[PATCH 1/2] mm: Warn once when a page is freed with PG_mlocked

[Ying Han <yinghan@google.com>]

I am resending this patch orginally from Mel, and the reason we spotted this
is due to the next patch where I am adding the mlock stat into per-memcg
meminfo. We found out that it is impossible to update the counter if the page
is in the freeing patch w/ mlocked bit set.

Then we started wondering if it is possible at all. It shouldn't happen that
freeing a mlocked page without going through munlock_vma_pages_all(). Looks
like it did happen few years ago, and here is the patch introduced it

commit 985737cf2ea096ea946aed82c7484d40defc71a8
Author: Lee Schermerhorn <lee.schermerhorn@hp.com>
Date:   Sat Oct 18 20:26:53 2008 -0700

    mlock: count attempts to free mlocked page

There are two ways to persue and I would like to ask people's opinion:

1. revert the patch totally and the page will get into bad_page(). Then we
get the report as well.

2. fix up the page like the patch does but put on warn_once() to report the
problem.

People might feel more confident by doing step by step which adding the
warn_on() first and then revert it later. So I resend the patch from Mel and
here is the patch:

When a page is freed with the PG_mlocked set, it is considered an unexpected
but recoverable situation. A counter records how often this event happens
but it is easy to miss that this event has occured at all. This patch warns
once when PG_mlocked is set to prompt debuggers to check the counter to
see how often it is happening.

Signed-off-by: Ying Han <yinghan@google.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/page_alloc.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a712fb9..4f905af 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -599,6 +599,11 @@ out:
  */
 static inline void free_page_mlock(struct page *page)
 {
+	WARN_ONCE(1, KERN_WARNING
+		"Page flag mlocked set for process %s at pfn:%05lx\n"
+		"page:%p flags:%#lx\n",
+		current->comm, page_to_pfn(page),
+		page, page->flags|__PG_MLOCKED);
 	__dec_zone_page_state(page, NR_MLOCK);
 	__count_vm_event(UNEVICTABLE_MLOCKFREED);
 }
-- 
1.7.7.3

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