[PATCH] mm: Warn once when a page is freed with PG_mlocked set (resend)

[PATCH] mm: Warn once when a page is freed with PG_mlocked set (resend)

[Mel Gorman]

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: Mel Gorman <mel@csn.ul.ie>
--- 
 mm/page_alloc.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index caa9268..f8902e7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -495,8 +495,16 @@ static inline void free_page_mlock(struct page *page)
 static void free_page_mlock(struct page *page) { }
 #endif
 
-static inline int free_pages_check(struct page *page)
-{
+static inline int free_pages_check(struct page *page, int wasMlocked)
+{
+	if (unlikely(wasMlocked)) {
+		WARN_ONCE(1, KERN_WARNING
+			"Page flag mlocked set for process %s at pfn:%05lx\n"
+			"page:%p flags:0x%lX\n",
+			current->comm, page_to_pfn(page),
+			page, page->flags|__PG_MLOCKED);
+	}
+
 	if (unlikely(page_mapcount(page) |
 		(page->mapping != NULL)  |
 		(atomic_read(&page->_count) != 0) |
@@ -562,7 +570,7 @@ static void __free_pages_ok(struct page *page, unsigned int order)
 	kmemcheck_free_shadow(page, order);
 
 	for (i = 0 ; i < (1 << order) ; ++i)
-		bad += free_pages_check(page + i);
+		bad += free_pages_check(page + i, wasMlocked);
 	if (bad)
 		return;
 
@@ -1027,7 +1035,7 @@ static void free_hot_cold_page(struct page *page, int cold)
 
 	if (PageAnon(page))
 		page->mapping = NULL;
-	if (free_pages_check(page))
+	if (free_pages_check(page, wasMlocked))
 		return;
 
 	if (!PageHighMem(page)) {

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Re: [PATCH] mm: Warn once when a page is freed with PG_mlocked set (resend)

[Johannes Weiner]

Hello Mel,

On Wed, Jul 15, 2009 at 11:48:34AM +0100, Mel Gorman wrote:
> 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: Mel Gorman <mel@csn.ul.ie>
> --- 
>  mm/page_alloc.c |   16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index caa9268..f8902e7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -495,8 +495,16 @@ static inline void free_page_mlock(struct page *page)
>  static void free_page_mlock(struct page *page) { }
>  #endif
>  
> -static inline int free_pages_check(struct page *page)
> -{
> +static inline int free_pages_check(struct page *page, int wasMlocked)
> +{
> +	if (unlikely(wasMlocked)) {
> +		WARN_ONCE(1, KERN_WARNING
> +			"Page flag mlocked set for process %s at pfn:%05lx\n"
> +			"page:%p flags:0x%lX\n",
> +			current->comm, page_to_pfn(page),
> +			page, page->flags|__PG_MLOCKED);

Since the warning is the only action in this branch, wouldn't
WARN_ONCE(wasMlocked, KERN_WARNING ...) be better?

	Hannes

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Re: [PATCH] mm: Warn once when a page is freed with PG_mlocked set (resend)

[Mel Gorman]

On Wed, Jul 15, 2009 at 02:45:09PM +0200, Johannes Weiner wrote:
> Hello Mel,
> 
> On Wed, Jul 15, 2009 at 11:48:34AM +0100, Mel Gorman wrote:
> > 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: Mel Gorman <mel@csn.ul.ie>
> > --- 
> >  mm/page_alloc.c |   16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index caa9268..f8902e7 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -495,8 +495,16 @@ static inline void free_page_mlock(struct page *page)
> >  static void free_page_mlock(struct page *page) { }
> >  #endif
> >  
> > -static inline int free_pages_check(struct page *page)
> > -{
> > +static inline int free_pages_check(struct page *page, int wasMlocked)
> > +{
> > +	if (unlikely(wasMlocked)) {
> > +		WARN_ONCE(1, KERN_WARNING
> > +			"Page flag mlocked set for process %s at pfn:%05lx\n"
> > +			"page:%p flags:0x%lX\n",
> > +			current->comm, page_to_pfn(page),
> > +			page, page->flags|__PG_MLOCKED);
> 
> Since the warning is the only action in this branch, wouldn't
> WARN_ONCE(wasMlocked, KERN_WARNING ...) be better?
> 

It was to have the unlikely() but now that I look at WARN_ONCE(), it
looks like it does the unlikely() part already. Will send another
version. Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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