This is v6. It follows Ben's review of v5: needs_ro_update() no longer
takes an ro value and no longer tries to avoid reading the paths' ro
state from sysfs. It recomputes force_readonly from all of the map's
paths on every call, which removes the special cases v5 needed. The
changes are listed at the end.
As agreed, tracking the ro state per path and dropping force_readonly
altogether, as Martin and Uday Shankar suggested, will be a separate
series on top of this fix.
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. dm_addmap_create() clears it as soon as it has read
it, and dm_addmap_reload() clears it once the reload has succeeded.
needs_ro_update() sets it for the reload done for a change uevent,
from the ro state of all of the map's paths in sysfs, 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 v5:
- needs_ro_update() no longer takes an ro value. As Ben suggested, it
sets force_readonly from map_has_ro_path() on every call. If the map
is already read-only and has to stay so, it clears the flag and
returns false, since dm has to reopen the paths for any read-write
reload anyway. Otherwise it returns true if the flag differs from
the map's current read-only state. This fixes the v5 case where the
map was read-write, a path was still read-only, and nothing was
reloaded, and it drops the "keep the flag after a failed read-only
reload" special case: the flag is recomputed on every change uevent,
so a failed reload is retried on the next one.
- uev_update_path() no longer reads the path's ro state itself, from
sysfs or from the uevent's DISK_RO. It just calls needs_ro_update().
The uevent still only tells us that something changed, and sysfs,
which the kernel updates before it sends the uevent, is the source
of the ro state. The "update path write_protect" message now prints
the new state from force_readonly.
- A path whose ro attribute can't be read in sysfs counts as not
read-only, instead of making uev_update_path() do nothing.
- The rest of the patch (dm_addmap_create(), dm_addmap_reload(),
update_map(), ev_add_path()) is unchanged from v5.
Changes since v4:
- dm_addmap_reload() no longer clears force_readonly up front and sets
it back on failure. It clears the flag only once the reload and
resume have succeeded, as Ben suggested. A failed reload leaves it
set, so the next reload tries read-only again.
- uev_update_path() takes the path's ro state from sysfs instead of
from the uevent's DISK_RO. The uevent only says that something
changed. The kernel updates the ro attribute before it sends the
uevent, so sysfs is current when the uevent is handled, while the
uevent itself may have been queued behind later changes.
Note that this changes behavior: any change uevent for a path in a
map, not only one with DISK_RO, can now trigger a read-only reload
if sysfs disagrees with the map. It also avoids the pointless
reload that 6e40bf92 describes when a rescan's DISK_RO=0 and
DISK_RO=1 uevents are handled after the rescan has finished, since
sysfs then shows read-only for both. The cost is a sysfs read of
each path of the map per change uevent, next to the pathinfo() call
uev_update_path() already makes.
- dm_addmap_create() is unchanged and keeps the read-only shortcut.
Changes since v3:
- If a reload fails, dm_addmap_reload() sets force_readonly back to
the value it had on entry. The map still has its old table, so a map
that was supposed to become read-only is tried again on the next
reload.
- Move the walk over the map's path groups into map_has_ro_path().
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.
- 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.
- 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.
Changes since v1:
- Set force_readonly in needs_ro_update() when it returns true,
instead of in uev_update_path().
- 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 | 18 ++++-
multipathd/main.c | 144 ++++++++++++++++++++++++---------------
2 files changed, 105 insertions(+), 57 deletions(-)
--
2.56.0
