On Thu, 2022-02-17 at 09:09 +1100, NeilBrown wrote: > On Thu, 17 Feb 2022, mwilck@xxxxxxxx wrote: > > From: Martin Wilck <mwilck@xxxxxxxx> > > > > device-mapper sets the flag DM_UDEV_DISABLE_OTHER_RULES_FLAG to 1 > > for > > devices which are unusable. They may be no set up yet, suspended, > > or > > otherwise unusable (e.g. multipath maps without usable path). This > > flag does not necessarily imply SYSTEMD_READY=0 and must therefore > > be tested separately. > > I really don't like this - looks like a hack. A Kludge. These are strong words. You didn't go into detail, so I'm assuming that your reasoning is that DM_UDEV_DISABLE_OTHER_RULES_FLAG is an internal flag of the device-mapper subsystem. Still, you can see that is's used both internally by dm, and by other subsystems: https://github.com/lvmteam/lvm2/blob/8dccc2314e2482370bc6e5cf007eb210994abdef/udev/13-dm-disk.rules.in#L15 https://github.com/g2p/bcache-tools/blob/a73679b22c333763597d39c72112ef5a53f55419/69-bcache.rules#L6 https://github.com/opensvc/multipath-tools/blob/d9d7ae9e2125116b465b4ff4d98ce65fe0eac3cc/kpartx/kpartx.rules#L10 Would you call these others "hacks", too? > Can you provide a reference to a detailed discussion that explains > why > SYSTEMD_READY=0 cannot be used? The main reason is that SYSTEMD_READY=0 is set too late, in 99-systemd- rules, and only on "add" events: https://github.com/systemd/systemd/blob/bfae960e53f6986f1c4d234ea82681d0acad57df/rules.d/99-systemd.rules.in#L18 I guess the device-mapper rules themselves could be setting SYSTEMD_READY="0". @Peter Rajnoha, do you want to comment on that? My concern wrt such a change would be possible side effects. Setting SYSTEMD_READY=0 on "change" events could actually be wrong, see below. I the case I was observing, there was a multipath device without valid paths, which had switched to queueing mode [*]. If this happens for whatever reason (and it could be something else, like a suspended DM device), IO on such a device hangs. IO that may hang must not be attempted from an udev rule. Therefore it makes sense that layers stacked on top of DM try to avoid it, and checking udev properties set by DM is a reasonable way to do that. The core of the problem is that there is no well-defined "API" specifying how different udev rule sets can communicate, iow which udev properties are well-defined enough to be consumed outside of the subsystem that defines them. SYSTEMD_READY is about the only "global" property. IMO it's somewhat overloaded: The actual semantics of SYSTEMD_READY=0 is "systemd shouldn't activate the associated device unit". Various udev rule sets use it with similar but not 100% identical semantics like "don't touch this" or "don't probe this". In the case I was looking at, the device had already been activated by systemd. Later, the device had lost all active paths and thus became unusable. We can't easily set SYSTEMD_READY=0 on such a device. Doing so would actually be dangerous, because systemd might remove the device. Moreover, while processing the udev rule, we just don't know if the problem is temporary or permanent. Other properties, like those set by the DM subsystem, are less well- defined. There's no official spec defining their names and semantics, and there are multiple flags which aren't easly differentiated (DM_UDEV_DISABLE_DISK_RULES_FLAG, DM_UDEV_DISABLE_OTHER_RULES_FLAG, DM_NOSCAN, DM_SUSPENDED, MPATH_DEVICE_READY). OTOH, most of these flags have been around for many years without changing, and thus have acquired the status of a semi-official API, which is actually used in other rule sets. In particular DM_UDEV_DISABLE_OTHER_RULES_FLAG has a few users, see above. I believe this is for good reason, and therefore I don't consider my patch a "hack". Regards Martin [*] How that came to pass is subtle by itself, and admittedly not fully understood. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel
