From ef4d43a8075891137ebc064e473d2e904971e933 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Fri, 22 Feb 2013 16:35:16 -0800 Subject: [PATCH] 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 Reported-by: Gerald Schaefer Tested-by: Gerald Schaefer Cc: Rik van Riel Cc: Petr Holasek Cc: Andrea Arcangeli Cc: Izik Eidus Cc: KOSAKI Motohiro Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Reading git-format-patch failed