[PATCH] pagemap: fix wrong KPF_THP on slab pages

[PATCH] pagemap: fix wrong KPF_THP on slab pages

[Naoya Horiguchi]

KPF_THP can be set on non-huge compound pages like slab pages, because
PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
and breaks user space applications which look for thp via /proc/kpageflags.
Currently thp is constructed only on anonymous pages, so this patch makes
KPF_THP be set when both of PageAnon and PageTransCompound are true.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 fs/proc/page.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git v3.6-rc6.orig/fs/proc/page.c v3.6-rc6/fs/proc/page.c
index 7fcd0d6..613102d 100644
--- v3.6-rc6.orig/fs/proc/page.c
+++ v3.6-rc6/fs/proc/page.c
@@ -115,7 +115,7 @@ u64 stable_page_flags(struct page *page)
 		u |= 1 << KPF_COMPOUND_TAIL;
 	if (PageHuge(page))
 		u |= 1 << KPF_HUGE;
-	else if (PageTransCompound(page))
+	else if (PageTransCompound(page) && PageAnon(page))
 		u |= 1 << KPF_THP;
 
 	/*
-- 
1.7.11.4

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[KOSAKI Motohiro]

On Tue, Sep 25, 2012 at 9:56 AM, Naoya Horiguchi
<n-horiguchi@ah.jp.nec.com> wrote:
> KPF_THP can be set on non-huge compound pages like slab pages, because
> PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> and breaks user space applications which look for thp via /proc/kpageflags.
> Currently thp is constructed only on anonymous pages, so this patch makes
> KPF_THP be set when both of PageAnon and PageTransCompound are true.

Indeed. Please add some comment too.

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Naoya Horiguchi]

On Tue, Sep 25, 2012 at 11:59:51AM -0400, KOSAKI Motohiro wrote:
> On Tue, Sep 25, 2012 at 9:56 AM, Naoya Horiguchi
> <n-horiguchi@ah.jp.nec.com> wrote:
> > KPF_THP can be set on non-huge compound pages like slab pages, because
> > PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> > and breaks user space applications which look for thp via /proc/kpageflags.
> > Currently thp is constructed only on anonymous pages, so this patch makes
> > KPF_THP be set when both of PageAnon and PageTransCompound are true.
> 
> Indeed. Please add some comment too.

Sure. I send revised one.

Thanks,
Naoya
---
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Date: Mon, 24 Sep 2012 16:28:30 -0400
Subject: [PATCH v2] pagemap: fix wrong KPF_THP on slab pages

KPF_THP can be set on non-huge compound pages like slab pages, because
PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
and breaks user space applications which look for thp via /proc/kpageflags.
Currently thp is constructed only on anonymous pages, so this patch makes
KPF_THP be set when both of PageAnon and PageTransCompound are true.

Changelog in v2:
  - add a comment in code

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 fs/proc/page.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 7fcd0d6..f7cd2f6c 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -115,7 +115,12 @@ u64 stable_page_flags(struct page *page)
 		u |= 1 << KPF_COMPOUND_TAIL;
 	if (PageHuge(page))
 		u |= 1 << KPF_HUGE;
-	else if (PageTransCompound(page))
+	/*
+	 * Since THP is relevant only for anonymous pages so far, we check it
+	 * explicitly with PageAnon. Otherwise thp is confounded with non-huge
+	 * compound pages like slab pages.
+	 */
+	else if (PageTransCompound(page) && PageAnon(page))
 		u |= 1 << KPF_THP;
 
 	/*
-- 
1.7.11.4

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[KOSAKI Motohiro]

On Tue, Sep 25, 2012 at 1:05 PM, Naoya Horiguchi
<n-horiguchi@ah.jp.nec.com> wrote:
> On Tue, Sep 25, 2012 at 11:59:51AM -0400, KOSAKI Motohiro wrote:
>> On Tue, Sep 25, 2012 at 9:56 AM, Naoya Horiguchi
>> <n-horiguchi@ah.jp.nec.com> wrote:
>> > KPF_THP can be set on non-huge compound pages like slab pages, because
>> > PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
>> > and breaks user space applications which look for thp via /proc/kpageflags.
>> > Currently thp is constructed only on anonymous pages, so this patch makes
>> > KPF_THP be set when both of PageAnon and PageTransCompound are true.
>>
>> Indeed. Please add some comment too.
>
> Sure. I send revised one.
>
> Thanks,
> Naoya
> ---
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Date: Mon, 24 Sep 2012 16:28:30 -0400
> Subject: [PATCH v2] pagemap: fix wrong KPF_THP on slab pages
>
> KPF_THP can be set on non-huge compound pages like slab pages, because
> PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> and breaks user space applications which look for thp via /proc/kpageflags.
> Currently thp is constructed only on anonymous pages, so this patch makes
> KPF_THP be set when both of PageAnon and PageTransCompound are true.
>
> Changelog in v2:
>   - add a comment in code
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  fs/proc/page.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 7fcd0d6..f7cd2f6c 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -115,7 +115,12 @@ u64 stable_page_flags(struct page *page)
>                 u |= 1 << KPF_COMPOUND_TAIL;
>         if (PageHuge(page))
>                 u |= 1 << KPF_HUGE;
> -       else if (PageTransCompound(page))
> +       /*
> +        * Since THP is relevant only for anonymous pages so far, we check it
> +        * explicitly with PageAnon. Otherwise thp is confounded with non-huge
> +        * compound pages like slab pages.
> +        */
> +       else if (PageTransCompound(page) && PageAnon(page))
>                 u |= 1 << KPF_THP;

Looks good to me.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[David Rientjes]

On Tue, 25 Sep 2012, Naoya Horiguchi wrote:

> KPF_THP can be set on non-huge compound pages like slab pages, because
> PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> and breaks user space applications which look for thp via /proc/kpageflags.
> Currently thp is constructed only on anonymous pages, so this patch makes
> KPF_THP be set when both of PageAnon and PageTransCompound are true.
> 
> Changelog in v2:
>   - add a comment in code
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Wouldn't PageTransCompound(page) && !PageHuge(page) && !PageSlab(page) be 
better for a future extension of thp support?

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Naoya Horiguchi]

On Tue, Sep 25, 2012 at 05:20:48PM -0700, David Rientjes wrote:
> On Tue, 25 Sep 2012, Naoya Horiguchi wrote:
> 
> > KPF_THP can be set on non-huge compound pages like slab pages, because
> > PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> > and breaks user space applications which look for thp via /proc/kpageflags.
> > Currently thp is constructed only on anonymous pages, so this patch makes
> > KPF_THP be set when both of PageAnon and PageTransCompound are true.
> > 
> > Changelog in v2:
> >   - add a comment in code
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> 
> Wouldn't PageTransCompound(page) && !PageHuge(page) && !PageSlab(page) be 
> better for a future extension of thp support?

Yes, this saves us an additional change when thp starts handling pagecaches.
Andrew, can you replace the previous version in -mm tree with new one below?

Thanks,
Naoya
---
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Date: Tue, 25 Sep 2012 21:30:25 -0400
Subject: [PATCH v3] kpageflags: fix wrong KPF_THP on slab pages

KPF_THP can be set on non-huge compound pages like slab pages, because
PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
and breaks user space applications which look for thp via /proc/kpageflags.
This patch rules out setting KPF_THP wrongly by additional PageSlab check.

Changelog in v3:
  - check PageSlab instead of PageAnon
  - fix patch subject

Changelog in v2:
  - add a comment in code

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 fs/proc/page.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 7fcd0d6..e36d1f3 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -115,7 +115,12 @@ u64 stable_page_flags(struct page *page)
 		u |= 1 << KPF_COMPOUND_TAIL;
 	if (PageHuge(page))
 		u |= 1 << KPF_HUGE;
-	else if (PageTransCompound(page))
+	/*
+	 * PageTransCompound can be true for slab pages because it just sees
+	 * PG_head/PG_head, so we need to check PageSlab to make sure the given
+	 * page is a thp, not a non-huge compound page.
+	 */
+	else if (PageTransCompound(page) && !PageSlab(page))
 		u |= 1 << KPF_THP;
 
 	/*
-- 
1.7.11.4

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[David Rientjes]

On Tue, 25 Sep 2012, Naoya Horiguchi wrote:

> KPF_THP can be set on non-huge compound pages like slab pages, because
> PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> and breaks user space applications which look for thp via /proc/kpageflags.
> This patch rules out setting KPF_THP wrongly by additional PageSlab check.
> 
> Changelog in v3:
>   - check PageSlab instead of PageAnon
>   - fix patch subject
> 
> Changelog in v2:
>   - add a comment in code
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Acked-by: David Rientjes <rientjes@google.com>

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Fengguang Wu]

On Tue, Sep 25, 2012 at 10:06:08PM -0400, Naoya Horiguchi wrote:
> On Tue, Sep 25, 2012 at 05:20:48PM -0700, David Rientjes wrote:
> > On Tue, 25 Sep 2012, Naoya Horiguchi wrote:
> > 
> > > KPF_THP can be set on non-huge compound pages like slab pages, because
> > > PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> > > and breaks user space applications which look for thp via /proc/kpageflags.
> > > Currently thp is constructed only on anonymous pages, so this patch makes
> > > KPF_THP be set when both of PageAnon and PageTransCompound are true.
> > > 
> > > Changelog in v2:
> > >   - add a comment in code
> > > 
> > > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > 
> > Wouldn't PageTransCompound(page) && !PageHuge(page) && !PageSlab(page) be 
> > better for a future extension of thp support?
> 
> Yes, this saves us an additional change when thp starts handling pagecaches.
> Andrew, can you replace the previous version in -mm tree with new one below?
> 
> Thanks,
> Naoya
> ---
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Date: Tue, 25 Sep 2012 21:30:25 -0400
> Subject: [PATCH v3] kpageflags: fix wrong KPF_THP on slab pages
> 
> KPF_THP can be set on non-huge compound pages like slab pages, because
> PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug

s/sees/checks/

> and breaks user space applications which look for thp via /proc/kpageflags.
> This patch rules out setting KPF_THP wrongly by additional PageSlab check.
> 
> Changelog in v3:
>   - check PageSlab instead of PageAnon
>   - fix patch subject
> 
> Changelog in v2:
>   - add a comment in code
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  fs/proc/page.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 7fcd0d6..e36d1f3 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -115,7 +115,12 @@ u64 stable_page_flags(struct page *page)
>  		u |= 1 << KPF_COMPOUND_TAIL;
>  	if (PageHuge(page))
>  		u |= 1 << KPF_HUGE;
> -	else if (PageTransCompound(page))
> +	/*
> +	 * PageTransCompound can be true for slab pages because it just sees

s/sees/checks/

> +	 * PG_head/PG_head, so we need to check PageSlab to make sure the given

PG_head/PG_head should be PG_head/PG_tail.

> +	 * page is a thp, not a non-huge compound page.
> +	 */
> +	else if (PageTransCompound(page) && !PageSlab(page))
>  		u |= 1 << KPF_THP;

Good catch!

Will this report THP for the various drivers that do __GFP_COMP
page allocations?

Thanks,
Fengguang

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Naoya Horiguchi]

On Wed, Sep 26, 2012 at 10:47:54AM +0800, Fengguang Wu wrote:
> On Tue, Sep 25, 2012 at 10:06:08PM -0400, Naoya Horiguchi wrote:
> > On Tue, Sep 25, 2012 at 05:20:48PM -0700, David Rientjes wrote:
> > > On Tue, 25 Sep 2012, Naoya Horiguchi wrote:
> > > 
> > > > KPF_THP can be set on non-huge compound pages like slab pages, because
> > > > PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> > > > and breaks user space applications which look for thp via /proc/kpageflags.
> > > > Currently thp is constructed only on anonymous pages, so this patch makes
> > > > KPF_THP be set when both of PageAnon and PageTransCompound are true.
> > > > 
> > > > Changelog in v2:
> > > >   - add a comment in code
> > > > 
> > > > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > > 
> > > Wouldn't PageTransCompound(page) && !PageHuge(page) && !PageSlab(page) be 
> > > better for a future extension of thp support?
> > 
> > Yes, this saves us an additional change when thp starts handling pagecaches.
> > Andrew, can you replace the previous version in -mm tree with new one below?
> > 
> > Thanks,
> > Naoya
> > ---
> > From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Date: Tue, 25 Sep 2012 21:30:25 -0400
> > Subject: [PATCH v3] kpageflags: fix wrong KPF_THP on slab pages
> > 
> > KPF_THP can be set on non-huge compound pages like slab pages, because
> > PageTransCompound only sees PG_head and PG_tail. Obviously this is a bug
> 
> s/sees/checks/
> 
> > and breaks user space applications which look for thp via /proc/kpageflags.
> > This patch rules out setting KPF_THP wrongly by additional PageSlab check.
> > 
> > Changelog in v3:
> >   - check PageSlab instead of PageAnon
> >   - fix patch subject
> > 
> > Changelog in v2:
> >   - add a comment in code
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > ---
> >  fs/proc/page.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/proc/page.c b/fs/proc/page.c
> > index 7fcd0d6..e36d1f3 100644
> > --- a/fs/proc/page.c
> > +++ b/fs/proc/page.c
> > @@ -115,7 +115,12 @@ u64 stable_page_flags(struct page *page)
> >  		u |= 1 << KPF_COMPOUND_TAIL;
> >  	if (PageHuge(page))
> >  		u |= 1 << KPF_HUGE;
> > -	else if (PageTransCompound(page))
> > +	/*
> > +	 * PageTransCompound can be true for slab pages because it just sees
> 
> s/sees/checks/
> 
> > +	 * PG_head/PG_head, so we need to check PageSlab to make sure the given
> 
> PG_head/PG_head should be PG_head/PG_tail.

