[PATCH 3.19.y-ckt 105/196] ext4: don't read blocks from disk after extents being swapped

From: Kamal Mostafa
Date: Tue Mar 08 2016 - 19:59:53 EST

3.19.8-ckt16 -stable review patch. If anyone has any objections, please let me know.


From: Eryu Guan <guaneryu@xxxxxxxxx>

commit bcff24887d00bce102e0857d7b0a8c44a40f53d1 upstream.

I notice ext4/307 fails occasionally on ppc64 host, reporting md5
checksum mismatch after moving data from original file to donor file.

The reason is that move_extent_per_page() calls __block_write_begin()
and block_commit_write() to write saved data from original inode blocks
to donor inode blocks, but __block_write_begin() not only maps buffer
heads but also reads block content from disk if the size is not block
size aligned. At this time the physical block number in mapped buffer
head is pointing to the donor file not the original file, and that
results in reading wrong data to page, which get written to disk in
following block_commit_write call.

This also can be reproduced by the following script on 1k block size ext4
on x86_64 host:


rm -f $donorfile $testfile

# reserve space for donor file, written by 0xaa and sync to disk to
xfs_io -fc "pwrite -S 0xaa 0 1m" -c "fsync" $donorfile

# create test file written by 0xbb
xfs_io -fc "pwrite -S 0xbb 0 1023" -c "fsync" $testfile

# compute initial md5sum
md5sum $testfile | tee md5sum.txt
# drop cache, force e4compact to read data from disk
echo 3 > /proc/sys/vm/drop_caches

# test defrag
echo "$testfile" | $e4compact -i -v -f $donorfile
# check md5sum
md5sum -c md5sum.txt

Fix it by creating & mapping buffer heads only but not reading blocks
from disk, because all the data in page is guaranteed to be up-to-date
in mext_page_mkuptodate().

Signed-off-by: Eryu Guan <guaneryu@xxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
fs/ext4/move_extent.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
index 370420b..7da8ac1 100644
--- a/fs/ext4/move_extent.c
+++ b/fs/ext4/move_extent.c
@@ -268,11 +268,12 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,
ext4_lblk_t orig_blk_offset, donor_blk_offset;
unsigned long blocksize = orig_inode->i_sb->s_blocksize;
unsigned int tmp_data_size, data_size, replaced_size;
- int err2, jblocks, retries = 0;
+ int i, err2, jblocks, retries = 0;
int replaced_count = 0;
int from = data_offset_in_page << orig_inode->i_blkbits;
int blocks_per_page = PAGE_CACHE_SIZE >> orig_inode->i_blkbits;
struct super_block *sb = orig_inode->i_sb;
+ struct buffer_head *bh = NULL;

* It needs twice the amount of ordinary journal buffers because
@@ -383,8 +384,16 @@ data_copy:
/* Perform all necessary steps similar write_begin()/write_end()
* but keeping in mind that i_size will not change */
- *err = __block_write_begin(pagep[0], from, replaced_size,
- ext4_get_block);
+ if (!page_has_buffers(pagep[0]))
+ create_empty_buffers(pagep[0], 1 << orig_inode->i_blkbits, 0);
+ bh = page_buffers(pagep[0]);
+ for (i = 0; i < data_offset_in_page; i++)
+ bh = bh->b_this_page;
+ for (i = 0; i < block_len_in_page; i++) {
+ *err = ext4_get_block(orig_inode, orig_blk_offset + i, bh, 0);
+ if (*err < 0)
+ break;
+ }
if (!*err)
*err = block_commit_write(pagep[0], from, from + replaced_size);