ksm: stop hotremove lockdep warning

Complaints are rare, but lockdep still does not understand the way
ksm_memory_callback(MEM_GOING_OFFLINE) takes ksm_thread_mutex, and holds
it until the ksm_memory_callback(MEM_OFFLINE): that appears to be a
problem because notifier callbacks are made under down_read of
blocking_notifier_head->rwsem (so first the mutex is taken while holding
the rwsem, then later the rwsem is taken while still holding the mutex);
but is not in fact a problem because mem_hotplug_mutex is held
throughout the dance.

There was an attempt to fix this with mutex_lock_nested(); but if that
happened to fool lockdep two years ago, apparently it does so no longer.

I had hoped to eradicate this issue in extending KSM page migration not
to need the ksm_thread_mutex.  But then realized that although the page
migration itself is safe, we do still need to lock out ksmd and other
users of get_ksm_page() while offlining memory - at some point between
MEM_GOING_OFFLINE and MEM_OFFLINE, the struct pages themselves may
vanish, and get_ksm_page()'s accesses to them become a violation.

So, give up on holding ksm_thread_mutex itself from MEM_GOING_OFFLINE to
MEM_OFFLINE, and add a KSM_RUN_OFFLINE flag, and wait_while_offlining()
checks, to achieve the same lockout without being caught by lockdep.
This is less elegant for KSM, but it's more important to keep lockdep
useful to other users - and I apologize for how long it took to fix.

Signed-off-by: Hugh Dickins <[email protected]>
Reported-by: Gerald Schaefer <[email protected]>
Tested-by: Gerald Schaefer <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Petr Holasek <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions