On Fri, 2013-01-25 at 18:10 -0800, Hugh Dickins wrote:
> 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.
>
> Reported-by: Gerald Schaefer <gerald.schae...@de.ibm.com>
> Signed-off-by: Hugh Dickins <hu...@google.com>
> ---
> mm/ksm.c | 55 +++++++++++++++++++++++++++++++++++++++--------------
> 1 file changed, 41 insertions(+), 14 deletions(-)
>
> --- mmotm.orig/mm/ksm.c 2013-01-25 14:37:06.880206290 -0800
> +++ mmotm/mm/ksm.c 2013-01-25 14:38:53.984208836 -0800
> @@ -226,7 +226,9 @@ static unsigned int ksm_merge_across_nod
> #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 *newpa
> #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 no
> 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
Why unmerge_ksm_pages beneath us is safe for ksm memory hotremove?
> + * 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 no
> /* 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
> */
>
> 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(
> 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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