[PATCH] mm: fix balloon_page_movable() page->flags check

[PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

This patch fixes the following crash by fixing and enhancing the way 
page->flags are tested to identify a ballooned page.

---8<---
BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
--->8---

The NULL pointer deref was taking place because balloon_page_movable()
page->flags tests were incomplete and we ended up 
inadvertently poking at private pages.

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 include/linux/balloon_compaction.h | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 68893bc..634a19b 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -107,6 +107,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping)
 }
 
 /*
+ * __balloon_page_flags - helper to perform balloon @page ->flags tests.
+ *
+ * As balloon pages are got from Buddy, and we do not play with page->flags
+ * at driver level (exception made when we get the page lock for compaction),
+ * therefore we can safely identify a ballooned page by checking if the
+ * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
+ * This approach also helps on skipping ballooned pages that are locked for
+ * compaction or release, thus mitigating their racy check at
+ * balloon_page_movable()
+ */
+#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
+static inline bool __balloon_page_flags(struct page *page)
+{
+	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
+}
+
+/*
  * __is_movable_balloon_page - helper to perform @page mapping->flags tests
  */
 static inline bool __is_movable_balloon_page(struct page *page)
@@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
 	 * Before dereferencing and testing mapping->flags, lets make sure
 	 * this is not a page that uses ->mapping in a different way
 	 */
-	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
-	    !page_mapped(page))
+	if (__balloon_page_flags(page) && !page_mapped(page) &&
+	    page_count(page) == 1)
 		return __is_movable_balloon_page(page);
 
 	return false;
-- 
1.7.11.7

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[PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

This patch fixes the following crash by fixing and enhancing the way 
page->flags are tested to identify a ballooned page.

---8<---
BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
--->8---

The NULL pointer deref was taking place because balloon_page_movable()
page->flags tests were incomplete and we ended up 
inadvertently poking at private pages.

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 include/linux/balloon_compaction.h | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 68893bc..634a19b 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -107,6 +107,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping)
 }
 
 /*
+ * __balloon_page_flags - helper to perform balloon @page ->flags tests.
+ *
+ * As balloon pages are got from Buddy, and we do not play with page->flags
+ * at driver level (exception made when we get the page lock for compaction),
+ * therefore we can safely identify a ballooned page by checking if the
+ * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
+ * This approach also helps on skipping ballooned pages that are locked for
+ * compaction or release, thus mitigating their racy check at
+ * balloon_page_movable()
+ */
+#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
+static inline bool __balloon_page_flags(struct page *page)
+{
+	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
+}
+
+/*
  * __is_movable_balloon_page - helper to perform @page mapping->flags tests
  */
 static inline bool __is_movable_balloon_page(struct page *page)
@@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
 	 * Before dereferencing and testing mapping->flags, lets make sure
 	 * this is not a page that uses ->mapping in a different way
 	 */
-	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
-	    !page_mapped(page))
+	if (__balloon_page_flags(page) && !page_mapped(page) &&
+	    page_count(page) == 1)
 		return __is_movable_balloon_page(page);
 
 	return false;
-- 
1.7.11.7

Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

On Tue, Nov 27, 2012 at 09:31:10PM -0200, Rafael Aquini wrote:
> This patch fixes the following crash by fixing and enhancing the way 
> page->flags are tested to identify a ballooned page.
> 
> ---8<---
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> --->8---
> 
> The NULL pointer deref was taking place because balloon_page_movable()
> page->flags tests were incomplete and we ended up 
> inadvertently poking at private pages.
> 
> Reported-by: Sasha Levin <levinsasha928@gmail.com>
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---

