[PATCHv2 0/2] Fix another VM_BUG_ON_PAGE(PageTail(page)) on mbind(2)

[PATCHv2 0/2] Fix another VM_BUG_ON_PAGE(PageTail(page)) on mbind(2)

[Kirill A. Shutemov]

Dmitry Vyukov reported yet another VM_BUG_ON_PAGE(PageTail(page)) bug from
isolate_lru_page().

The fisrt patch fixes the bug by filter out non migratable VMAs in
queue_pages_test_walk(). There's no point to queue pages from non-migratable
VMA even for MPOL_MF_STRICT.

The second patch replace VM_BUG_ON_PAGE() with WARN_RATELIMIT() in
isolate_lru_page(). Most attempts to isolate tail pages are not fatal, as
these pages usually are not on LRU and will not be isolated.

v2:
 - address feedback from Michal Hocko;

Kirill A. Shutemov (2):
  mempolicy: do not try to queue pages from !vma_migratable()
  mm: downgrade VM_BUG in isolate_lru_page() to warning

 mm/mempolicy.c | 14 +++++---------
 mm/vmscan.c    |  2 +-
 2 files changed, 6 insertions(+), 10 deletions(-)

-- 
2.7.0

[PATCHv2 0/2] Fix another VM_BUG_ON_PAGE(PageTail(page)) on mbind(2)

[Kirill A. Shutemov]

Dmitry Vyukov reported yet another VM_BUG_ON_PAGE(PageTail(page)) bug from
isolate_lru_page().

The fisrt patch fixes the bug by filter out non migratable VMAs in
queue_pages_test_walk(). There's no point to queue pages from non-migratable
VMA even for MPOL_MF_STRICT.

The second patch replace VM_BUG_ON_PAGE() with WARN_RATELIMIT() in
isolate_lru_page(). Most attempts to isolate tail pages are not fatal, as
these pages usually are not on LRU and will not be isolated.

v2:
 - address feedback from Michal Hocko;

Kirill A. Shutemov (2):
  mempolicy: do not try to queue pages from !vma_migratable()
  mm: downgrade VM_BUG in isolate_lru_page() to warning

 mm/mempolicy.c | 14 +++++---------
 mm/vmscan.c    |  2 +-
 2 files changed, 6 insertions(+), 10 deletions(-)

-- 
2.7.0

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[PATCHv2 1/2] mempolicy: do not try to queue pages from !vma_migratable()

[Kirill A. Shutemov]

Maybe I miss some point, but I don't see a reason why we try to queue
pages from non migratable VMAs.

This testcase steps on VM_BUG_ON_PAGE() in isolate_lru_page():

	#include <fcntl.h>
	#include <unistd.h>
	#include <stdio.h>
	#include <sys/mman.h>
	#include <numaif.h>

	#define SIZE 0x2000

	int foo;

	int main()
	{
		int fd;
		char *p;
		unsigned long mask = 2;

		fd = open("/dev/sg0", O_RDWR);
		p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
		/* Faultin pages */
		foo = p[0] + p[0x1000];
		mbind(p, SIZE, MPOL_BIND, &mask, 4, MPOL_MF_MOVE | MPOL_MF_STRICT);
		return 0;
	}

The only case when we can queue pages from such VMA is MPOL_MF_STRICT
plus MPOL_MF_MOVE or MPOL_MF_MOVE_ALL for VMA which has pages on LRU,
but gfp mask is not sutable for migaration (see mapping_gfp_mask() check
in vma_migratable()). That's looks like a bug to me.

Let's filter out non-migratable vma at start of queue_pages_test_walk()
and go to queue_pages_pte_range() only if MPOL_MF_MOVE or
MPOL_MF_MOVE_ALL flag is set.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
---
 mm/mempolicy.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 27d135408a22..4c4187c0e1de 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -548,8 +548,7 @@ retry:
 			goto retry;
 		}
 
-		if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
-			migrate_page_add(page, qp->pagelist, flags);
+		migrate_page_add(page, qp->pagelist, flags);
 	}
 	pte_unmap_unlock(pte - 1, ptl);
 	cond_resched();
@@ -625,7 +624,7 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 	unsigned long endvma = vma->vm_end;
 	unsigned long flags = qp->flags;
 
