Re: [PATCH] mm:add VM_BUG_ON() for page_mapcount()

Re: [PATCH] mm:add VM_BUG_ON() for page_mapcount()

[Hillf Danton]

> 
> This patch add VM_BUG_ON() for slab page,
> because _mapcount is an union with slab struct in struct page,
> avoid access _mapcount if this page is a slab page.
> Also remove the unneeded bracket.
> 
> Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
> ---
>  include/linux/mm.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 11b65cf..34124c4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -373,7 +373,8 @@ static inline void reset_page_mapcount(struct page *page)
> 
>  static inline int page_mapcount(struct page *page)
>  {
> -	return atomic_read(&(page)->_mapcount) + 1;
> +	VM_BUG_ON(PageSlab(page));

s/ VM_BUG_ON/ VM_BUG_ON_PAGE/ ?

> +	return atomic_read(&page->_mapcount) + 1;
>  }
> 

RE: [PATCH] mm:add VM_BUG_ON() for page_mapcount()

[Wang, Yalin]

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> -----Original Message-----
> From: Hillf Danton [mailto:hillf.zj@alibaba-inc.com]
> Sent: Monday, December 08, 2014 5:33 PM
> To: Wang, Yalin
> Cc: linux-kernel; linux-mm@kvack.org; linux-arm-kernel@lists.infradead.org;
> Andrew Morton; Hillf Danton
> Subject: Re: [PATCH] mm:add VM_BUG_ON() for page_mapcount()
> 
> >
> > This patch add VM_BUG_ON() for slab page, because _mapcount is an
> > union with slab struct in struct page, avoid access _mapcount if this
> > page is a slab page.
> > Also remove the unneeded bracket.
> >
> > Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
> > ---
> >  include/linux/mm.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h index
> > 11b65cf..34124c4 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -373,7 +373,8 @@ static inline void reset_page_mapcount(struct page
> > *page)
> >
> >  static inline int page_mapcount(struct page *page)  {
> > -	return atomic_read(&(page)->_mapcount) + 1;
> > +	VM_BUG_ON(PageSlab(page));
> 
> s/ VM_BUG_ON/ VM_BUG_ON_PAGE/ ?
Yes, I will send again .
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[PATCH V2] mm:add VM_BUG_ON() for page_mapcount()

[Wang, Yalin]

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This patch add VM_BUG_ON_PAGE() for slab page,
because _mapcount is an union with slab struct in struct page,
avoid access _mapcount if this page is a slab page.
Also remove the unneeded bracket.

Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
---
 include/linux/mm.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b464611..a117527 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -449,7 +449,8 @@ static inline void page_mapcount_reset(struct page *page)
 
 static inline int page_mapcount(struct page *page)
 {
-	return atomic_read(&(page)->_mapcount) + 1;
+	VM_BUG_ON_PAGE(PageSlab(page), page);
+	return atomic_read(&page->_mapcount) + 1;
 }
 
 static inline int page_count(struct page *page)
-- 
2.1.3
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[PATCH V3] mm:add VM_BUG_ON_PAGE() for page_mapcount()

[Wang, Yalin]

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This patch add VM_BUG_ON_PAGE() for slab page,
because _mapcount is an union with slab struct in struct page,
avoid access _mapcount if this page is a slab page.
Also remove the unneeded bracket.

Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
---
 include/linux/mm.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b464611..a117527 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -449,7 +449,8 @@ static inline void page_mapcount_reset(struct page *page)
 
 static inline int page_mapcount(struct page *page)
 {
-	return atomic_read(&(page)->_mapcount) + 1;
+	VM_BUG_ON_PAGE(PageSlab(page), page);
+	return atomic_read(&page->_mapcount) + 1;
 }
 
 static inline int page_count(struct page *page)
-- 
2.1.3
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Re: [PATCH V3] mm:add VM_BUG_ON_PAGE() for page_mapcount()

[Kirill A. Shutemov]

On Mon, Dec 08, 2014 at 05:59:46PM +0800, Wang, Yalin wrote:
> This patch add VM_BUG_ON_PAGE() for slab page,
> because _mapcount is an union with slab struct in struct page,
> avoid access _mapcount if this page is a slab page.
> Also remove the unneeded bracket.
> 
> Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
-- 
 Kirill A. Shutemov

Re: [PATCH V3] mm:add VM_BUG_ON_PAGE() for page_mapcount()

[Hillf Danton]

> 
> This patch add VM_BUG_ON_PAGE() for slab page,
> because _mapcount is an union with slab struct in struct page,
> avoid access _mapcount if this page is a slab page.
> Also remove the unneeded bracket.
> 
> Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
> ---
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>

>  include/linux/mm.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b464611..a117527 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -449,7 +449,8 @@ static inline void page_mapcount_reset(struct page *page)
> 
>  static inline int page_mapcount(struct page *page)
>  {
> -	return atomic_read(&(page)->_mapcount) + 1;
> +	VM_BUG_ON_PAGE(PageSlab(page), page);
> +	return atomic_read(&page->_mapcount) + 1;
>  }
> 
>  static inline int page_count(struct page *page)
> --
> 2.1.3

Re: [PATCH V3] mm:add VM_BUG_ON_PAGE() for page_mapcount()

[Vlastimil Babka]

On 12/08/2014 10:59 AM, Wang, Yalin wrote:
> This patch add VM_BUG_ON_PAGE() for slab page,
> because _mapcount is an union with slab struct in struct page,
> avoid access _mapcount if this page is a slab page.
> Also remove the unneeded bracket.
>
> Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
> ---
>   include/linux/mm.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b464611..a117527 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -449,7 +449,8 @@ static inline void page_mapcount_reset(struct page *page)
>
>   static inline int page_mapcount(struct page *page)
>   {
> -	return atomic_read(&(page)->_mapcount) + 1;
> +	VM_BUG_ON_PAGE(PageSlab(page), page);
> +	return atomic_read(&page->_mapcount) + 1;
>   }
>

I think this might theoretically trigger on the following code in 
compaction's isolate_migratepages_block():

/*
   * Migration will fail if an anonymous page is pinned in memory,
   * so avoid taking lru_lock and isolating it unnecessarily in an
   * admittedly racy check.
   */
if (!page_mapping(page) &&
     page_count(page) > page_mapcount(page))
	continue;

This is done after PageLRU() was positive, but the lru_lock might be not 
taken yet. So, there's some time window during which the page might have 
been reclaimed from LRU and become a PageSlab(page). !page_mapping(page) 
will be true in that case so it will proceed with page_mapcount(page) 
test and trigger the VM_BUG_ON.

(That test was added by DavidR year ago in commit 
119d6d59dcc0980dcd581fdadb6b2033b512a473)

Vlastimil





>   static inline int page_count(struct page *page)
>