linux-ext4 - [PATCH] ext4: ensure LARGE_FILE feature when mounting delalloc

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Message-ID: <542C7331.4070200@redhat.com>
Date:	Wed, 01 Oct 2014 16:33:37 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: [PATCH] ext4: ensure LARGE_FILE feature when mounting delalloc

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

It simplifies things if we ensure that when we are running
with delalloc, we have LARGE_FILE set already; that way we
don't have to potentially set it later during a file write.

For any fs of sufficient size, LARGE_FILE is usually set
simply due to the size of the resize inode.  And for ext4,
HUGE_FILE is set by default.

LARGE_FILE is a decades-old compatibility flag, so at this
point there is little risk of backwards compatibility problems
by enabling it when the filesystem is mounted as ext4.

So just set LARGE_FILE if we are mounted delalloc, if it's
not set already, and be done with it.

Signed-off-by: Eric Sandeen <sandeen@...hat.com>
--- 

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 0b28b36..8e56d7e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3576,6 +3576,20 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 			clear_opt(sb, DELALLOC);
 	}
 
+	/*
+	 * Adding the LARGE_FILES feature to the superblock adds
+	 * unnecessary complication to journal credit calculations
+	 * when delalloc is enabled.  This is a decades-old feature,
+	 * so just enable it now to simplify things.
+	 */
+	if (test_opt(sb, DELALLOC) && !(sb->s_flags & MS_RDONLY) &&
+	    EXT4_HAS_COMPAT_FEATURE(sb, EXT4_FEATURE_COMPAT_HAS_JOURNAL) &&
+	    !EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_LARGE_FILE)) {
+		ext4_update_dynamic_rev(sb);
+		EXT4_SET_RO_COMPAT_FEATURE(sb,
+					   EXT4_FEATURE_RO_COMPAT_LARGE_FILE);
+	}
+
 	sb->s_flags = (sb->s_flags & ~MS_POSIXACL) |
 		(test_opt(sb, POSIX_ACL) ? MS_POSIXACL : 0);
 

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