Re: ext4: Always verify extent tree blocks

On Tue, Aug 30, 2011 at 09:08:59AM +0800, 黃一軒 (I-Hsuan Huang) wrote:

Dear Darrick,

Thanks for your correction.
Would you please kindly give me some idea about how to prevent extent
attribute block error?
I would like to try journal_checksum.

First you'd have to figure out how the extent attribute got corrupted (bad
disk, bad cable, bad memory, bad software....) and then we could put together a
plan to fix it.  The ext4 metadata checksumming patches (which are nearly ready
to be sent out) would at least help with silent corruption.

--D

Thanks,
IH

2011/8/30 Darrick J. Wong [off-list ref]

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On Mon, Aug 29, 2011 at 01:40:11PM +0800, 黃一軒 (I-Hsuan Huang) wrote:

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Dear Darrick,

I still met the following failure symptom with your patch.

<2>[    3.311674] EXT4-fs error (device mmcblk0p23): ext4_iget: inode
#22944: (comm init) bad extended attribute block 173879342

The validation code looks like not work.
Is changing the flag enough to force validating extent header?

Extended attribute blocks are not extent tree blocks.

The patch in question only made ext4 complain consistently if the extent
tree
got corrupt.

--D

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Thanks,
IH Huang

2011/8/25 Darrick J. Wong [off-list ref]

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On Wed, Aug 24, 2011 at 05:54:15PM +0800, 黃一軒 (I-Hsuan Huang) wrote:

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Dear Darrick,

I have read your ext4 patch on Patchwork and have a question about

your

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ext4

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patch to validate the extent header.
Is you patch related to the Bug 20992:
https://bugzilla.kernel.org/show_bug.cgi?id=20992

Yes, I think it would fix that bug.  That said, I'm pretty close to

posting

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a
patch set to implement full metadata checksuming, which includes a

better

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solution than what's in patchwork.  Basically it implements a new
bufferhead
flag that tracks whether or not a metadata buffer has been validated,

and

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skips
the checks if it has.

Obviously that won't help you if you overwrite the filesystem while

it's

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mounted... but oh well, at least it'll fix your case.  It was meant to

fix

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the
case where the first time a bad extent block gets read it fails the

check

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and
complains, but subsequent accesses of the same block don't even check.

--D

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Ted mentioned that

*Yep, looks like a bug alright.

From what I can tell, you were in the middle of async I/O, at the

time

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when

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the
disk was corrupted.  The problem seemed to come after the I/O was

completed,

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and  ext4_convert_unwritten_extents() was trying to set the

initialized

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bit

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on
the extent tree.  At that point the extent tree must have gotten

corrupted

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on
disk, and this seriously confused the extent conversion code, which

ended

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up

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passing 0 to ext4_ext_put_in_cache() as the length of the extent, and

that

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tripped the BUG_ON in ext4_ext_put_in_cache().

How did you corrupt the file system while it was mounted?   Was it

via

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some

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dd
to the disk device directly?

We do have code that checks to make sure the extent tree is sane, but

we

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skip
it if the data was already in the buffer cache, to save CPU costs.

 But

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if

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you
wrote to the disk device directly, it would have gone through the

buffer

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cache,
since the extent tree was already cached, we would have skipped the
validation
step, and that could be the explanation for how the bug got

triggered.

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If so, I'm loathe to turn on the validation check unconditionally,

since

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that
would kill performance.  I can probably change the BUG_ON in
ext4_put_in_cache() to rather set the cache state to "invalid", which

would

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at
least prevent the BUG_ON.  The filesystem was probably well and truly
trashed,
though, so sooner or later the ext4 fs code would have hit something

to

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cause
it to be very unhappy.  Hoepfully it would be an ext4_error() call to

mark

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the
file system as corrupted, as opposed to another BUG_ON.
*
Have a nice day.

Thanks,
Randall

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