-	if (vma->vm_flags & VM_PFNMAP)
+	if (!vma_migratable(vma))
 		return 1;
 
 	if (endvma > end)
@@ -644,16 +643,13 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 
 	if (flags & MPOL_MF_LAZY) {
 		/* Similar to task_numa_work, skip inaccessible VMAs */
-		if (vma_migratable(vma) &&
-			vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
+		if (vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
 			change_prot_numa(vma, start, endvma);
 		return 1;
 	}
 
-	if ((flags & MPOL_MF_STRICT) ||
-	    ((flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) &&
-	     vma_migratable(vma)))
-		/* queue pages from current vma */
+	/* queue pages from current vma */
+	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
 		return 0;
 	return 1;
 }
-- 
2.7.0

[PATCHv2 1/2] mempolicy: do not try to queue pages from !vma_migratable()

[Kirill A. Shutemov]

Maybe I miss some point, but I don't see a reason why we try to queue
pages from non migratable VMAs.

This testcase steps on VM_BUG_ON_PAGE() in isolate_lru_page():

	#include <fcntl.h>
	#include <unistd.h>
	#include <stdio.h>
	#include <sys/mman.h>
	#include <numaif.h>

	#define SIZE 0x2000

	int foo;

	int main()
	{
		int fd;
		char *p;
		unsigned long mask = 2;

		fd = open("/dev/sg0", O_RDWR);
		p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
		/* Faultin pages */
		foo = p[0] + p[0x1000];
		mbind(p, SIZE, MPOL_BIND, &mask, 4, MPOL_MF_MOVE | MPOL_MF_STRICT);
		return 0;
	}

The only case when we can queue pages from such VMA is MPOL_MF_STRICT
plus MPOL_MF_MOVE or MPOL_MF_MOVE_ALL for VMA which has pages on LRU,
but gfp mask is not sutable for migaration (see mapping_gfp_mask() check
in vma_migratable()). That's looks like a bug to me.

Let's filter out non-migratable vma at start of queue_pages_test_walk()
and go to queue_pages_pte_range() only if MPOL_MF_MOVE or
MPOL_MF_MOVE_ALL flag is set.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
---
 mm/mempolicy.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 27d135408a22..4c4187c0e1de 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -548,8 +548,7 @@ retry:
 			goto retry;
 		}
 
-		if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
-			migrate_page_add(page, qp->pagelist, flags);
+		migrate_page_add(page, qp->pagelist, flags);
 	}
 	pte_unmap_unlock(pte - 1, ptl);
 	cond_resched();
@@ -625,7 +624,7 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 	unsigned long endvma = vma->vm_end;
 	unsigned long flags = qp->flags;
 
-	if (vma->vm_flags & VM_PFNMAP)
+	if (!vma_migratable(vma))
 		return 1;
 
 	if (endvma > end)
@@ -644,16 +643,13 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 
 	if (flags & MPOL_MF_LAZY) {
 		/* Similar to task_numa_work, skip inaccessible VMAs */
-		if (vma_migratable(vma) &&
-			vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
+		if (vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
 			change_prot_numa(vma, start, endvma);
 		return 1;
 	}
 
-	if ((flags & MPOL_MF_STRICT) ||
-	    ((flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) &&
-	     vma_migratable(vma)))
-		/* queue pages from current vma */
+	/* queue pages from current vma */
+	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
 		return 0;
 	return 1;
 }
-- 
2.7.0

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[PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Kirill A. Shutemov]

Calling isolate_lru_page() is wrong and shouldn't happen, but it not
nessesary fatal: the page just will not be isolated if it's not on LRU.

Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index eb3dd37ccd7c..71b1c29948db 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
 	int ret = -EBUSY;
 
 	VM_BUG_ON_PAGE(!page_count(page), page);
-	VM_BUG_ON_PAGE(PageTail(page), page);
+	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
 
 	if (PageLRU(page)) {
 		struct zone *zone = page_zone(page);
-- 
2.7.0

[PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Kirill A. Shutemov]

Calling isolate_lru_page() is wrong and shouldn't happen, but it not
nessesary fatal: the page just will not be isolated if it's not on LRU.

Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index eb3dd37ccd7c..71b1c29948db 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
 	int ret = -EBUSY;
 
 	VM_BUG_ON_PAGE(!page_count(page), page);
-	VM_BUG_ON_PAGE(PageTail(page), page);
+	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
 
 	if (PageLRU(page)) {
 		struct zone *zone = page_zone(page);
-- 
2.7.0

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Re: [PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Andrew Morton]

On Tue,  2 Feb 2016 19:21:01 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> Calling isolate_lru_page() is wrong and shouldn't happen, but it not
> nessesary fatal: the page just will not be isolated if it's not on LRU.
> 
> Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eb3dd37ccd7c..71b1c29948db 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
>  	int ret = -EBUSY;
>  
>  	VM_BUG_ON_PAGE(!page_count(page), page);
> -	VM_BUG_ON_PAGE(PageTail(page), page);
> +	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
>  
>  	if (PageLRU(page)) {
>  		struct zone *zone = page_zone(page);

Confused.  I thought mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page.patch:

--- a/mm/vmscan.c~mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page
+++ a/mm/vmscan.c
@@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
 	int ret = -EBUSY;
 
 	VM_BUG_ON_PAGE(!page_count(page), page);
-	VM_BUG_ON_PAGE(PageTail(page), page);
+	VM_BUG_ON_PAGE(PageLRU(page) && PageTail(page), page);
 
 	if (PageLRU(page)) {
 		struct zone *zone = page_zone(page);

was better.  We *know* that we sometimes encounter LRU pages here and
we know that we handle them correctly.  So why scare users by blurting
out a warning about something for which we won't be taking any action?

Re: [PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Andrew Morton]

On Tue,  2 Feb 2016 19:21:01 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> Calling isolate_lru_page() is wrong and shouldn't happen, but it not
> nessesary fatal: the page just will not be isolated if it's not on LRU.
> 
> Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eb3dd37ccd7c..71b1c29948db 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
>  	int ret = -EBUSY;
>  
>  	VM_BUG_ON_PAGE(!page_count(page), page);
> -	VM_BUG_ON_PAGE(PageTail(page), page);
> +	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
>  
>  	if (PageLRU(page)) {
>  		struct zone *zone = page_zone(page);

Confused.  I thought mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page.patch:

--- a/mm/vmscan.c~mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page
+++ a/mm/vmscan.c
@@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
 	int ret = -EBUSY;
 
 	VM_BUG_ON_PAGE(!page_count(page), page);
-	VM_BUG_ON_PAGE(PageTail(page), page);
+	VM_BUG_ON_PAGE(PageLRU(page) && PageTail(page), page);
 
 	if (PageLRU(page)) {
 		struct zone *zone = page_zone(page);

was better.  We *know* that we sometimes encounter LRU pages here and
we know that we handle them correctly.  So why scare users by blurting
out a warning about something for which we won't be taking any action?

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Re: [PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Kirill A. Shutemov]

On Tue, Feb 02, 2016 at 12:58:44PM -0800, Andrew Morton wrote:
> On Tue,  2 Feb 2016 19:21:01 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> 
> > Calling isolate_lru_page() is wrong and shouldn't happen, but it not
> > nessesary fatal: the page just will not be isolated if it's not on LRU.
> > 
> > Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  mm/vmscan.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index eb3dd37ccd7c..71b1c29948db 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
> >  	int ret = -EBUSY;
> >  
> >  	VM_BUG_ON_PAGE(!page_count(page), page);
> > -	VM_BUG_ON_PAGE(PageTail(page), page);
> > +	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
> >  
> >  	if (PageLRU(page)) {
> >  		struct zone *zone = page_zone(page);
> 
> Confused.  I thought mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page.patch:
> 
> --- a/mm/vmscan.c~mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page
> +++ a/mm/vmscan.c
> @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
>  	int ret = -EBUSY;
>  
>  	VM_BUG_ON_PAGE(!page_count(page), page);
> -	VM_BUG_ON_PAGE(PageTail(page), page);
> +	VM_BUG_ON_PAGE(PageLRU(page) && PageTail(page), page);
>  
>  	if (PageLRU(page)) {
>  		struct zone *zone = page_zone(page);
> 
> was better.  We *know* that we sometimes encounter LRU pages here and
> we know that we handle them correctly.  So why scare users by blurting
> out a warning about something for which we won't be taking any action?

We will.

If we try to isolate tail page something went wrong. It just shouldn't
happen. Compound pages should be isolated by head page as only whole
compound page is on LRU, not subpages.

