[Bug 15420] New: EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[Bug 15420] New: EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[bugzilla-daemon]

http://bugzilla.kernel.org/show_bug.cgi?id=15420

           Summary: EXT4_USE_FOR_EXT23 causes wrong free space calculation
                    on ext2 and ext3
           Product: File System
           Version: 2.5
    Kernel Version: 2.6.33
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: ext4
        AssignedTo: fs_ext4@kernel-bugs.osdl.org
        ReportedBy: thomas@archlinux.org
        Regression: No


When enabling the EXT4_USE_FOR_EXT23 option in 2.6.33. The free space
calculation on ext2 and ext3 becomes incorrect. Steps to reproduce:

1) Create ext2 or ext3 file system with mkfs
2) Mount it on a kernel with EXT4_USE_FOR_EXT23 enabled
3) Launch "watch -n1 df -h /path/to/mountpoint" in another terminal
4) Download a file with wget

The "used space" on the ext2/3 will now increase by about 500MB per second.
Once you stop the download and type "sync" (or wait for a while), the values
shown for free/used space return to expected values. This is always
reproducible.

We can reproduce this on x86_32 and x86_64 with the Arch Linux distribution
kernel, as well as my own personal Core2-optimized 64 Bit kernel.

.config x86_32:
http://repos.archlinux.org/wsvn/packages/kernel26/trunk/config?rev=70484
.config x86_64:
http://repos.archlinux.org/wsvn/packages/kernel26/trunk/config.x86_64?rev=70484

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

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--- Comment #1 from Thomas Bächler <thomas@archlinux.org>  2010-03-01 18:19:33 ---
Created an attachment (id=25293)
 --> (http://bugzilla.kernel.org/attachment.cgi?id=25293)
.config for my personal kernel

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[bugzilla-daemon]

http://bugzilla.kernel.org/show_bug.cgi?id=15420





--- Comment #2 from Thomas Bächler <thomas@archlinux.org>  2010-03-09 21:27:19 ---
Ping? Anyone actually reading this?

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[bugzilla-daemon]

http://bugzilla.kernel.org/show_bug.cgi?id=15420


Eric Sandeen <sandeen@redhat.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sandeen@redhat.com




--- Comment #3 from Eric Sandeen <sandeen@redhat.com>  2010-03-09 21:35:05 ---
Yes, but we are reading a lot of things ;)

Is it du, or df, or both, which shows the "wrong" amount until sync?

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

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http://bugzilla.kernel.org/show_bug.cgi?id=15420





--- Comment #4 from Thomas Bächler <thomas@archlinux.org>  2010-03-09 22:07:23 ---
It is definitely df.

I didn't test du, and have no test case right now (and the other guys who had
the problem are not reachable right now). I'll make another test with du as
soon as I get to it.

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[bugzilla-daemon]

http://bugzilla.kernel.org/show_bug.cgi?id=15420


Theodore Tso <tytso@mit.edu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tytso@mit.edu




--- Comment #5 from Theodore Tso <tytso@mit.edu>  2010-03-10 03:12:30 ---
This sounds like the block reservation code for delayed allocation is
over-aggressive about estimating how many blocks are need for the indirect
blocks in the case of indirect-mapped inodes (which would be what you get when
mounting an ext2 filesystem using the ext4 file system driver).

This should be an aesthetic issue only, but it would be nice to have this be
fixed.

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Re: [Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[Jan Kara]

> http://bugzilla.kernel.org/show_bug.cgi?id=15420
> 
> 
> Theodore Tso <tytso@mit.edu> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |tytso@mit.edu
> 
> 
> 
> 
> --- Comment #5 from Theodore Tso <tytso@mit.edu>  2010-03-10 03:12:30 ---
> This sounds like the block reservation code for delayed allocation is
> over-aggressive about estimating how many blocks are need for the indirect
> blocks in the case of indirect-mapped inodes (which would be what you get when
> mounting an ext2 filesystem using the ext4 file system driver).
> 
> This should be an aesthetic issue only, but it would be nice to have this be
> fixed.
  Since bugzilla seems to be down now, I'll reply by email:
Won't it make sence to use 'nodelalloc' by default when ext4 driver is used
for ext2 or ext3 filesystem? Because delayed allocation has some implications
people need not expect - like the overestimation of needed blocks, or changed
semantics of ordered data mode etc...
  So something like the patch below?

									Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs
---
>From c5fa50757c0b81f661532b6f833946bd9294e439 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Wed, 10 Mar 2010 12:23:03 +0100
Subject: [PATCH] ext4: Don't use delayed allocation by default when used instead of ext[23]

When ext4 driver is used to mount a filesystem instead of ext2 or ext3 driver,
do not enable delayed allocation by default since it has some unobvious
consequences (different semantics of data=ordered mode, overestimation of
needed metadata blocks for allocation).

