]);
-		kfree(pages);
+		goto repeat;
 	}
 
-	physical = (__u64)newex->ec_start << blksize_bits;
-	length =   (__u64)newex->ec_len << blksize_bits;
-
-	if (ex && ext4_ext_is_uninitialized(ex))
-		flags |= FIEMAP_EXTENT_UNWRITTEN;
-
-	if (next == EXT_MAX_BLOCKS)
-		flags |= FIEMAP_EXTENT_LAST;
+	for (index = 0; index < ret; index++)
+		page_cache_release(pages[index]);
+	kfree(pages);
 
-	ret = fiemap_fill_next_extent(fieinfo, logical, physical,
-					length, flags);
-	if (ret < 0)
-		return ret;
-	if (ret == 1)
-		return EXT_BREAK;
-	return EXT_CONTINUE;
+	return 1;
 }
 /* fiemap flags we can handle specified here */
 #define EXT4_FIEMAP_FLAGS	(FIEMAP_FLAG_SYNC|FIEMAP_FLAG_XATTR)
@@ -4991,6 +4988,7 @@  out_mutex:
 	mutex_unlock(&inode->i_mutex);
 	return err;
 }
+
 int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		__u64 start, __u64 len)
 {
@@ -5021,8 +5019,8 @@  int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		 * Walk the extent tree gathering extent information.
 		 * ext4_ext_fiemap_cb will push extents back to user.
 		 */
-		error = ext4_ext_walk_space(inode, start_blk, len_blks,
-					  ext4_ext_fiemap_cb, fieinfo);
+		error = ext4_fill_fiemap_extents(inode, start_blk,
+						 len_blks, fieinfo);
 	}
 
 	return error;