If we see tail page here it's most probably from broken driver which
forgot to set VM_IO. With setting VM_IO on such VMA we would avoid useless
scan through pte in them and save some time.

Or maybe something else is broken. Like we forgot to split THP before
migration.

-- 
 Kirill A. Shutemov

Re: [PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Kirill A. Shutemov]

On Tue, Feb 02, 2016 at 12:58:44PM -0800, Andrew Morton wrote:
> On Tue,  2 Feb 2016 19:21:01 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> 
> > Calling isolate_lru_page() is wrong and shouldn't happen, but it not
> > nessesary fatal: the page just will not be isolated if it's not on LRU.
> > 
> > Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  mm/vmscan.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index eb3dd37ccd7c..71b1c29948db 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
> >  	int ret = -EBUSY;
> >  
> >  	VM_BUG_ON_PAGE(!page_count(page), page);
> > -	VM_BUG_ON_PAGE(PageTail(page), page);
> > +	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
> >  
> >  	if (PageLRU(page)) {
> >  		struct zone *zone = page_zone(page);
> 
> Confused.  I thought mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page.patch:
> 
> --- a/mm/vmscan.c~mm-fix-bogus-vm_bug_on_page-in-isolate_lru_page
> +++ a/mm/vmscan.c
> @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
>  	int ret = -EBUSY;
>  
>  	VM_BUG_ON_PAGE(!page_count(page), page);
> -	VM_BUG_ON_PAGE(PageTail(page), page);
> +	VM_BUG_ON_PAGE(PageLRU(page) && PageTail(page), page);
>  
>  	if (PageLRU(page)) {
>  		struct zone *zone = page_zone(page);
> 
> was better.  We *know* that we sometimes encounter LRU pages here and
> we know that we handle them correctly.  So why scare users by blurting
> out a warning about something for which we won't be taking any action?

We will.

If we try to isolate tail page something went wrong. It just shouldn't
happen. Compound pages should be isolated by head page as only whole
compound page is on LRU, not subpages.

If we see tail page here it's most probably from broken driver which
forgot to set VM_IO. With setting VM_IO on such VMA we would avoid useless
scan through pte in them and save some time.

Or maybe something else is broken. Like we forgot to split THP before
migration.

-- 
 Kirill A. Shutemov

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Re: [PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Michal Hocko]

On Tue 02-02-16 19:21:01, Kirill A. Shutemov wrote:
> Calling isolate_lru_page() is wrong and shouldn't happen, but it not
> nessesary fatal: the page just will not be isolated if it's not on LRU.
> 
> Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().

This will trigger for !CONFIG_DEBUG_VM as well which I am not sure is
necessary. I guess isolate_lru_page is not such a hot path so this would
be acceptable.

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

Anyway
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eb3dd37ccd7c..71b1c29948db 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
>  	int ret = -EBUSY;
>  
>  	VM_BUG_ON_PAGE(!page_count(page), page);
> -	VM_BUG_ON_PAGE(PageTail(page), page);
> +	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
>  
>  	if (PageLRU(page)) {
>  		struct zone *zone = page_zone(page);
> -- 
> 2.7.0
> 

-- 
Michal Hocko
SUSE Labs

Re: [PATCHv2 2/2] mm: downgrade VM_BUG in isolate_lru_page() to warning

[Michal Hocko]

On Tue 02-02-16 19:21:01, Kirill A. Shutemov wrote:
> Calling isolate_lru_page() is wrong and shouldn't happen, but it not
> nessesary fatal: the page just will not be isolated if it's not on LRU.
> 
> Let's downgrade the VM_BUG_ON_PAGE() to WARN_RATELIMIT().

This will trigger for !CONFIG_DEBUG_VM as well which I am not sure is
necessary. I guess isolate_lru_page is not such a hot path so this would
be acceptable.

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

Anyway
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eb3dd37ccd7c..71b1c29948db 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1443,7 +1443,7 @@ int isolate_lru_page(struct page *page)
>  	int ret = -EBUSY;
>  
>  	VM_BUG_ON_PAGE(!page_count(page), page);
> -	VM_BUG_ON_PAGE(PageTail(page), page);
> +	WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");
>  
>  	if (PageLRU(page)) {
>  		struct zone *zone = page_zone(page);
> -- 
> 2.7.0
> 

-- 
Michal Hocko
SUSE Labs

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