Re: [PATCH] blk: missing add of padded bytes to io completion bytecount

From: Boaz Harrosh
Date: Wed Mar 05 2008 - 07:53:45 EST

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On Wed, Mar 05 2008 at 14:33 +0200, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> On Wed, Mar 05 2008, Boaz Harrosh wrote:
>> On Wed, Mar 05 2008 at 2:26 +0200, FUJITA Tomonori <fujita.tomonori@xxxxxxxxxxxxx> wrote:
>>> On Wed, 05 Mar 2008 08:33:05 +0900
>>> Tejun Heo <htejun@xxxxxxxxx> 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@xxxxxxxxxxx>
>> 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@xxxxxxxxxxx>
>> Signed-off-by: Benny Halevy <bhalevy@xxxxxxxxxxx>
>> ---
>> 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@xxxxxxxxxxx>
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@xxxxxxxxxxx>
Signed-off-by: Benny Halevy <bhalevy@xxxxxxxxxxx>
---
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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Next message: Ahmed S. Darwish: "Re: [PATCH BUGFIX -rc3] Smack: Don't register smackfs if we're notloaded"
Previous message: Jeff Garzik: "Re: [PATCH RFC] ahci: fix SB600 h/w errata issue"
In reply to: Jens Axboe: "Re: [PATCH] blk: missing add of padded bytes to io completion byte count"
Next in thread: Jens Axboe: "Re: [PATCH] blk: missing add of padded bytes to io completion byte count"
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