On Thu, Apr 25 2013 at 10:50am -0400, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > On Thu, 25 Apr 2013, Mike Snitzer wrote: > > > On Thu, Apr 25 2013 at 9:48am -0400, > > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > > > > > > > > > On Mon, 22 Apr 2013, Mike Snitzer wrote: > > > > > > > I spoke with Hannes at LSF, to address the potential crashes in the > > > > endio path (e.g. stpg_endio) we'd have to bump the scsi_dh_data kref > > > > where appropriate (e.g. for ALUA kref_get in submit_stpg and kref_put in > > > > stpg_endio). > > > > > > > > But that is just the tip of the iceberg relative to scsi_dh lifetime. > > > > Seems we've been playing it pretty fast and loose with scsi_dh issued > > > > requests vs detach for quite some time. > > > > > > > > I'm now inclined to not care about this issue. Take away is: don't > > > > switch the device handler (attach the correct one from the start). > > > > > > I did a patch that disables device handler switching and it was NACKed by > > > Hannes. The problem that he pointed out was - when we load SCSI device > > > handler modules, they attach automatically to SCSI devices they think they > > > belong to. The user then can't set the desired device handler in multipath > > > configuration because a different handler is already attached. > > > > The handler that is automatically attached _should_ be the correct > > handler. We now have the .match() hook for scsi_dh and it has made for > > reliable scsi_dh attachment of the correct handler. > > The EMC devices work with both ALUA and EMC handlers - so there is no one > "correct" handler, the correct handler is the one that the user specified > in multipath configuration. > > > > So we need a functionality to change device handlers. > > > > I really cannot think of a sequence where the scsi_dh .match() will > > attach the incorrect handler. This is why I added the > > "retain_attached_hw_handler" feature to mpath (commit a58a935d5). > > The automatic handler assigment can't change existing handler. > > But if one handler was automatically selected and the user selects a > different handler in multipath configuration, the handler is changed. > > > > (or maybe stop the scsi device handlers from attaching automatically, but > > > it would surely generate a lot of other regressions) > > > > The need to support changing device handlers (via multipath table load) > > is overblown/historic. > > So - do you mean that we make "retain_attached_hw_handler" the default > option and don't allow the user to change existing device handler in > multipath configuration? > > That's what my patch did and it was NACKed by Hannes. The problem there is > that behavior depends on module loading order - if you activate multipath > with "EMC" option, it activates the EMC handler. If you load the ALUA > module and activate multipath with "EMC" option, it stays with the ALUA > handler. .match allows for correct scsi_dh selection in the decision of alua vs emc (alua has the tpgs bit set) -- but both scsi_dh modules must be loaded. If the incorrect handler is getting attached then it is either a bug in the .match method (for the handler that should've been attached) or the storage isn't configured how the user thought and they need to adjust/reconfigure to have it be like they expected. Either way we really _could_ impose not allowing the scsi_dh handler to be changed (by multipath) -- which is why I Acked your patch. There is always the scsi_dh sysfs interface to allow the user to change the scsi_dh (and possibly shoot themselves in the foot). -- 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