Here it is Andrew, sorry by the lagged reply

Cheers!
--Rafael


>  include/linux/balloon_compaction.h | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 68893bc..634a19b 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -107,6 +107,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping)
>  }
>  
>  /*
> + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> + *
> + * As balloon pages are got from Buddy, and we do not play with page->flags
> + * at driver level (exception made when we get the page lock for compaction),
> + * therefore we can safely identify a ballooned page by checking if the
> + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> + * This approach also helps on skipping ballooned pages that are locked for
> + * compaction or release, thus mitigating their racy check at
> + * balloon_page_movable()
> + */
> +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
> +static inline bool __balloon_page_flags(struct page *page)
> +{
> +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
> +}
> +
> +/*
>   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
>   */
>  static inline bool __is_movable_balloon_page(struct page *page)
> @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
>  	 * Before dereferencing and testing mapping->flags, lets make sure
>  	 * this is not a page that uses ->mapping in a different way
>  	 */
> -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> -	    !page_mapped(page))
> +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> +	    page_count(page) == 1)
>  		return __is_movable_balloon_page(page);
>  
>  	return false;
> -- 
> 1.7.11.7
> 

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Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

On Tue, Nov 27, 2012 at 09:31:10PM -0200, Rafael Aquini wrote:
> This patch fixes the following crash by fixing and enhancing the way 
> page->flags are tested to identify a ballooned page.
> 
> ---8<---
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> --->8---
> 
> The NULL pointer deref was taking place because balloon_page_movable()
> page->flags tests were incomplete and we ended up 
> inadvertently poking at private pages.
> 
> Reported-by: Sasha Levin <levinsasha928@gmail.com>
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---

Here it is Andrew, sorry by the lagged reply

Cheers!
--Rafael


>  include/linux/balloon_compaction.h | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 68893bc..634a19b 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -107,6 +107,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping)
>  }
>  
>  /*
> + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> + *
> + * As balloon pages are got from Buddy, and we do not play with page->flags
> + * at driver level (exception made when we get the page lock for compaction),
> + * therefore we can safely identify a ballooned page by checking if the
> + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> + * This approach also helps on skipping ballooned pages that are locked for
> + * compaction or release, thus mitigating their racy check at
> + * balloon_page_movable()
> + */
> +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
> +static inline bool __balloon_page_flags(struct page *page)
> +{
> +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
> +}
> +
> +/*
>   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
>   */
>  static inline bool __is_movable_balloon_page(struct page *page)
> @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
>  	 * Before dereferencing and testing mapping->flags, lets make sure
>  	 * this is not a page that uses ->mapping in a different way
>  	 */
> -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> -	    !page_mapped(page))
> +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> +	    page_count(page) == 1)
>  		return __is_movable_balloon_page(page);
>  
>  	return false;
> -- 
> 1.7.11.7
> 

Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Andrew Morton]

On Tue, 27 Nov 2012 21:31:10 -0200
Rafael Aquini <aquini@redhat.com> wrote:

> This patch fixes the following crash by fixing and enhancing the way 
> page->flags are tested to identify a ballooned page.
> 
> ---8<---
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> --->8---
> 
> The NULL pointer deref was taking place because balloon_page_movable()
> page->flags tests were incomplete and we ended up 
> inadvertently poking at private pages.
> 
> ....
>
>  /*
> + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> + *
> + * As balloon pages are got from Buddy, and we do not play with page->flags
> + * at driver level (exception made when we get the page lock for compaction),
> + * therefore we can safely identify a ballooned page by checking if the
> + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> + * This approach also helps on skipping ballooned pages that are locked for
> + * compaction or release, thus mitigating their racy check at
> + * balloon_page_movable()
> + */
> +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)

hm, this seems a bit fragile.

What's actually going on here?  You're assuming that a page fresh from
buddy has all flags 0..NR_PAGEFLAGS cleared?

That may be true, I'm unsure.  Please take a look at
PAGE_FLAGS_CHECK_AT_FREE and PAGE_FLAGS_CHECK_AT_PREP and work out why
the heck these aren't the same thing!

Either way around, doing

	#define BALLOON_PAGE_FLAGS_MASK PAGE_FLAGS_CHECK_AT_PREP

seems rather more maintainable.

> +static inline bool __balloon_page_flags(struct page *page)
> +{
> +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;

We have a neater way of doing the scalar-to-boolean conversion:

	return !(page->flags & BALLOON_PAGE_FLAGS_MASK);

> +}
> +
> +/*
>   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
>   */
>  static inline bool __is_movable_balloon_page(struct page *page)
> @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
>  	 * Before dereferencing and testing mapping->flags, lets make sure
>  	 * this is not a page that uses ->mapping in a different way
>  	 */
> -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> -	    !page_mapped(page))
> +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> +	    page_count(page) == 1)
>  		return __is_movable_balloon_page(page);
>  
>  	return false;
>
> ...
>

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Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Andrew Morton]

On Tue, 27 Nov 2012 21:31:10 -0200
Rafael Aquini <aquini@redhat.com> wrote:

> This patch fixes the following crash by fixing and enhancing the way 
> page->flags are tested to identify a ballooned page.
> 
> ---8<---
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> --->8---
> 
> The NULL pointer deref was taking place because balloon_page_movable()
> page->flags tests were incomplete and we ended up 
> inadvertently poking at private pages.
> 
> ....
>
>  /*
> + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> + *
> + * As balloon pages are got from Buddy, and we do not play with page->flags
> + * at driver level (exception made when we get the page lock for compaction),
> + * therefore we can safely identify a ballooned page by checking if the
> + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> + * This approach also helps on skipping ballooned pages that are locked for
> + * compaction or release, thus mitigating their racy check at
> + * balloon_page_movable()
> + */
> +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)

hm, this seems a bit fragile.

What's actually going on here?  You're assuming that a page fresh from
buddy has all flags 0..NR_PAGEFLAGS cleared?

That may be true, I'm unsure.  Please take a look at
PAGE_FLAGS_CHECK_AT_FREE and PAGE_FLAGS_CHECK_AT_PREP and work out why
the heck these aren't the same thing!

Either way around, doing

	#define BALLOON_PAGE_FLAGS_MASK PAGE_FLAGS_CHECK_AT_PREP

seems rather more maintainable.

> +static inline bool __balloon_page_flags(struct page *page)
> +{
> +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;

We have a neater way of doing the scalar-to-boolean conversion:

	return !(page->flags & BALLOON_PAGE_FLAGS_MASK);

> +}
> +
> +/*
>   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
>   */
>  static inline bool __is_movable_balloon_page(struct page *page)
> @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
>  	 * Before dereferencing and testing mapping->flags, lets make sure
>  	 * this is not a page that uses ->mapping in a different way
>  	 */
> -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> -	    !page_mapped(page))
> +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> +	    page_count(page) == 1)
>  		return __is_movable_balloon_page(page);
>  
>  	return false;
>
> ...
>

Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

On Tue, Nov 27, 2012 at 03:52:01PM -0800, Andrew Morton wrote:
> On Tue, 27 Nov 2012 21:31:10 -0200
> Rafael Aquini <aquini@redhat.com> wrote:
> 
> > This patch fixes the following crash by fixing and enhancing the way 
> > page->flags are tested to identify a ballooned page.
> > 
> > ---8<---
> > BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> > IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> > --->8---
> > 
> > The NULL pointer deref was taking place because balloon_page_movable()
> > page->flags tests were incomplete and we ended up 
> > inadvertently poking at private pages.
> > 
> > ....
> >
> >  /*
> > + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> > + *
> > + * As balloon pages are got from Buddy, and we do not play with page->flags
> > + * at driver level (exception made when we get the page lock for compaction),
> > + * therefore we can safely identify a ballooned page by checking if the
> > + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> > + * This approach also helps on skipping ballooned pages that are locked for
> > + * compaction or release, thus mitigating their racy check at
> > + * balloon_page_movable()
> > + */
> > +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
> 
> hm, this seems a bit fragile.
> 
> What's actually going on here?  You're assuming that a page fresh from
> buddy has all flags 0..NR_PAGEFLAGS cleared?
>
 
Yes, thats the idea, as when we get pages from freelists, they are all checked 
by prep_new_page()->check_new_page() before buffered_rmqueue() returns them.
By the path we use to get balloon pages, if the page has any of 0..NR_PAGEFLAGS
flags set at the alloc time it's regarded as bad_page by check_new_page(),
and we go after another victim.


