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Subject[PATCH 21/93] ext4: fix lost truncate due to race with writeback
Date
3.8.13.10 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@suse.cz>

commit 90e775b71ac4e685898c7995756fe58c135adaa6 upstream.

The following race can lead to a loss of i_disksize update from truncate
thus resulting in a wrong inode size if the inode size isn't updated
again before inode is reclaimed:

ext4_setattr() mpage_map_and_submit_extent()
EXT4_I(inode)->i_disksize = attr->ia_size;
... ...
disksize = ((loff_t)mpd->first_page) << PAGE_CACHE_SHIFT
/* False because i_size isn't
* updated yet */
if (disksize > i_size_read(inode))
/* True, because i_disksize is
* already truncated */
if (disksize > EXT4_I(inode)->i_disksize)
/* Overwrite i_disksize
* update from truncate */
ext4_update_i_disksize()
i_size_write(inode, attr->ia_size);

For other places updating i_disksize such race cannot happen because
i_mutex prevents these races. Writeback is the only place where we do
not hold i_mutex and we cannot grab it there because of lock ordering.

We fix the race by doing both i_disksize and i_size update in truncate
atomically under i_data_sem and in mpage_map_and_submit_extent() we move
the check against i_size under i_data_sem as well.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
[ kamal: backport to 3.8 (context) ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
fs/ext4/ext4.h | 24 ++++++++++++++++++++----
fs/ext4/inode.c | 17 ++++++++++++-----
2 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index bbcd6a0..a4903ff 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2341,16 +2341,32 @@ do { \
#define EXT4_FREECLUSTERS_WATERMARK 0
#endif

+/* Update i_disksize. Requires i_mutex to avoid races with truncate */
static inline void ext4_update_i_disksize(struct inode *inode, loff_t newsize)
{
- /*
- * XXX: replace with spinlock if seen contended -bzzz
- */
+ WARN_ON_ONCE(S_ISREG(inode->i_mode) &&
+ !mutex_is_locked(&inode->i_mutex));
+ down_write(&EXT4_I(inode)->i_data_sem);
+ if (newsize > EXT4_I(inode)->i_disksize)
+ EXT4_I(inode)->i_disksize = newsize;
+ up_write(&EXT4_I(inode)->i_data_sem);
+}
+
+/*
+ * Update i_disksize after writeback has been started. Races with truncate
+ * are avoided by checking i_size under i_data_sem.
+ */
+static inline void ext4_wb_update_i_disksize(struct inode *inode, loff_t newsize)
+{
+ loff_t i_size;
+
down_write(&EXT4_I(inode)->i_data_sem);
+ i_size = i_size_read(inode);
+ if (newsize > i_size)
+ newsize = i_size;
if (newsize > EXT4_I(inode)->i_disksize)
EXT4_I(inode)->i_disksize = newsize;
up_write(&EXT4_I(inode)->i_data_sem);
- return ;
}

struct ext4_group_info {
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 158145b..fe1fada 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1666,10 +1666,8 @@ static void mpage_da_map_and_submit(struct mpage_da_data *mpd)
* Update on-disk size along with block allocation.
*/
disksize = ((loff_t) next + blks) << mpd->inode->i_blkbits;
- if (disksize > i_size_read(mpd->inode))
- disksize = i_size_read(mpd->inode);
if (disksize > EXT4_I(mpd->inode)->i_disksize) {
- ext4_update_i_disksize(mpd->inode, disksize);
+ ext4_wb_update_i_disksize(mpd->inode, disksize);
err = ext4_mark_inode_dirty(handle, mpd->inode);
if (err)
ext4_error(mpd->inode->i_sb,
@@ -4449,18 +4447,27 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
error = ext4_orphan_add(handle, inode);
orphan = 1;
}
+ down_write(&EXT4_I(inode)->i_data_sem);
EXT4_I(inode)->i_disksize = attr->ia_size;
rc = ext4_mark_inode_dirty(handle, inode);
if (!error)
error = rc;
+ /*
+ * We have to update i_size under i_data_sem together
+ * with i_disksize to avoid races with writeback code
+ * running ext4_wb_update_i_disksize().
+ */
+ if (!error)
+ i_size_write(inode, attr->ia_size);
+ up_write(&EXT4_I(inode)->i_data_sem);
ext4_journal_stop(handle);
if (error) {
ext4_orphan_del(NULL, inode);
goto err_out;
}
- }
+ } else
+ i_size_write(inode, attr->ia_size);

- i_size_write(inode, attr->ia_size);
/*
* Blocks are going to be removed from the inode. Wait
* for dio in flight. Temporarily disable
--
1.8.1.2


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