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From: Alan Stern <stern@rowland.harvard.edu>
To: Martin Kepplinger <martin.kepplinger@puri.sm>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>,
	Bart Van Assche <bvanassche@acm.org>,
	Can Guo <cang@codeaurora.org>,
	martin.petersen@oracle.com, linux-scsi@vger.kernel.org,
	linux-kernel@vger.kernel.org, kernel@puri.sm
Subject: Re: [PATCH] scsi: sd: add runtime pm to open / release
Date: Sat, 8 Aug 2020 11:05:42 -0400
Message-ID: <20200808150542.GB256751@rowland.harvard.edu> (raw)
In-Reply-To: <b0abab28-880e-4b88-eb3c-9ffd927d1ed9@puri.sm>

On Sat, Aug 08, 2020 at 08:59:09AM +0200, Martin Kepplinger wrote:
> On 07.08.20 16:30, Alan Stern wrote:
> > On Fri, Aug 07, 2020 at 11:51:21AM +0200, Martin Kepplinger wrote:
> >> it's really strange: below is the change I'm trying. Of course that's
> >> only for testing the functionality, nothing how a patch could look like.
> >>
> >> While I remember it had worked, now (weirdly since I tried that mounting
> >> via fstab) it doesn't anymore!
> >>
> >> What I understand (not much): I handle the error with "retry" via the
> >> new flag, but scsi_decide_disposition() returns SUCCESS because of "no
> >> more retries"; but it's the first and only time it's called.
> > 
> > Are you saying that scmd->allowed is set to 0?  Or is scsi_notry_cmd() 
> > returning a nonzero value?  Whichever is true, why does it happen that 
> > way?
> 
> scsi_notry_cmd() is returning 1. (it's retry 1 of 5 allowed).
> 
> why is it returning 1? REQ_FAILFAST_DEV is set. It's DID_OK, then "if
> (status_byte(scmd->result) != CHECK_CONDITION)" appearently is not true,
> then at the end it returns 1 because of REQ_FAILFAST_DEV.
> 
> that seems to come from the block layer. why and when? could I change
> that so that the scsi error handling stays in control?

The only place I see where that flag might get set is in 
blk_mq_bio_to_request() in block/blk-mq.c, which does:

	if (bio->bi_opf & REQ_RAHEAD)
		rq->cmd_flags |= REQ_FAILFAST_MASK;

So apparently read-ahead reads are supposed to fail fast (i.e., without 
retries), presumably because they are optional after all.

> > What is the failing command?  Is it a READ(10)?
> 
> Not sure how I'd answer that, but here's the test to trigger the error:
> 
> mount /dev/sda1 /mnt
> cd /mnt
> ls
> cp file ~/ (if ls "works" and doesn't yet trigger the error)
> 
> and that's the (familiar looking) logs when doing so. again: despite the
> mentioned workaround in scsi_error and the new expected_media_change
> flag *is* set and gets cleared, as it should be. REQ_FAILFAST_DEV seems
> to override what I want to do is scsi_error:
> 
> [   55.557629] sd 0:0:0:0: [sda] tag#0 UNKNOWN(0x2003) Result:
> hostbyte=0x00 driverbyte=0x08 cmd_age=0s
> [   55.557639] sd 0:0:0:0: [sda] tag#0 Sense Key : 0x6 [current]
> [   55.557646] sd 0:0:0:0: [sda] tag#0 ASC=0x28 ASCQ=0x0
> [   55.557657] sd 0:0:0:0: [sda] tag#0 CDB: opcode=0x28 28 00 00 08 fc
> e0 00 00 01 00

Yes, 0x28 is READ(10).  Likely this is a read-ahead request, although I 
don't know how we can tell for sure.

> [   55.557666] blk_update_request: I/O error, dev sda, sector 589024 op
> 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
> [   55.568899] sd 0:0:0:0: [sda] tag#0 device offline or changed
> [   55.574691] blk_update_request: I/O error, dev sda, sector 589025 op
> 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
> [   55.585756] sd 0:0:0:0: [sda] tag#0 device offline or changed
> [   55.591562] blk_update_request: I/O error, dev sda, sector 589026 op
> 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
> [   55.602274] sd 0:0:0:0: [sda] tag#0 device offline or changed
> (... goes on with the same)

Is such a drastic response really appropriate for the failure of a 
read-ahead request?  It seems like a more logical response would be to 
let the request fail but keep the device online.

