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    SubjectRe: [PATCH] blk: missing add of padded bytes to io completion byte count
    On Wed, Mar 05 2008 at 14:33 +0200, Jens Axboe <jens.axboe@oracle.com> wrote:
    > On Wed, Mar 05 2008, Boaz Harrosh wrote:
    >> On Wed, Mar 05 2008 at 2:26 +0200, FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:
    >>> On Wed, 05 Mar 2008 08:33:05 +0900
    >>> Tejun Heo <htejun@gmail.com> wrote:
    >>>
    >>>> FUJITA Tomonori wrote:
    >>>>> Hmm, does SCSI mid-layer need to care about how many bytes the block
    >>>>> layer allocates? I don't think that extra_len is NOT good_bytes.
    >>>>>
    >>>>> I think that the block layer had better take care about it (fix
    >>>>> __end_that_request_first?).
    >>>> Yeah, probably calling completion functions w/o bytes count is the right
    >>>> thing to do but what I was talking about was what could break when the
    >>>> semantics of rq->data_len changed. If we keep rq->data_len() ==
    >>>> sum(sg), we keep it business as usual for all the rest except for the
    >>>> device application layer if we don't we do the reverse and SCSI midlayer
    >>>> completion was a good example, I think.
    >>> sglist is a low-level I/O representation for device drivers. SCSI
    >>> midlayer should not care about sglist. We should not fix SCSI midlayer
    >>> for rq->data_len != sum(sg) change (so I can't agree with your
    >>> diagrams in another mail).
    >>>
    >>> When if we change a rule, we need to fix something.
    >>>
    >>> If we keep rq->data_len == sum(sg), we need to fix the device
    >>> application layer. If we keep rq->data_len == the true data length, we
    >>> need to fix the low-level drivers.
    >>>
    >>> Now I'm fine with the commit e97a294ef6938512b655b1abf17656cf2b26f709
    >>> since we are in -rc stages. But I plan to send a patch to revert it
    >>> and fix this issue in the block layer. I'd like to test it in -mm for
    >>> a while.
    >> No this commit is a serious bug, and the only fix is like you suggested
    >> in __end_that_request_first. This is because it breaks that scsi-ml loop
    >> where scsi_bufflen() can be less then blk_rq_bytes(). In that case this
    >> commit is a data corruption.
    >>
    >>> Only sglist stuff in SCSI midlayer is scsi_req_map_sg now. As you
    >>> know, we really want to remove it.
    >>>
    >>>
    >>>> Things going the other way is fine with me but I at least want to hear a
    >>>> valid rationale. Till now all I got is "because that's the true size"
    >>>> which doesn't really make much sense to me.
    >>> Most of users of request structure care about only the real data
    >>> length, don't care about padding and drain length. Why do they bother
    >>> to use a helper function to get the real data length?
    >>> --
    >> Submitted is the right fix to this problem, as pointed out by TOMO.
    >> Please test it solves the CD burning problem.
    >> (The patch includes the revert of commit e97a294e)
    >> ---
    >> From: Boaz Harrosh <bharrosh@panasas.com>
    >> Date: Wed, 5 Mar 2008 12:07:12 +0200
    >> Subject: [PATCH] blk: missing add of padded bytes to io completion byte count
    >>
    >> the commit e97a294ef6938512b655b1abf17656cf2b26f709 was very wrong. This is
    >> because scsi-ml supports the ability to split a request into smaller chunks,
    >> in which case scsi_bufflen() is smaller then request length. Then at completion
    >> time the remainder can be issued as a new scsi command. In that case the above
    >> commit is a data corruption.
    >
    > We needed something for -rc4, so it had to be rushed a bit...
    >
    >> Also in this fix all users of block layer are taken care of, and not only
    >> scsi devices.
    >>
    >> Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
    >> Signed-off-by: Benny Halevy <bhalevy@panasas.com>
    >> ---
    >> block/blk-core.c | 4 ++++
    >> drivers/scsi/scsi.c | 2 +-
    >> 2 files changed, 5 insertions(+), 1 deletions(-)
    >>
    >> diff --git a/block/blk-core.c b/block/blk-core.c
    >> index 2a438a9..37fcccc 100644
    >> --- a/block/blk-core.c
    >> +++ b/block/blk-core.c
    >> @@ -1549,6 +1549,9 @@ static int __end_that_request_first(struct request *req, int error,
    >> nr_bytes >> 9, req->sector);
    >> }
    >>
    >> + if (nr_bytes >= blk_rq_bytes(req))
    >> + nr_bytes += req->extra_len;
    >> +
    >> total_bytes = bio_nbytes = 0;
    >> while ((bio = req->bio) != NULL) {
    >> int nbytes;
    >> @@ -1616,6 +1619,7 @@ static int __end_that_request_first(struct request *req, int error,
    >> if (!req->bio)
    >> return 0;
    >>
    >> + BUG_ON(total_bytes >= blk_rq_bytes(req));
    >
    > Make that a WARN_ON() first please. It's indeed a bug, but it wont be
    > critical and it's not fair killing everything since this padding stuff
    > is so fresh and may still need a tweak or two.
    >
    > I'd be fine with making it a BUG_ON() post 2.6.25.
    >
    Updated, you are absolutely right, thanks.

