Re: Wrong DIF guard tag on ext2 write

On Tue, Jun 01, 2010 at 12:30:42PM +0200, Christof Schmitt wrote:
On Mon, May 31, 2010 at 06:30:05PM +0300, Boaz Harrosh wrote:
On 05/31/2010 06:01 PM, James Bottomley wrote:
On Mon, 2010-05-31 at 10:20 -0400, Martin K. Petersen wrote:
"Christof" == Christof Schmitt <christof.schmitt@xxxxxxxxxx> writes:

Christof> Since the guard tags are created in Linux, it seems that the
Christof> data attached to the write request changes between the
Christof> generation in bio_integrity_generate and the call to
Christof> sd_prep_fn.

Yep, known bug. Page writeback locking is messed up for buffer_head
users. The extNfs folks volunteered to look into this a while back but
I don't think they have found the time yet.


Christof> Using ext3 or ext4 instead of ext2 does not show the problem.

Last I looked there were still code paths in ext3 and ext4 that
permitted pages to be changed during flight. I guess you've just been
lucky.

Pages have always been modifiable in flight. The OS guarantees they'll
be rewritten, so the drivers can drop them if it detects the problem.
This is identical to the iscsi checksum issue (iscsi adds a checksum
because it doesn't trust TCP/IP and if the checksum is generated in
software, there's time between generation and page transmission for the
alteration to occur). The solution in the iscsi case was not to
complain if the page is still marked dirty.


And also why RAID1 and RAID4/5/6 need the data bounced. I wish VFS
would prevent data writing given a device queue flag that requests
it. So all these devices and modes could just flag the VFS/filesystems
that: "please don't allow concurrent writes, otherwise I need to copy data"

From what Chris Mason has said before, all the mechanics are there, and it's
what btrfs is doing. Though I don't know how myself?

I also tested with btrfs and invalid guard tags in writes have been
encountered as well (again in 2.6.34). The only difference is that no
error was reported to userspace, although this might be a
configuration issue.

This would be a btrfs bug. We have strict checks in place that are
supposed to prevent buffers changing while in flight. What was the
workload that triggered this problem?


What is the best strategy to continue with the invalid guard tags on
write requests? Should this be fixed in the filesystems?


Long term, I think the filesystems shouldn't be changing pages in
flight. Bouncing just hurts way too much.

-chris
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Relevant Pages

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