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/super.c |   20 +++++++++++---------
 1 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ce84a6e..7fc9ab3 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -68,7 +68,16 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf);
 static int ext4_unfreeze(struct super_block *sb);
 static void ext4_write_super(struct super_block *sb);
 static int ext4_freeze(struct super_block *sb);
+static int ext4_get_sb(struct file_system_type *fs_type, int flags,
+		       const char *dev_name, void *data, struct vfsmount *mnt);
 
+static struct file_system_type ext4_fs_type = {
+	.owner		= THIS_MODULE,
+	.name		= "ext4",
+	.get_sb		= ext4_get_sb,
+	.kill_sb	= kill_block_super,
+	.fs_flags	= FS_REQUIRES_DEV,
+};
 
 ext4_fsblk_t ext4_block_bitmap(struct super_block *sb,
 			       struct ext4_group_desc *bg)
@@ -2539,7 +2548,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	 * enable delayed allocation by default
 	 * Use -o nodelalloc to turn it off
 	 */
-	set_opt(sbi->s_mount_opt, DELALLOC);
+	if (sb->s_bdev->bd_holder == &ext4_fs_type)
+		set_opt(sbi->s_mount_opt, DELALLOC);
 
 	if (!parse_options((char *) data, sb, &journal_devnum,
 			   &journal_ioprio, NULL, 0))
@@ -4122,14 +4132,6 @@ static inline void register_as_ext3(void) { }
 static inline void unregister_as_ext3(void) { }
 #endif
 
-static struct file_system_type ext4_fs_type = {
-	.owner		= THIS_MODULE,
-	.name		= "ext4",
-	.get_sb		= ext4_get_sb,
-	.kill_sb	= kill_block_super,
-	.fs_flags	= FS_REQUIRES_DEV,
-};

[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[bugzilla-daemon]

http://bugzilla.kernel.org/show_bug.cgi?id=15420





--- Comment #6 from Jan Kara <jack@suse.cz>  2010-03-10 12:25:21 ---
> http://bugzilla.kernel.org/show_bug.cgi?id=15420
> 
> 
> Theodore Tso <tytso@mit.edu> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |tytso@mit.edu
> 
> 
> 
> 
> --- Comment #5 from Theodore Tso <tytso@mit.edu>  2010-03-10 03:12:30 ---
> This sounds like the block reservation code for delayed allocation is
> over-aggressive about estimating how many blocks are need for the indirect
> blocks in the case of indirect-mapped inodes (which would be what you get when
> mounting an ext2 filesystem using the ext4 file system driver).
> 
> This should be an aesthetic issue only, but it would be nice to have this be
> fixed.
  Since bugzilla seems to be down now, I'll reply by email:
Won't it make sence to use 'nodelalloc' by default when ext4 driver is used
for ext2 or ext3 filesystem? Because delayed allocation has some implications
people need not expect - like the overestimation of needed blocks, or changed
semantics of ordered data mode etc...
  So something like the patch below?

                                    Honza

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

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--- Comment #7 from Thomas Bächler <thomas@archlinux.org>  2010-03-10 12:36:12 ---
(In reply to comment #5)
> This sounds like the block reservation code for delayed allocation is
> over-aggressive about estimating how many blocks are need for the indirect
> blocks in the case of indirect-mapped inodes (which would be what you get when
> mounting an ext2 filesystem using the ext4 file system driver).
> 
> This should be an aesthetic issue only, but it would be nice to have this be
> fixed.

It is not purely aesthetic, lots of applications abort their operation if space
is supposedly running low.

However, as this doesn't seem to be high priority for anyone here, we should
probably consider disabling EXT4_USE_FOR_EXT23 in our distribution kernel for
the time being.

(In reply to comment #6)
> Won't it make sence to use 'nodelalloc' by default when ext4 driver is used
> for ext2 or ext3 filesystem? Because delayed allocation has some implications
> people need not expect - like the overestimation of needed blocks, or changed
> semantics of ordered data mode etc...

Don't see a patch here.

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[bugzilla-daemon]

http://bugzilla.kernel.org/show_bug.cgi?id=15420





--- Comment #8 from Theodore Tso <tytso@mit.edu>  2010-03-10 14:16:46 ---
What applications do you actually call statfs() and check the amount of space
that you have?  I'm not aware of any.

Does the problem go away if you add the mount option "nodelalloc"?   If it
does, that's Jan's patch is probably the right workaround.  It looks like
Bugzilla stripped it off (it's in the e-mail version of his reply).  I'll add
it to this bug, and you can try it out as well.

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

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--- Comment #9 from Theodore Tso <tytso@mit.edu>  2010-03-10 14:18:25 ---
Created an attachment (id=25448)
 --> (http://bugzilla.kernel.org/attachment.cgi?id=25448)
Possible patch from Jan Kara

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

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--- Comment #10 from Thomas Bächler <thomas@archlinux.org>  2010-03-10 14:56:42 ---
I don't know the exact application that was used, but it one guy said he was
unable to work because many applications would constantly complain that there
was not enough space and thus abort. I'll ask again.

I'll test this later and report back.

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

[bugzilla-daemon]

http://bugzilla.kernel.org/show_bug.cgi?id=15420





--- Comment #11 from Eric Sandeen <sandeen@redhat.com>  2010-03-10 15:11:28 ---
If delalloc doesn't work with indirect files on ext3 filesystems, then we need
to stop telling people they can migrate ext3 to ext4 simply by turning on
extents etc, and mounting as ext4.

