[3.16.y-ckt extended stable] Patch "ext4: fix reservation overflow in ext4_da_write_begin" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    ext4: fix reservation overflow in ext4_da_write_begin

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt1.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 8035ac9178f8bb7ed9879e75cb891aa76c50d0c1 Mon Sep 17 00:00:00 2001
From: Eric Sandeen <sandeen at redhat.com>
Date: Sat, 11 Oct 2014 19:51:17 -0400
Subject: ext4: fix reservation overflow in ext4_da_write_begin

commit 0ff8947fc5f700172b37cbca811a38eb9cb81e08 upstream.

Delalloc write journal reservations only reserve 1 credit,
to update the inode if necessary.  However, it may happen
once in a filesystem's lifetime that a file will cross
the 2G threshold, and require the LARGE_FILE feature to
be set in the superblock as well, if it was not set already.

This overruns the transaction reservation, and can be
demonstrated simply on any ext4 filesystem without the LARGE_FILE
feature already set:

dd if=/dev/zero of=testfile bs=1 seek=2147483646 count=1 \
	conv=notrunc of=testfile
sync
dd if=/dev/zero of=testfile bs=1 seek=2147483647 count=1 \
	conv=notrunc of=testfile

leads to:

EXT4-fs: ext4_do_update_inode:4296: aborting transaction: error 28 in __ext4_handle_dirty_super
EXT4-fs error (device loop0) in ext4_do_update_inode:4301: error 28
EXT4-fs error (device loop0) in ext4_reserve_inode_write:4757: Readonly filesystem
EXT4-fs error (device loop0) in ext4_dirty_inode:4876: error 28
EXT4-fs error (device loop0) in ext4_da_write_end:2685: error 28

Adjust the number of credits based on whether the flag is
already set, and whether the current write may extend past the
LARGE_FILE limit.

Signed-off-by: Eric Sandeen <sandeen at redhat.com>
Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Reviewed-by: Andreas Dilger <adilger at dilger.ca>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/ext4/inode.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 2b06bac72c94..05b441641a33 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2631,6 +2631,20 @@ static int ext4_nonda_switch(struct super_block *sb)
 	return 0;
 }

+/* We always reserve for an inode update; the superblock could be there too */
+static int ext4_da_write_credits(struct inode *inode, loff_t pos, unsigned len)
+{
+	if (likely(EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb,
+				EXT4_FEATURE_RO_COMPAT_LARGE_FILE)))
+		return 1;
+
+	if (pos + len <= 0x7fffffffULL)
+		return 1;
+
+	/* We might need to update the superblock to set LARGE_FILE */
+	return 2;
+}
+
 static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
 			       loff_t pos, unsigned len, unsigned flags,
 			       struct page **pagep, void **fsdata)
@@ -2681,7 +2695,8 @@ retry_grab:
 	 * of file which has an already mapped buffer.
 	 */
 retry_journal:
-	handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, 1);
+	handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE,
+				ext4_da_write_credits(inode, pos, len));
 	if (IS_ERR(handle)) {
 		page_cache_release(page);
 		return PTR_ERR(handle);
--
2.1.0