On 02/25/2018 07:29 PM, Douglas Gilbert wrote:
> The SCSI PRE-FETCH (10 or 16) command is present both on hard disks
> and some SSDs. It is useful when the address of the next block(s) to
> be read is known but it is not following the LBA of the current READ
> (so read-ahead won't help). It returns two "good" SCSI Status values.
> If the requested blocks have fitted (or will most likely fit (when
> the IMMED bit is set)) into the disk's cache, it returns CONDITION
> MET. If it didn't (or will not) fit then it returns GOOD status.
>
> The primary goal of this patch is to stop the SCSI subsystem treating
> the CONDITION MET SCSI status as an error. The current state makes the
> PRE-FETCH command effectively unusable via pass-throughs. The hunt to
> find where the erroneous decision was made led to the
> scsi_io_completion() function in scsi_lib.c . This is a complicated
> function made more complicated by the intertwining of good and error
> (or special case) processing paths.
>
> Future work: if the sd driver was to use the PRE-FETCH command,
> decide whether it and/or the block layer needs to know about the
> two different "good" statuses. If so a mechanism would be needed
> to do that.
>
> ChangeLog to v2:
> - further restructure the code, place some early non-zero
> result processing in a new helper function:
> scsi_io_completion_nz_result()
> - this reduces the number of error checks on the zero result
> path (the fast path) at the expense of some extra work for
> the non-zero result processing
> - rename some variables to make the code a little clearer
>
> ChangeLog to v1:
> - expand scsi_status_is_good() to check for CONDITION MET
> - add another corner case in scsi_io_completion() adjacent
> to the one for the RECOVERED ERROR sense key case. That
> is another "non-error"
> - structure code so extra checks are only on the error
> path (i.e. when cmd->result is non-zero)
>
> This patch is against mkp's 4.17/scsi-queue branch. It also applies
> to lk 4.15.x where it was re-tested on a SAS SSD.
>
> Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
> ---
> drivers/scsi/scsi_lib.c | 140 +++++++++++++++++++++++++++++-------------------
> include/scsi/scsi.h | 2 +
> 2 files changed, 87 insertions(+), 55 deletions(-)
>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index aea5a1ae318b..e1e974f08d52 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -738,6 +738,73 @@ static blk_status_t __scsi_error_from_host_byte(struct scsi_cmnd *cmd,
> }
> }
>
> +/* Helper for scsi_io_completion() when cmd->result is non-zero. */
> +static int scsi_io_completion_nz_result(struct scsi_cmnd *cmd,
> + blk_status_t *blk_statp)
> +{
> + bool sense_valid;
> + bool about_current;
> + int result = cmd->result;
> + struct request *req = cmd->request;
> + struct scsi_sense_hdr sshdr;
> +
> + sense_valid = scsi_command_normalize_sense(cmd, &sshdr);
> + about_current = sense_valid ? !scsi_sense_is_deferred(&sshdr) : true;
> +
> + if (blk_rq_is_passthrough(req)) {
> + if (sense_valid) {
> + /*
> + * SG_IO wants current and deferred errors
> + */
> + scsi_req(req)->sense_len =
> + min(8 + cmd->sense_buffer[7],
> + SCSI_SENSE_BUFFERSIZE);
> + }
> + if (about_current)
> + *blk_statp = __scsi_error_from_host_byte(cmd, result);
> + } else if (blk_rq_bytes(req) == 0 && about_current) {
> + /*
> + * Flush commands do not transfers any data, and thus cannot use
> + * good_bytes != blk_rq_bytes(req) as the signal for an error.
> + * This sets the error explicitly for the problem case.
> + */
> + *blk_statp = __scsi_error_from_host_byte(cmd, result);
> + }
> + /*
> + * Recovered errors need reporting, but they're always treated as
> + * success, so fiddle the result code here. For passthrough requests
> + * we already took a copy of the original into sreq->result which
> + * is what gets returned to the user
> + */
> + if (sense_valid && (sshdr.sense_key == RECOVERED_ERROR)) {
> + /*
> + * if ATA PASS-THROUGH INFORMATION AVAILABLE skip
> + * print since caller wants ATA registers. Only occurs
> + * on SCSI ATA PASS_THROUGH commands when CK_COND=1
> + */
> + if ((sshdr.asc == 0x0) && (sshdr.ascq == 0x1d))
> + ;
> + else if (!(req->rq_flags & RQF_QUIET))
> + scsi_print_sense(cmd);
> + result = 0;
> + *blk_statp = BLK_STS_OK;
> + /* for passthrough, blk_stat may be set */
> + }
> + /*
> + * Another corner case: the SCSI status byte is non-zero but 'good'.