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

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--- Comment #12 from Theodore Tso <tytso@mit.edu>  2010-03-10 15:28:12 ---
Well, let's figure out what actually broke, and why.   The vast majority of
files are not appended to after they are first written, I suspect the migration
story still works.   And this issue with over-aggressive estimation for space
needed for indirect blocks is probably something recent in 2.6.33, and I think
we can fix it --- and even if it isn't, I'd really like to know what
applications are checking statfs and aborting as a result, and under what
situations this is happening.

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[Bug 15420] EXT4_USE_FOR_EXT23 causes wrong free space calculation on ext2 and ext3

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--- Comment #13 from Theodore Tso <tytso@mit.edu>  2010-03-10 15:44:46 ---
OK, I can confirm that the problem is with delayed allocation and indirect
files.   While downloading a 70 meg kernel source tar ball, the amount of space
shown as being used by df goes as high as 1 gigabyte (50% of the space on my
disk, about 25% of my available free memory) before dropping back down. as the
blocks get allocated.   Mounting with nodelalloc makes the problem go away.
Using extents, the df result will occasionally be as much as 1-2 megs but it is
much smaller in practice.

Using nodelalloc for ext2/3 mounts may make sense not just as a workaround, but
to help protect users using crappy desktop applications that don't know how to
use fsync().  (They'll still get screwed by btrfs, but the lazy application
writers and Phoronix they can complain to the btrfs developers about how they
are incompetent, and it's no longer my problem at that point.  :-)

We should try to improve the estimation logic for indirect blocks (I remember
being in a hurry to write the code to work around the regression caused by the
quota bugfix that has caused us so much heartache), but I don't think this is a
problem for people who are migrating from ext3 to ext4, since it's rare that
they will be extending an already existing indirect-mapped file.  The problem
with EXT4_USE_FOR_EXT23 is that they are _not_ migrating, and we're seeing a
weakness in delalloc with indirect block files; a weakness that was only
introduced recently, and one that I hope we can fix.   But for other reasons
mentioned above, we probably want to turn on nodelalloc anyway...

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Jan Kara <jack@suse.cz> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jack@suse.cz




--- Comment #14 from Jan Kara <jack@suse.cz>  2010-03-11 16:00:10 ---
Indeed, the logic in ext4_indirect_calc_metadata_amount seems to be broken.
Attached patch fixes it for me...

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--- Comment #15 from Jan Kara <jack@suse.cz>  2010-03-11 16:01:19 ---
Created an attachment (id=25477)
 --> (http://bugzilla.kernel.org/attachment.cgi?id=25477)
Fix logic in ext4_indirect_calc_metadata_amount

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--- Comment #16 from Jan Kara <jack@suse.cz>  2010-03-11 16:10:46 ---
As for comment #13 - I fully agree with Ted. When admin does "mount -t ext3
...", I think we should give him as close behavior to ext3 as possible by the
principle of the least surprise - and that means nodelalloc. If user wishes
better performance etc, he can always mount the filesystem as ext4 explicitely.
Also I remember some discussions with KDE guys when they were at some point
considering detecting filesystem type and avoid doing fsync if it was ext3 -
hopefully, I convinced them not to do it but I bet someone else had the same
idea and actually implemented it. Not that I'd have too much pity with broken
applications but I prefer to avoid breaking them silently like this...

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--- Comment #17 from Eric Sandeen <sandeen@redhat.com>  2010-03-11 16:16:46 ---
(In reply to comment #16)
> As for comment #13 - I fully agree with Ted. When admin does "mount -t ext3
> ...", I think we should give him as close behavior to ext3 as possible by the
> principle of the least surprise - and that means nodelalloc. If user wishes
> better performance etc, he can always mount the filesystem as ext4 explicitely.

Under what circumstances, then, do we recommend mounting ext3 with the ext4
driver?

The user will get different allocation behavior as well; will that also be a
surprise?

I just want to be clear in our recommendations about this stuff...

> Also I remember some discussions with KDE guys when they were at some point
> considering detecting filesystem type and avoid doing fsync if it was ext3 -
> hopefully, I convinced them not to do it but I bet someone else had the same
> idea and actually implemented it. Not that I'd have too much pity with broken
> applications but I prefer to avoid breaking them silently like this...

I think by having this config option at -all- we have really muddied the
waters, to be honest ... but I suppose this is all better hashed outon the
list, not in this bug.  :)

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--- Comment #18 from Theodore Tso <tytso@mit.edu>  2010-03-11 17:42:06 ---
On Thu, Mar 11, 2010 at 04:00:27PM +0000, bugzilla-daemon@bugzilla.kernel.org
wrote:
> Indeed, the logic in ext4_indirect_calc_metadata_amount seems to be broken.
> Attached patch fixes it for me...

Nice catch.  Yeah, I really screwed that up!

I'll get that into the ext4 patch queue, quickly, and get that pushed
to Linus.

                    - Ted

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