> That may be true, I'm unsure.  Please take a look at
> PAGE_FLAGS_CHECK_AT_FREE and PAGE_FLAGS_CHECK_AT_PREP and work out why
> the heck these aren't the same thing!

Humm, I can't think of why, either... As I've followed the prep path, I didn't
notice that difference. I'll look at it closer, though.


>
> Either way around, doing
> 
> 	#define BALLOON_PAGE_FLAGS_MASK PAGE_FLAGS_CHECK_AT_PREP
> 
> seems rather more maintainable.

As usual, your suggestion is far way better than my orinal proposal :)


> 
> > +static inline bool __balloon_page_flags(struct page *page)
> > +{
> > +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
> 
> We have a neater way of doing the scalar-to-boolean conversion:
> 
> 	return !(page->flags & BALLOON_PAGE_FLAGS_MASK);
> 

ditto! :)


Do you want me to resubmit this patch with the changes you suggested?


Cheers!
Rafael

> > +}
> > +
> > +/*
> >   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
> >   */
> >  static inline bool __is_movable_balloon_page(struct page *page)
> > @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
> >  	 * Before dereferencing and testing mapping->flags, lets make sure
> >  	 * this is not a page that uses ->mapping in a different way
> >  	 */
> > -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> > -	    !page_mapped(page))
> > +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> > +	    page_count(page) == 1)
> >  		return __is_movable_balloon_page(page);
> >  
> >  	return false;
> >
> > ...
> >

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Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

On Tue, Nov 27, 2012 at 03:52:01PM -0800, Andrew Morton wrote:
> On Tue, 27 Nov 2012 21:31:10 -0200
> Rafael Aquini <aquini@redhat.com> wrote:
> 
> > This patch fixes the following crash by fixing and enhancing the way 
> > page->flags are tested to identify a ballooned page.
> > 
> > ---8<---
> > BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> > IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> > --->8---
> > 
> > The NULL pointer deref was taking place because balloon_page_movable()
> > page->flags tests were incomplete and we ended up 
> > inadvertently poking at private pages.
> > 
> > ....
> >
> >  /*
> > + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> > + *
> > + * As balloon pages are got from Buddy, and we do not play with page->flags
> > + * at driver level (exception made when we get the page lock for compaction),
> > + * therefore we can safely identify a ballooned page by checking if the
> > + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> > + * This approach also helps on skipping ballooned pages that are locked for
> > + * compaction or release, thus mitigating their racy check at
> > + * balloon_page_movable()
> > + */
> > +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
> 
> hm, this seems a bit fragile.
> 
> What's actually going on here?  You're assuming that a page fresh from
> buddy has all flags 0..NR_PAGEFLAGS cleared?
>
 
Yes, thats the idea, as when we get pages from freelists, they are all checked 
by prep_new_page()->check_new_page() before buffered_rmqueue() returns them.
By the path we use to get balloon pages, if the page has any of 0..NR_PAGEFLAGS
flags set at the alloc time it's regarded as bad_page by check_new_page(),
and we go after another victim.


> That may be true, I'm unsure.  Please take a look at
> PAGE_FLAGS_CHECK_AT_FREE and PAGE_FLAGS_CHECK_AT_PREP and work out why
> the heck these aren't the same thing!

Humm, I can't think of why, either... As I've followed the prep path, I didn't
notice that difference. I'll look at it closer, though.


>
> Either way around, doing
> 
> 	#define BALLOON_PAGE_FLAGS_MASK PAGE_FLAGS_CHECK_AT_PREP
> 
> seems rather more maintainable.

As usual, your suggestion is far way better than my orinal proposal :)


> 
> > +static inline bool __balloon_page_flags(struct page *page)
> > +{
> > +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
> 
> We have a neater way of doing the scalar-to-boolean conversion:
> 
> 	return !(page->flags & BALLOON_PAGE_FLAGS_MASK);
> 

ditto! :)


Do you want me to resubmit this patch with the changes you suggested?


Cheers!
Rafael

> > +}
> > +
> > +/*
> >   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
> >   */
> >  static inline bool __is_movable_balloon_page(struct page *page)
> > @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
> >  	 * Before dereferencing and testing mapping->flags, lets make sure
> >  	 * this is not a page that uses ->mapping in a different way
> >  	 */
> > -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> > -	    !page_mapped(page))
> > +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> > +	    page_count(page) == 1)
> >  		return __is_movable_balloon_page(page);
> >  
> >  	return false;
> >
> > ...
> >

Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Andrew Morton]

On Tue, 27 Nov 2012 22:34:10 -0200 Rafael Aquini <aquini@redhat.com> wrote:

> Do you want me to resubmit this patch with the changes you suggested?

oh, I think I can reach that far.  How's this look?

From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check-fix

use PAGE_FLAGS_CHECK_AT_PREP, s/__balloon_page_flags/page_flags_cleared/, small cleanups

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/balloon_compaction.h |   21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff -puN include/linux/balloon_compaction.h~mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check-fix include/linux/balloon_compaction.h
--- a/include/linux/balloon_compaction.h~mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check-fix
+++ a/include/linux/balloon_compaction.h
@@ -41,6 +41,7 @@
 #ifndef _LINUX_BALLOON_COMPACTION_H
 #define _LINUX_BALLOON_COMPACTION_H
 #include <linux/pagemap.h>
+#include <linux/page-flags.h>
 #include <linux/migrate.h>
 #include <linux/gfp.h>
 #include <linux/err.h>
@@ -109,18 +110,16 @@ static inline void balloon_mapping_free(
 /*
  * __balloon_page_flags - helper to perform balloon @page ->flags tests.
  *
- * As balloon pages are got from Buddy, and we do not play with page->flags
+ * As balloon pages are obtained from buddy and we do not play with page->flags
  * at driver level (exception made when we get the page lock for compaction),
- * therefore we can safely identify a ballooned page by checking if the
- * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
- * This approach also helps on skipping ballooned pages that are locked for
- * compaction or release, thus mitigating their racy check at
- * balloon_page_movable()
+ * we can safely identify a ballooned page by checking if the
+ * PAGE_FLAGS_CHECK_AT_PREP page->flags are all cleared.  This approach also
+ * helps us skip ballooned pages that are locked for compaction or release, thus
+ * mitigating their racy check at balloon_page_movable()
  */
-#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
-static inline bool __balloon_page_flags(struct page *page)
+static inline bool page_flags_cleared(struct page *page)
 {
-	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
+	return !(page->flags & PAGE_FLAGS_CHECK_AT_PREP);
 }
 
 /*
@@ -149,10 +148,10 @@ static inline bool __is_movable_balloon_
 static inline bool balloon_page_movable(struct page *page)
 {
 	/*
-	 * Before dereferencing and testing mapping->flags, lets make sure
+	 * Before dereferencing and testing mapping->flags, let's make sure
 	 * this is not a page that uses ->mapping in a different way
 	 */
-	if (__balloon_page_flags(page) && !page_mapped(page) &&
+	if (page_flags_cleared(page) && !page_mapped(page) &&
 	    page_count(page) == 1)
 		return __is_movable_balloon_page(page);
 
_

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Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Andrew Morton]

On Tue, 27 Nov 2012 22:34:10 -0200 Rafael Aquini <aquini@redhat.com> wrote:

> Do you want me to resubmit this patch with the changes you suggested?

oh, I think I can reach that far.  How's this look?