> + * Example: PRE-FETCH command returns SAM_STAT_CONDITION_MET when
> + * it is able to fit nominated LBs in its cache (and SAM_STAT_GOOD
> + * if it can't fit). Treat SAM_STAT_CONDITION_MET and the related
> + * intermediate statuses (both obsolete in SAM-4) as good.
> + */
> + if (status_byte(result) && scsi_status_is_good(result)) {
> + result = 0;
> + /* for passthrough error may be set */
> + *blk_statp = BLK_STS_OK;
> + }
> + return result;
> +}
> +
> /*
> * Function: scsi_io_completion()
> *
Hmm. Can't we return blk_stat from this function, and adjusting the
'result' value after it with an if-clause like
if (blk_stat == BLK_STS_OK)
result = 0;
That would cleanup up the function and avoid having (essentially) two
return values.
The only problem here is that __scsi_error_from_hostbyte() will return
BLK_STS_IOERR if result == 0; doubt that is intended.
And I guess it'll affect this issue, too.
Mind sending a separate patch for that?
> @@ -772,33 +839,23 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
> int result = cmd->result;
> struct request_queue *q = cmd->device->request_queue;
> struct request *req = cmd->request;
> - blk_status_t error = BLK_STS_OK;
> + blk_status_t blk_stat = BLK_STS_OK; /* enum, BLK_STS_OK is 0 */
> struct scsi_sense_hdr sshdr;
> - bool sense_valid = false;
> - int sense_deferred = 0, level = 0;
> + bool sense_valid_and_current = false;
> + int level = 0;
> enum {ACTION_FAIL, ACTION_REPREP, ACTION_RETRY,
> ACTION_DELAYED_RETRY} action;
> unsigned long wait_for = (cmd->allowed + 1) * req->timeout;
>
> if (result) {
> - sense_valid = scsi_command_normalize_sense(cmd, &sshdr);
> - if (sense_valid)
> - sense_deferred = scsi_sense_is_deferred(&sshdr);
> + /* sense not about current command is termed: deferred */
> + if (scsi_command_normalize_sense(cmd, &sshdr) &&
> + !scsi_sense_is_deferred(&sshdr))
> + sense_valid_and_current = true;
> + result = scsi_io_completion_nz_result(cmd, &blk_stat);
> }
>
> if (blk_rq_is_passthrough(req)) {
> - if (result) {
> - if (sense_valid) {
> - /*
> - * SG_IO wants current and deferred errors
> - */
> - scsi_req(req)->sense_len =
> - min(8 + cmd->sense_buffer[7],
> - SCSI_SENSE_BUFFERSIZE);
> - }
> - if (!sense_deferred)
> - error = __scsi_error_from_host_byte(cmd, result);
> - }
> /*
> * __scsi_error_from_host_byte may have reset the host_byte
> */
> @@ -816,13 +873,6 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
> BUG();
> return;
> }
> - } else if (blk_rq_bytes(req) == 0 && result && !sense_deferred) {
> - /*
> - * Flush commands do not transfers any data, and thus cannot use
> - * good_bytes != blk_rq_bytes(req) as the signal for an error.
> - * This sets the error explicitly for the problem case.
