Is it safe for kthreadd to drain_all_pages?

[Hugh Dickins <hughd@google.com>]

Hi Mel,

I suspect that it's not safe for kthreadd to drain_all_pages();
but I haven't studied flush_work() etc, so don't really know what
I'm talking about: hoping that you will jump to a realization.

4.11-rc has been giving me hangs after hours of swapping load.  At
first they looked like memory leaks ("fork: Cannot allocate memory");
but for no good reason I happened to do "cat /proc/sys/vm/stat_refresh"
before looking at /proc/meminfo one time, and the stat_refresh stuck
in D state, waiting for completion of flush_work like many kworkers.
kthreadd waiting for completion of flush_work in drain_all_pages().

But I only noticed that pattern later: originally tried to bisect
rc1 before rc2 came out, but underestimated how long to wait before
deciding a stage good - I thought 12 hours, but would now say 2 days.
Too late for bisection, I suspect your drain_all_pages() changes.

(I've also found order:0 page allocation stalls in /var/log/messages,
148804ms a nice example: which suggest that these hangs are perhaps a
condition it can sometimes get out of itself.  None with the patch.)

Patch below has been running well for 36 hours now:
a bit too early to be sure, but I think it's time to turn to you.


[PATCH] mm: don't let kthreadd drain_all_pages

4.11-rc has been giving me hangs after many hours of swapping load: most
kworkers waiting for completion of a flush_work, kthreadd waiting for
completion of flush_work in drain_all_pages (while doing copy_process).
I suspect that kthreadd should not be allowed to drain_all_pages().

Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/page_alloc.c |    2 ++
 1 file changed, 2 insertions(+)

--- 4.11-rc5/mm/page_alloc.c	2017-03-13 09:08:37.743209168 -0700
+++ linux/mm/page_alloc.c	2017-04-04 00:33:44.086867413 -0700
@@ -2376,6 +2376,8 @@ void drain_all_pages(struct zone *zone)
 	/* Workqueues cannot recurse */
 	if (current->flags & PF_WQ_WORKER)
 		return;
+	if (current == kthreadd_task)
+		return;
 
 	/*
 	 * Do not drain if one is already in progress unless it's specific to

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