    Will you commit below patch for 2.6.25? I know that, at the time, I have
    seen this scsi-ml-loop in action on a sata drive here in the lab, on an
    x86_64 machine. The current solution will silently corrupt data, which
    is very hard to find.

    Boaz
    ---
    From: Boaz Harrosh <bharrosh@panasas.com>
    Date: Wed, 5 Mar 2008 12:07:12 +0200
    Subject: [PATCH] blk: missing add of padded bytes to io completion byte count

    the commit e97a294ef6938512b655b1abf17656cf2b26f709 was very wrong. This is
    because scsi-ml supports the ability to split a request into smaller chunks,
    in which case scsi_bufflen() is smaller then request length. Then at completion
    time the remainder can be issued as a new scsi command. In that case the above
    commit is a data corruption.

    Also in this fix all users of block layer are taken care of, and not only
    scsi devices.

    Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
    Signed-off-by: Benny Halevy <bhalevy@panasas.com>
    ---
    block/blk-core.c | 4 ++++
    drivers/scsi/scsi.c | 2 +-
    2 files changed, 5 insertions(+), 1 deletions(-)

    diff --git a/block/blk-core.c b/block/blk-core.c
    index 2a438a9..c82e68a 100644
    --- a/block/blk-core.c
    +++ b/block/blk-core.c
    @@ -1549,6 +1549,9 @@ static int __end_that_request_first(struct request *req, int error,
    nr_bytes >> 9, req->sector);
    }

    + if (nr_bytes >= blk_rq_bytes(req))
    + nr_bytes += req->extra_len;
    +
    total_bytes = bio_nbytes = 0;
    while ((bio = req->bio) != NULL) {
    int nbytes;
    @@ -1616,6 +1619,7 @@ static int __end_that_request_first(struct request *req, int error,
    if (!req->bio)
    return 0;

    + WARN_ON(total_bytes >= blk_rq_bytes(req));
    /*
    * if the request wasn't completed, update state
    */
    diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
    index e5c6f6a..fecba05 100644
    --- a/drivers/scsi/scsi.c
    +++ b/drivers/scsi/scsi.c
    @@ -757,7 +757,7 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
    "Notifying upper driver of completion "
    "(result %x)\n", cmd->result));

    - good_bytes = scsi_bufflen(cmd) + cmd->request->extra_len;
    + good_bytes = scsi_bufflen(cmd);
    if (cmd->request->cmd_type != REQ_TYPE_BLOCK_PC) {
    drv = scsi_cmd_to_driver(cmd);
    if (drv->done)
    --
    1.5.3.3


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