This is v3. It follows Martin's review of v2: needs_ro_update() now
fetches missing dm info instead of assuming a reload is needed, and
force_readonly is cleared when a load reads it rather than only when
the load succeeds. The changes are listed at the end.
Ben, I dropped your Reviewed-by because the needs_ro_update() logic
changed. The no-dm-info case reverses the "assume it needs to be
reloaded" behavior from 6e40bf92, so please check that you're OK
with it.
A multipath map can stay read-only after write protect is cleared on
the storage, even though all of its paths are read-write. It happens
when the map's paths are removed and re-added while write protect is
set:
1. Write protect is set. The map is reloaded read-only.
uev_update_path() sets force_readonly for that reload and clears it
afterwards.
2. All of the map's paths are removed, and new paths are added while
write protect is still set. ev_add_path() sees a read-only path and
sets force_readonly = 1. Nothing ever clears it.
3. Write protect is cleared on the new paths, one after another. Each
path's DISK_RO=0 uevent is ignored while any path of the map is
still read-only.
4. The last path's uevent reloads the map. The reload sees the stale
force_readonly = 1 and loads the map read-only, logging "reloaded
(retval 0)".
force_readonly only works in one direction: ev_add_path() can set it
but never clears it. If the new paths had been added already writable,
ev_add_path() wouldn't have set the flag and the map wouldn't have
stuck. The paths have to be added while write protected and then
cleared in place.
After step 4 nothing reloads the map again. A rescan doesn't change the
paths' ro state, so it generates no DISK_RO uevent. And since
multipathd now sees the map as read-only, needs_ro_update() also
ignores the next DISK_RO=1 uevent for it.
If two paths' uevents happen to be handled after all paths are
writable, the map does recover: uev_update_path() clears
force_readonly after its reload, so the second reload is read-write.
That makes the problem look intermittent.
Example
-------
We hit this on RHEL 9 device-mapper-multipath 0.8.7, whose
ev_add_path() and uev_update_path() handle force_readonly the same way
as current master. The host's iSCSI sessions were torn down and
re-established while the array was write protecting the volume. From
the messages log, trimmed:
05:48:27 ...114d8: reload [0 51200 multipath 0 1 alua 1 1
service-time 0 6 1 135:272 1 66:624 1 69:720 1 129:560 1
132:656 1 135:752 1]
05:48:27 sdsd: map ...114d8 reloaded (retval 0)
05:51:04 kernel: sd 24:0:0:48: [sdaer] Write Protect is on
05:51:05 ...114d8: remaining active paths: 0
05:51:07 kernel: sd 29:0:0:48: [sdaoi] Write Protect is on
(and four more new paths)
06:09:01 kernel: sd 24:0:0:48: [sdaer] Write Protect is off
06:09:07 kernel: sd 29:0:0:48: [sdaoi] Write Protect is off
06:09:10 sdaoi: update path write_protect to '0' (uevent)
06:09:10 ...114d8: reload [0 51200 multipath 0 1 alua 1 1
service-time 0 6 1 67:880 1 70:992 1 130:848 1 133:960 1
65:1072 1 67:1056 1]
06:09:10 sdaoi: map ...114d8 reloaded (retval 0)
The state afterwards, as recorded by the test:
dm-106: ro
sdaer: rw sdagu: rw sdaix: rw sdala: rw sdand: rw sdaoi: rw
The patch
---------
The patch sets force_readonly right before each load from the state of
the map's paths, and dm_addmap_create() and dm_addmap_reload() clear it
as soon as they have read it. needs_ro_update() sets it for the reload
done for a DISK_RO uevent, and ev_add_path() and update_map() compute
it from all of the map's paths. With the patch, the 06:09:10 reload
would have found no read-only path and loaded the map read-write.
Changes since v2:
- In needs_ro_update(), fetch the dm info with dm_get_info() if the
map has none, and return false if that fails, instead of assuming a
reload is needed. This also removes the separate path walk for that
case.
- Clear force_readonly in dm_addmap_create() and dm_addmap_reload() as
soon as they have read it, instead of in dm_addmap() on success. A
failed load can no longer leave the flag set for a later, unrelated
reload, such as one for a path group change. needs_ro_update() no
longer needs to clear the flag itself.
- I didn't add per-path tracking of the ro state. force_readonly is
only computed from sysfs right before a load, which is rare, so the
cost of reading sysfs there is small. Per-path tracking could be a
follow-up if you want it.
Changes since v1:
- Set force_readonly in needs_ro_update() when it returns true,
instead of in uev_update_path(). The DISK_RO=1 case uses the uevent
again, rather than rereading sysfs.
- Clear force_readonly in dm_addmap() on success, as for need_reload,
instead of after the loads in ev_add_path() and uev_update_path().
- In ev_add_path(), only compute force_readonly when need_do_map is
set.
- Walk mpp->pg rather than mpp->paths in needs_ro_update(), since
mpp->paths may be stale there. ev_add_path() and update_map() still
use mpp->paths: both call adopt_paths() first, and mpp->pg doesn't
contain the new paths until setup_map().
- Move sysfs_get_ro() above has_ro_path() instead of adding a forward
declaration. The function is unchanged.
Brian Bunker (1):
multipathd: don't leave maps read-only after write protect is cleared
libmultipath/devmapper.c | 17 +++++--
multipathd/main.c | 104 +++++++++++++++++++++++----------------
2 files changed, 75 insertions(+), 46 deletions(-)
--
2.56.0
