On Mon 09-03-20 16:42:43, Vlastimil Babka wrote:

On 3/9/20 4:19 PM, Oleksandr Natalenko wrote:

quoted

On Mon, Mar 09, 2020 at 04:08:15PM +0100, Michal Hocko wrote:

quoted

On Mon 09-03-20 14:11:17, Oleksandr Natalenko wrote:

quoted

On Fri, Mar 06, 2020 at 05:08:18PM +0100, Vlastimil Babka wrote:

[...]

quoted

quoted

Dunno, it's nice to react to signals quickly, for any proces that gets them, no?

So, do you mean something like this?

===

diff --git a/mm/ksm.c b/mm/ksm.c
index 363ec8189561..b39c237cfcf4 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c

@@ -849,7 +849,8 @@ static int unmerge_ksm_pages(struct vm_area_struct *vma,
 	for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
 		if (ksm_test_exit(vma->vm_mm))
 			break;
-		if (signal_pending(current))
+		if (signal_pending(current) ||
+		    signal_pending(rcu_dereference(vma->vm_mm->owner)))
 			err = -ERESTARTSYS;
 		else
 			err = break_ksm(vma, addr);

===

This is broken because mm might be attached to different tasks.
AFAIU this check is meant to allow quick backoff of the _calling_
process so that it doesn't waste time when the context is killed
already. I do not understand why should we care about any other context
here? What is the actual problem this would solve?

I agree with you, but still trying to understand what does Vlastimil mean
:).

Well you wondered if we should stop caring about current, and I said that
probably wouldn't be nice.
As for caring about the other task, patch 3/7 does that for
(MADV_COLD|MADV_PAGEOUT) so I just pointed out that the KSM case doesn't. AFAIU
if we don't check the signals, we might be blocking the killed task from exiting?

I would have to double check but I do not think this would be a problem
because the remote task should take mmget to prevent address space to
vanish under its feet. That should also rule out the exclusive mmap_sem
usage from the exit path.
-- 
Michal Hocko
SUSE Labs