From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check-fix

use PAGE_FLAGS_CHECK_AT_PREP, s/__balloon_page_flags/page_flags_cleared/, small cleanups

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/balloon_compaction.h |   21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff -puN include/linux/balloon_compaction.h~mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check-fix include/linux/balloon_compaction.h
--- a/include/linux/balloon_compaction.h~mm-introduce-a-common-interface-for-balloon-pages-mobility-mm-fix-balloon_page_movable-page-flags-check-fix
+++ a/include/linux/balloon_compaction.h
@@ -41,6 +41,7 @@
 #ifndef _LINUX_BALLOON_COMPACTION_H
 #define _LINUX_BALLOON_COMPACTION_H
 #include <linux/pagemap.h>
+#include <linux/page-flags.h>
 #include <linux/migrate.h>
 #include <linux/gfp.h>
 #include <linux/err.h>
@@ -109,18 +110,16 @@ static inline void balloon_mapping_free(
 /*
  * __balloon_page_flags - helper to perform balloon @page ->flags tests.
  *
- * As balloon pages are got from Buddy, and we do not play with page->flags
+ * As balloon pages are obtained from buddy and we do not play with page->flags
  * at driver level (exception made when we get the page lock for compaction),
- * therefore we can safely identify a ballooned page by checking if the
- * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
- * This approach also helps on skipping ballooned pages that are locked for
- * compaction or release, thus mitigating their racy check at
- * balloon_page_movable()
+ * we can safely identify a ballooned page by checking if the
+ * PAGE_FLAGS_CHECK_AT_PREP page->flags are all cleared.  This approach also
+ * helps us skip ballooned pages that are locked for compaction or release, thus
+ * mitigating their racy check at balloon_page_movable()
  */
-#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
-static inline bool __balloon_page_flags(struct page *page)
+static inline bool page_flags_cleared(struct page *page)
 {
-	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
+	return !(page->flags & PAGE_FLAGS_CHECK_AT_PREP);
 }
 
 /*
@@ -149,10 +148,10 @@ static inline bool __is_movable_balloon_
 static inline bool balloon_page_movable(struct page *page)
 {
 	/*
-	 * Before dereferencing and testing mapping->flags, lets make sure
+	 * Before dereferencing and testing mapping->flags, let's make sure
 	 * this is not a page that uses ->mapping in a different way
 	 */
-	if (__balloon_page_flags(page) && !page_mapped(page) &&
+	if (page_flags_cleared(page) && !page_mapped(page) &&
 	    page_count(page) == 1)
 		return __is_movable_balloon_page(page);
 
_

Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

On Tue, Nov 27, 2012 at 05:15:44PM -0800, Andrew Morton wrote:
> On Tue, 27 Nov 2012 22:34:10 -0200 Rafael Aquini <aquini@redhat.com> wrote:
> 
> > Do you want me to resubmit this patch with the changes you suggested?
> 
> oh, I think I can reach that far.  How's this look?
>

It looks great to me.

Just a small nitpick, 
here __balloon_page_flags should be changed to page_flags_cleared too:
> @@ -109,18 +110,16 @@ static inline void balloon_mapping_free(
>  /*
>   * __balloon_page_flags - helper to perform balloon @page ->flags tests.
>   *

Thanks!
--Rafael

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Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini]

On Tue, Nov 27, 2012 at 05:15:44PM -0800, Andrew Morton wrote:
> On Tue, 27 Nov 2012 22:34:10 -0200 Rafael Aquini <aquini@redhat.com> wrote:
> 
> > Do you want me to resubmit this patch with the changes you suggested?
> 
> oh, I think I can reach that far.  How's this look?
>

It looks great to me.

Just a small nitpick, 
here __balloon_page_flags should be changed to page_flags_cleared too:
> @@ -109,18 +110,16 @@ static inline void balloon_mapping_free(
>  /*
>   * __balloon_page_flags - helper to perform balloon @page ->flags tests.
>   *

Thanks!
--Rafael