Re: [PATCH RESEND] scsi: Set sg_tablesize to 1, for LLDDs that set SG_NONE

[Hannes Reinecke <hare@suse.de>]

On 11/10/2015 11:45 PM, Manoj Kumar wrote:
> Oops while testing blk_mq over the new cxlflash driver.
> 
> [ 2960.817172] Oops: Kernel access of bad area, sig: 11 [#5]
> [ 2960.817309] NIP  __blk_mq_run_hw_queue+0x278/0x4c0
> [ 2960.817313] LR __blk_mq_run_hw_queue+0x2bc/0x4c0
> [ 2960.817314] Call Trace:
> [ 2960.817320] __blk_mq_run_hw_queue+0x2bc/0x4c0 (unreliable)
> [ 2960.817324] blk_mq_run_hw_queue+0xd8/0x100
> [ 2960.817329] blk_mq_insert_requests+0x14c/0x1f0
> [ 2960.817333] blk_mq_flush_plug_list+0x150/0x190
> [ 2960.817338] blk_flush_plug_list+0x11c/0x2b0
> [ 2960.817344] blk_finish_plug+0x58/0x80
> [ 2960.817348] __do_page_cache_readahead+0x1c0/0x2e0
> [ 2960.817352] force_page_cache_readahead+0x68/0xd0
> [ 2960.817356] generic_file_read_iter+0x43c/0x6a0
> [ 2960.817359] blkdev_read_iter+0x68/0xa0
> [ 2960.817361] __vfs_read+0x11c/0x180
> [ 2960.817364] vfs_read+0xa4/0x1c0
> [ 2960.817366] SyS_read+0x6c/0x110
> [ 2960.817369] system_call+0x38/0xb4
> 
> The root cause of the problem was this low level device driver(LLDD),
> in this case cxlflash, does not support scatter-gather and hence had
> set it's sg_tablesize to SG_NONE (value of 0). In reality the tablesize
> is of length 1. This value of SG_NONE does not cause any problems with
> the standard block driver stack but causes issues for blk_mq, as shown
> above. Since quite a few of the legacy LLDDs are setting sg_tablesize
> to SG_NONE, it was preferable to override the LLDD provided value in
> scsi_host_alloc().
> 
> Signed-off-by: Manoj N. Kumar <manoj@linux.vnet.ibm.com>
> Signed-off-by: Youngjae Lee <leeyo@linux.vnet.ibm.com>
> Reviewed-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
> ---
>  drivers/scsi/hosts.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index 8bb173e..bd13c9d 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -413,7 +413,7 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
>  	shost->hostt = sht;
>  	shost->this_id = sht->this_id;
>  	shost->can_queue = sht->can_queue;
> -	shost->sg_tablesize = sht->sg_tablesize;
> +	shost->sg_tablesize = (sht->sg_tablesize ? sht->sg_tablesize : 1);
>  	shost->sg_prot_tablesize = sht->sg_prot_tablesize;
>  	shost->cmd_per_lun = sht->cmd_per_lun;
>  	shost->unchecked_isa_dma = sht->unchecked_isa_dma;
> 
Shouldn't we rather fixup blk-mq to properly support SG_NONE?
Silently converting SG_NONE (=0) to 1 has a fair chance of breaking
non-mq enabled setups, which happily work with SG_NONE currently.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@suse.de			               +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)
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