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    SubjectRe: Wrong DIF guard tag on ext2 write
    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 <> 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?

    > James


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