Commit ef4d43a8 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

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: default avatarHugh Dickins <hughd@google.com>
Reported-by: default avatarGerald Schaefer <gerald.schaefer@de.ibm.com>
Tested-by: default avatarGerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Petr Holasek <pholasek@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Izik Eidus <izik.eidus@ravellosystems.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9c620e2b
......@@ -226,7 +226,9 @@ static unsigned int ksm_merge_across_nodes = 1;
#define KSM_RUN_STOP 0
#define KSM_RUN_MERGE 1
#define KSM_RUN_UNMERGE 2
static unsigned int ksm_run = KSM_RUN_STOP;
#define KSM_RUN_OFFLINE 4
static unsigned long ksm_run = KSM_RUN_STOP;
static void wait_while_offlining(void);
static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
static DEFINE_MUTEX(ksm_thread_mutex);
......@@ -1700,6 +1702,7 @@ static int ksm_scan_thread(void *nothing)
while (!kthread_should_stop()) {
mutex_lock(&ksm_thread_mutex);
wait_while_offlining();
if (ksmd_should_run())
ksm_do_scan(ksm_thread_pages_to_scan);
mutex_unlock(&ksm_thread_mutex);
......@@ -2056,6 +2059,22 @@ void ksm_migrate_page(struct page *newpage, struct page *oldpage)
#endif /* CONFIG_MIGRATION */
#ifdef CONFIG_MEMORY_HOTREMOVE
static int just_wait(void *word)
{
schedule();
return 0;
}
static void wait_while_offlining(void)
{
while (ksm_run & KSM_RUN_OFFLINE) {
mutex_unlock(&ksm_thread_mutex);
wait_on_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE),
just_wait, TASK_UNINTERRUPTIBLE);
mutex_lock(&ksm_thread_mutex);
}
}
static void ksm_check_stable_tree(unsigned long start_pfn,
unsigned long end_pfn)
{
......@@ -2098,15 +2117,15 @@ static int ksm_memory_callback(struct notifier_block *self,
switch (action) {
case MEM_GOING_OFFLINE:
/*
* Keep it very simple for now: just lock out ksmd and
* MADV_UNMERGEABLE while any memory is going offline.
* mutex_lock_nested() is necessary because lockdep was alarmed
* that here we take ksm_thread_mutex inside notifier chain
* mutex, and later take notifier chain mutex inside
* ksm_thread_mutex to unlock it. But that's safe because both
* are inside mem_hotplug_mutex.
* Prevent ksm_do_scan(), unmerge_and_remove_all_rmap_items()
* and remove_all_stable_nodes() while memory is going offline:
* it is unsafe for them to touch the stable tree at this time.
* But unmerge_ksm_pages(), rmap lookups and other entry points
* which do not need the ksm_thread_mutex are all safe.
*/
mutex_lock_nested(&ksm_thread_mutex, SINGLE_DEPTH_NESTING);
mutex_lock(&ksm_thread_mutex);
ksm_run |= KSM_RUN_OFFLINE;
mutex_unlock(&ksm_thread_mutex);
break;
case MEM_OFFLINE:
......@@ -2122,11 +2141,20 @@ static int ksm_memory_callback(struct notifier_block *self,
/* fallthrough */
case MEM_CANCEL_OFFLINE:
mutex_lock(&ksm_thread_mutex);
ksm_run &= ~KSM_RUN_OFFLINE;
mutex_unlock(&ksm_thread_mutex);
smp_mb(); /* wake_up_bit advises this */
wake_up_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE));
break;
}
return NOTIFY_OK;
}
#else
static void wait_while_offlining(void)
{
}
#endif /* CONFIG_MEMORY_HOTREMOVE */
#ifdef CONFIG_SYSFS
......@@ -2189,7 +2217,7 @@ KSM_ATTR(pages_to_scan);
static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
char *buf)
{
return sprintf(buf, "%u\n", ksm_run);
return sprintf(buf, "%lu\n", ksm_run);
}
static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
......@@ -2212,6 +2240,7 @@ static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
*/
mutex_lock(&ksm_thread_mutex);
wait_while_offlining();
if (ksm_run != flags) {
ksm_run = flags;
if (flags & KSM_RUN_UNMERGE) {
......@@ -2254,6 +2283,7 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj,
return -EINVAL;
mutex_lock(&ksm_thread_mutex);
wait_while_offlining();
if (ksm_merge_across_nodes != knob) {
if (ksm_pages_shared || remove_all_stable_nodes())
err = -EBUSY;
......@@ -2366,10 +2396,7 @@ static int __init ksm_init(void)
#endif /* CONFIG_SYSFS */
#ifdef CONFIG_MEMORY_HOTREMOVE
/*
* Choose a high priority since the callback takes ksm_thread_mutex:
* later callbacks could only be taking locks which nest within that.
*/
/* There is no significance to this priority 100 */
hotplug_memory_notifier(ksm_memory_callback, 100);
#endif
return 0;
......
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