> - */
> - error = __scsi_error_from_host_byte(cmd, result);
> }
>
> /* no bidi support for !blk_rq_is_passthrough yet */
> @@ -836,40 +886,20 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
> "%u sectors total, %d bytes done.\n",
> blk_rq_sectors(req), good_bytes));
>
> - /*
> - * Recovered errors need reporting, but they're always treated as
> - * success, so fiddle the result code here. For passthrough requests
> - * we already took a copy of the original into sreq->result which
> - * is what gets returned to the user
> - */
> - if (sense_valid && (sshdr.sense_key == RECOVERED_ERROR)) {
> - /* if ATA PASS-THROUGH INFORMATION AVAILABLE skip
> - * print since caller wants ATA registers. Only occurs on
> - * SCSI ATA PASS_THROUGH commands when CK_COND=1
> - */
> - if ((sshdr.asc == 0x0) && (sshdr.ascq == 0x1d))
> - ;
> - else if (!(req->rq_flags & RQF_QUIET))
> - scsi_print_sense(cmd);
> - result = 0;
> - /* for passthrough error may be set */
> - error = BLK_STS_OK;
> - }
> -
> /*
> * special case: failed zero length commands always need to
> * drop down into the retry code. Otherwise, if we finished
> * all bytes in the request we are done now.
> */
> - if (!(blk_rq_bytes(req) == 0 && error) &&
> - !scsi_end_request(req, error, good_bytes, 0))
> + if (!(blk_rq_bytes(req) == 0 && blk_stat) &&
> + !scsi_end_request(req, blk_stat, good_bytes, 0))
> return;
>
> /*
> * Kill remainder if no retrys.
> */
> - if (error && scsi_noretry_cmd(cmd)) {
> - if (scsi_end_request(req, error, blk_rq_bytes(req), 0))
> + if (blk_stat && scsi_noretry_cmd(cmd)) {
> + if (scsi_end_request(req, blk_stat, blk_rq_bytes(req), 0))
> BUG();
> return;
> }
> @@ -881,7 +911,7 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
> if (result == 0)
> goto requeue;
>
> - error = __scsi_error_from_host_byte(cmd, result);
> + blk_stat = __scsi_error_from_host_byte(cmd, result);
>
> if (host_byte(result) == DID_RESET) {
> /* Third party bus reset or reset for error recovery
> @@ -889,7 +919,7 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
> * happens.
> */
> action = ACTION_RETRY;
> - } else if (sense_valid && !sense_deferred) {
> + } else if (sense_valid_and_current) {
> switch (sshdr.sense_key) {
> case UNIT_ATTENTION:
> if (cmd->device->removable) {
> @@ -925,18 +955,18 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
> action = ACTION_REPREP;
> } else if (sshdr.asc == 0x10) /* DIX */ {
> action = ACTION_FAIL;
> - error = BLK_STS_PROTECTION;
> + blk_stat = BLK_STS_PROTECTION;
> /* INVALID COMMAND OPCODE or INVALID FIELD IN CDB */
> } else if (sshdr.asc == 0x20 || sshdr.asc == 0x24) {
> action = ACTION_FAIL;
> - error = BLK_STS_TARGET;
> + blk_stat = BLK_STS_TARGET;
> } else
> action = ACTION_FAIL;
> break;
> case ABORTED_COMMAND:
> action = ACTION_FAIL;
> if (sshdr.asc == 0x10) /* DIF */
> - error = BLK_STS_PROTECTION;
> + blk_stat = BLK_STS_PROTECTION;
> break;
> case NOT_READY:
> /* If the device is in the process of becoming
> @@ -999,7 +1029,7 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
> scsi_print_command(cmd);
> }
> }
> - if (!scsi_end_request(req, error, blk_rq_err_bytes(req), 0))
> + if (!scsi_end_request(req, blk_stat, blk_rq_err_bytes(req), 0))
> return;
> /*FALLTHRU*/
> case ACTION_REPREP:
> diff --git a/include/scsi/scsi.h b/include/scsi/scsi.h
> index cb85eddb47ea..eb7853c1a23b 100644
> --- a/include/scsi/scsi.h
> +++ b/include/scsi/scsi.h
> @@ -47,6 +47,8 @@ static inline int scsi_status_is_good(int status)
> */
> status &= 0xfe;
> return ((status == SAM_STAT_GOOD) ||
> + (status == SAM_STAT_CONDITION_MET) ||
> + /* Next two "intermediate" statuses are obsolete in SAM-4 */
> (status == SAM_STAT_INTERMEDIATE) ||
> (status == SAM_STAT_INTERMEDIATE_CONDITION_MET) ||
> /* FIXME: this is obsolete in SAM-3 */
>
Other than that it's a nice cleanup.
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@xxxxxxx +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