Of course, that would only solve part of your problem -- your log would 
still get filled with those "I/O error" messages even though they 
wouldn't be fatal.  Probably a better approach would be to make the new 
expecting_media_change flag override scsi_no_retry_cmd().

But this is not my area of expertise.  Maybe someone else will have more 
to say.

Alan Stern

  reply index

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-23 11:10 Martin Kepplinger
2020-06-24 13:33 ` Bart Van Assche
2020-06-25  8:16   ` Martin Kepplinger
2020-06-25 14:52     ` Alan Stern
2020-06-26  3:53     ` Bart Van Assche
2020-06-26 15:07     ` Bart Van Assche
2020-06-26 15:44       ` Alan Stern
2020-06-28  2:37         ` Bart Van Assche
2020-06-28 13:10           ` Alan Stern
2020-06-29  9:42         ` Martin Kepplinger
2020-06-29 16:15           ` Alan Stern
2020-06-29 16:56             ` Bart Van Assche
2020-06-29 17:40               ` Alan Stern
2020-06-30  3:33             ` Martin Kepplinger
2020-06-30 13:38               ` Alan Stern
2020-06-30 15:59             ` Bart Van Assche
2020-06-30 18:02               ` Alan Stern
2020-06-30 19:23                 ` Bart Van Assche
2020-06-30 19:38                   ` Alan Stern
2020-06-30 23:31                     ` Bart Van Assche
2020-07-01  0:49                       ` Alan Stern
2020-07-06 16:41                         ` Alan Stern
2020-07-28  7:02                           ` Martin Kepplinger
2020-07-28 20:02                             ` Alan Stern
2020-07-29 14:12                               ` Martin Kepplinger
2020-07-29 14:32                                 ` Alan Stern
2020-07-29 14:44                                   ` Martin K. Petersen
2020-07-29 14:56                                     ` Alan Stern
2020-07-29 14:46                                   ` James Bottomley
2020-07-29 14:53                                     ` James Bottomley
2020-07-29 15:40                                       ` Martin Kepplinger
2020-07-29 15:44                                         ` James Bottomley
2020-07-29 16:43                                           ` Martin Kepplinger
2020-07-29 18:25                                             ` Alan Stern
2020-07-29 18:29                                               ` James Bottomley
2020-07-30  8:52                                                 ` Martin Kepplinger
2020-07-30  8:54                                                   ` Martin Kepplinger
2020-07-30 15:10                                                   ` Alan Stern
2020-08-04  9:39                                                     ` Martin Kepplinger
2020-08-07  9:51                                                       ` Martin Kepplinger
2020-08-07 14:30                                                         ` Alan Stern
2020-08-08  6:59                                                           ` Martin Kepplinger
2020-08-08 15:05                                                             ` Alan Stern [this message]
2020-08-09  9:20                                                               ` Martin Kepplinger
2020-08-09 15:26                                                                 ` Alan Stern
2020-08-10 12:03                                                                   ` Martin Kepplinger
2020-08-10 14:13                                                                     ` Alan Stern
2020-08-11  7:55                                                                       ` Martin Kepplinger
2020-08-11 13:48                                                                         ` Alan Stern
2020-08-23 14:57                                                                         ` [PATCH] block: Fix bug in runtime-resume handling Alan Stern
2020-08-24 17:48                                                                           ` Bart Van Assche
2020-08-24 20:13                                                                             ` Alan Stern
2020-08-26  7:48                                                                               ` Martin Kepplinger
2020-08-27 17:42                                                                                 ` Martin Kepplinger
2020-08-27 20:29                                                                                   ` Alan Stern
2020-08-29  7:24                                                                                     ` Martin Kepplinger
2020-08-29 15:26                                                                                       ` Alan Stern
2020-08-29 16:33                                                                                         ` Martin Kepplinger
2020-08-29 18:56                                                                                           ` Alan Stern
2020-08-30  0:38                                                                                             ` Bart Van Assche
2020-08-30  1:06                                                                                               ` Alan Stern
2020-07-29 15:40                                       ` [PATCH] scsi: sd: add runtime pm to open / release Alan Stern
2020-07-29 15:49                                         ` James Bottomley
2020-07-29 16:17                                           ` Alan Stern
2020-07-29 15:52                                         ` Martin Kepplinger
2020-07-29 18:10                                   ` Douglas Gilbert
2020-07-30  8:05             ` Martin Kepplinger
2020-07-30 15:14               ` Alan Stern

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