[PATCH 4.4 148/163] ext4: fix races between buffered IO and collapse / insert range

Greg Kroah-Hartman
2016-05-03T00:33:07Z

From: Jan Kara <jack@suse.com>

commit 32ebffd3bbb4162da5ff88f9a35dd32d0a28ea70 upstream.

Current code implementing FALLOC_FL_COLLAPSE_RANGE and
FALLOC_FL_INSERT_RANGE is prone to races with buffered writes and page
faults. If buffered write or write via mmap manages to squeeze between
filemap_write_and_wait_range() and truncate_pagecache() in the fallocate
implementations, the written data is simply discarded by
truncate_pagecache() although it should have been shifted.

Fix the problem by moving filemap_write_and_wait_range() call inside
i_mutex and i_mmap_sem. That way we are protected against races with
both buffered writes and page faults.

Signed-off-by: Jan Kara <jack@suse.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/ext4/extents.c | 59 ++++++++++++++++++++++++++++--------------------------
1 file changed, 31 insertions(+), 28 deletions(-)

--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -5487,21 +5487,7 @@ int ext4_collapse_range(struct inode *in
return ret;
}

- /*
- * Need to round down offset to be aligned with page size boundary
- * for page size > block size.
- */
- ioffset = round_down(offset, PAGE_SIZE);
-
- /* Write out all dirty pages */
- ret = filemap_write_and_wait_range(inode->i_mapping, ioffset,
- LLONG_MAX);
- if (ret)
- return ret;
-
- /* Take mutex lock */
mutex_lock(&inode->i_mutex);
-
/*
* There is no need to overlap collapse range with EOF, in which case
* it is effectively a truncate operation
@@ -5526,6 +5512,27 @@ int ext4_collapse_range(struct inode *in
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
+ /*
+ * Need to round down offset to be aligned with page size boundary
+ * for page size > block size.
+ */
+ ioffset = round_down(offset, PAGE_SIZE);
+ /*
+ * Write tail of the last page before removed range since it will get
+ * removed from the page cache below.
+ */
+ ret = filemap_write_and_wait_range(inode->i_mapping, ioffset, offset);
+ if (ret)
+ goto out_mmap;
+ /*
+ * Write data that will be shifted to preserve them when discarding
+ * page cache below. We are also protected from pages becoming dirty
+ * by i_mmap_sem.
+ */
+ ret = filemap_write_and_wait_range(inode->i_mapping, offset + len,
+ LLONG_MAX);
+ if (ret)
+ goto out_mmap;
truncate_pagecache(inode, ioffset);

credits = ext4_writepage_trans_blocks(inode);
@@ -5626,21 +5633,7 @@ int ext4_insert_range(struct inode *inod
return ret;
}

- /*
- * Need to round down to align start offset to page size boundary
- * for page size > block size.
- */
- ioffset = round_down(offset, PAGE_SIZE);
-
- /* Write out all dirty pages */
- ret = filemap_write_and_wait_range(inode->i_mapping, ioffset,
- LLONG_MAX);
- if (ret)
- return ret;
-
- /* Take mutex lock */
mutex_lock(&inode->i_mutex);
-
/* Currently just for extent based files */
if (!ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
ret = -EOPNOTSUPP;
@@ -5668,6 +5661,16 @@ int ext4_insert_range(struct inode *inod
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
+ /*
+ * Need to round down to align start offset to page size boundary
+ * for page size > block size.
+ */
+ ioffset = round_down(offset, PAGE_SIZE);
+ /* Write out all dirty pages */
+ ret = filemap_write_and_wait_range(inode->i_mapping, ioffset,
+ LLONG_MAX);
+ if (ret)
+ goto out_mmap;
truncate_pagecache(inode, ioffset);

credits = ext4_writepage_trans_blocks(inode);