ksm: unmerge is an origin of OOMs

Just as the swapoff system call allocates many pages of RAM to various
processes, perhaps triggering OOM, so "echo 2 >/sys/kernel/mm/ksm/run"
(unmerge) is liable to allocate many pages of RAM to various processes,
perhaps triggering OOM; and each is normally run from a modest admin
process (swapoff or shell), easily repeated until it succeeds.

So treat unmerge_and_remove_all_rmap_items() in the same way that we treat
try_to_unuse(): generalize PF_SWAPOFF to PF_OOM_ORIGIN, and bracket both
with that, to ask the OOM killer to kill them first, to prevent them from
spawning more and more OOM kills.
Signed-off-by: NHugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: NIzik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -1720,7 +1720,7 @@ extern cputime_t task_gtime(struct task_struct *p);
 #define PF_FROZEN	0x00010000	/* frozen for system suspend */
 #define PF_FSTRANS	0x00020000	/* inside a filesystem transaction */
 #define PF_KSWAPD	0x00040000	/* I am kswapd */
-#define PF_SWAPOFF	0x00080000	/* I am in swapoff */
+#define PF_OOM_ORIGIN	0x00080000	/* Allocating much memory to others */
 #define PF_LESS_THROTTLE 0x00100000	/* Throttle me less: I clean memory */
 #define PF_KTHREAD	0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE	0x00400000	/* randomize virtual address space */

mm/ksm.c

@@ -1557,7 +1557,9 @@ static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
 	if (ksm_run != flags) {
 		ksm_run = flags;
 		if (flags & KSM_RUN_UNMERGE) {
+			current->flags |= PF_OOM_ORIGIN;
 			err = unmerge_and_remove_all_rmap_items();
+			current->flags &= ~PF_OOM_ORIGIN;
 			if (err) {
 				ksm_run = KSM_RUN_STOP;
 				count = err;

mm/oom_kill.c

@@ -79,7 +79,7 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
 	/*
 	 * swapoff can easily use up all memory, so kill those first.
 	 */
-	if (p->flags & PF_SWAPOFF)
+	if (p->flags & PF_OOM_ORIGIN)
 		return ULONG_MAX;
 
 	/*

mm/swapfile.c

@@ -1575,9 +1575,9 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->flags &= ~SWP_WRITEOK;
 	spin_unlock(&swap_lock);
 
-	current->flags |= PF_SWAPOFF;
+	current->flags |= PF_OOM_ORIGIN;
 	err = try_to_unuse(type);
-	current->flags &= ~PF_SWAPOFF;
+	current->flags &= ~PF_OOM_ORIGIN;
 
 	if (err) {
 		/* re-insert swap space back into swap_list */