On Thu, Nov 26, 2015 at 4:13 PM, Hannes Reinecke <hare@xxxxxxx> wrote: > Hi all, > > while investigating the crash in scsi_lib.c I found a rather curious > behaviour for QUEUE_FLAG_NO_SG_MERGE. > > While the flag is evaluated in blk_recalc_rq_segments and > blk_recount_segments (resulting in nr_phys_segments being > computed based on that flag) it is completely ignored > during blk_rq_map_sg() or the actual merging itself. Yes, I guess Jens introduced the flag for decreasing CPU consumption when comuputing segments, but it is still ignored by blk_rq_map_sg(), but it may not be used by some drivers. After bio splitting is introduced, the flag is also ignored when computing segments. > > This typically shouldn't be an issue, seeing that with > QUEUE_FLAG_NO_SG_MERGE nr_phys_segments will always be > larger than the actual segment count. > > However, it still makes me wonder: > What is the point of having a QUEUE_FLAG_NO_SG_MERGE > which doesn't work as advertised? > Or, to be precise, which only works for blk-mq? > Should we make it work for non-block-mq, too? Thanks bio splitting, this flag has little effect on performance now, so I think it can be removed if Jens has no objection. Thanks, > > > Cheers, > > Hannes > -- > Dr. Hannes Reinecke zSeries & Storage > 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) > -- > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
