On 12/13/2013 09:49 AM, Bob Liu wrote:

On 12/13/2013 05:05 AM, Sasha Levin wrote:

quoted

On 12/12/2013 07:41 AM, Vlastimil Babka wrote:

quoted

On 12/12/2013 06:03 AM, Bob Liu wrote:

quoted

On 12/12/2013 11:16 AM, Sasha Levin wrote:

quoted

On 12/11/2013 05:59 PM, Vlastimil Babka wrote:

quoted

On 12/09/2013 09:26 PM, Sasha Levin wrote:

quoted

On 12/09/2013 12:12 PM, Vlastimil Babka wrote:

quoted

On 12/09/2013 06:05 PM, Sasha Levin wrote:

quoted

On 12/09/2013 04:34 AM, Vlastimil Babka wrote:

quoted

Hello, I will look at it, thanks.
Do you have specific reproduction instructions?

Not really, the fuzzer hit it once and I've been unable to trigger
it again. Looking at
the piece of code involved it might have had something to do with
hugetlbfs, so I'll crank
up testing on that part.

Thanks. Do you have trinity log and the .config file? I'm currently
unable to even boot linux-next
with my config/setup due to a GPF.
Looking at code I wouldn't expect that it could encounter a tail
page, without first encountering a
head page and skipping the whole huge page. At least in THP case, as
TLB pages should be split when
a vma is split. As for hugetlbfs, it should be skipped for
mlock/munlock operations completely. One
of these assumptions is probably failing here...

If it helps, I've added a dump_page() in case we hit a tail page
there and got:

[  980.172299] page:ffffea003e5e8040 count:0 mapcount:1
mapping:          (null) index:0
x0
[  980.173412] page flags: 0x2fffff80008000(tail)

I can also add anything else in there to get other debug output if
you think of something else useful.

Please try the following. Thanks in advance.

[  428.499889] page:ffffea003e5c0040 count:0 mapcount:4
mapping:          (null) index:0x0
[  428.499889] page flags: 0x2fffff80008000(tail)
[  428.499889] start=140117131923456 pfn=16347137
orig_start=140117130543104 page_increm
=1 vm_start=140117130543104 vm_end=140117134688256 vm_flags=135266419
[  428.499889] first_page pfn=16347136
[  428.499889] page:ffffea003e5c0000 count:204 mapcount:44
mapping:ffff880fb5c466c1 inde
x:0x7f6f8fe00
[  428.499889] page flags:
0x2fffff80084068(uptodate|lru|active|head|swapbacked)

   From this print, it looks like the page is still a huge page.
One situation I guess is a huge page which isn't PageMlocked and passed
to munlock_vma_page(). I'm not sure whether this will happen.

Yes that's quite likely the case. It's not illegal to happen I would say.

quoted

Please take a try this patch.

I've made a simpler version that does away with the ugly page_mask
thing completely.
Please try that as well. Thanks.

Also when working on this I think I found another potential but much
rare problem
when munlock_vma_page races with a THP split. That would however
manifest such that
part of the former tail pages would stay PageMlocked. But that still
needs more thought.
The bug at hand should however be fixed by this patch.

Yup, this patch seems to fix the issue previously reported.

However, I'll piggyback another thing that popped up now that the vm
could run for a while which
also seems to be caused by the original patch. It looks like a pretty
straightforward deadlock, but

Sigh, put one down, patch it around... :)

Looks like put_page() in __munlock_pagevec() need to get the
zone->lru_lock which is already held when entering __munlock_pagevec().

I've come to the same conclusion, however:

How about fix like this?

That unfortunately removes most of the purpose of this function which was to avoid repeated locking. 

Please try this patch.

-------8<-------
From: Vlastimil Babka <redacted>
Date: Fri, 13 Dec 2013 10:03:25 +0100
Subject: [PATCH] Deadlock in __munlock_pagevec candidate fix

---
 mm/mlock.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index a34dfdc..c97273e 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c

@@ -281,10 +281,12 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
 {
 	int i;
 	int nr = pagevec_count(pvec);
-	int delta_munlocked = -nr;
+	int delta_munlocked;
 	struct pagevec pvec_putback;
 	int pgrescued = 0;
 
+	pagevec_init(&pvec_putback, 0);
+
 	/* Phase 1: page isolation */
 	spin_lock_irq(&zone->lru_lock);
 	for (i = 0; i < nr; i++) {

@@ -313,16 +315,22 @@ skip_munlock:
 			/*
 			 * We won't be munlocking this page in the next phase
 			 * but we still need to release the follow_page_mask()
-			 * pin.
+			 * pin. We cannot do it under lru_lock however. If it's
+			 * the last pin, __page_cache_release would deadlock.
 			 */
+			pagevec_add(&pvec_putback, pvec->pages[i]);
 			pvec->pages[i] = NULL;
-			put_page(page);
-			delta_munlocked++;
 		}
 	}
+	delta_munlocked = -nr + pagevec_count(&pvec_putback);
 	__mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
 	spin_unlock_irq(&zone->lru_lock);
 
+	/* Now we can release pins of pages that we are not munlocking */
+	for (i = 0; i < pagevec_count(&pvec_putback); i++) {
+		put_page(pvec_putback.pages[i]);
+	}
+
 	/* Phase 2: page munlock */
 	pagevec_init(&pvec_putback, 0);
 	for (i = 0; i < nr; i++) {

-- 
1.8.4


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