Ah, sorry for my carelessness.

> > +	 * page is a thp, not a non-huge compound page.
> > +	 */
> > +	else if (PageTransCompound(page) && !PageSlab(page))
> >  		u |= 1 << KPF_THP;
> 
> Good catch!
> 
> Will this report THP for the various drivers that do __GFP_COMP
> page allocations?

I'm afraid it will. I think of checking PageLRU as an alternative,
but it needs compound_head() to report tail pages correctly.
In this context, pages are not pinned or locked, so it's unsafe to
use compound_head() because it can return a dangling pointer.
Maybe it's a thp's/hugetlbfs's (not kpageflags specific) problem,
so going forward with compound_head() expecting that it will be
fixed in the future work can be an option.

Thanks,
Naoya

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Naoya Horiguchi]

On Wed, Sep 26, 2012 at 12:02:34AM -0400, Naoya Horiguchi wrote:
...
> > > +	 * page is a thp, not a non-huge compound page.
> > > +	 */
> > > +	else if (PageTransCompound(page) && !PageSlab(page))
> > >  		u |= 1 << KPF_THP;
> > 
> > Good catch!
> > 
> > Will this report THP for the various drivers that do __GFP_COMP
> > page allocations?
> 
> I'm afraid it will. I think of checking PageLRU as an alternative,
> but it needs compound_head() to report tail pages correctly.
> In this context, pages are not pinned or locked, so it's unsafe to
> use compound_head() because it can return a dangling pointer.
> Maybe it's a thp's/hugetlbfs's (not kpageflags specific) problem,
> so going forward with compound_head() expecting that it will be
> fixed in the future work can be an option.

It seems that compound_trans_head() solves this problem, so I'll
simply use it.

Naoya

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Fengguang Wu]

On Wed, Sep 26, 2012 at 02:06:08AM -0400, Naoya Horiguchi wrote:
> On Wed, Sep 26, 2012 at 12:02:34AM -0400, Naoya Horiguchi wrote:
> ...
> > > > +	 * page is a thp, not a non-huge compound page.
> > > > +	 */
> > > > +	else if (PageTransCompound(page) && !PageSlab(page))
> > > >  		u |= 1 << KPF_THP;
> > > 
> > > Good catch!
> > > 
> > > Will this report THP for the various drivers that do __GFP_COMP
> > > page allocations?
> > 
> > I'm afraid it will. I think of checking PageLRU as an alternative,
> > but it needs compound_head() to report tail pages correctly.
> > In this context, pages are not pinned or locked, so it's unsafe to
> > use compound_head() because it can return a dangling pointer.
> > Maybe it's a thp's/hugetlbfs's (not kpageflags specific) problem,
> > so going forward with compound_head() expecting that it will be
> > fixed in the future work can be an option.
> 
> It seems that compound_trans_head() solves this problem, so I'll
> simply use it.

Naoya, in fact I didn't quite catch your concerns. Why not just test

        PageTransCompound(page) && PageLRU(page)

The whole page flag report thing is inherently racy and it's fine to
report wrong values due to races. The "__GFP_COMP reported as THP",
however, should be avoided because it will make consistent wrong
reporting of page flags.

Thanks,
Fengguang

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Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

[Naoya Horiguchi]

On Wed, Sep 26, 2012 at 03:38:41PM +0800, Fengguang Wu wrote:
> On Wed, Sep 26, 2012 at 02:06:08AM -0400, Naoya Horiguchi wrote:
> > On Wed, Sep 26, 2012 at 12:02:34AM -0400, Naoya Horiguchi wrote:
> > ...
> > > > > +	 * page is a thp, not a non-huge compound page.
> > > > > +	 */
> > > > > +	else if (PageTransCompound(page) && !PageSlab(page))
> > > > >  		u |= 1 << KPF_THP;
> > > > 
> > > > Good catch!
> > > > 
> > > > Will this report THP for the various drivers that do __GFP_COMP
> > > > page allocations?
> > > 
> > > I'm afraid it will. I think of checking PageLRU as an alternative,
> > > but it needs compound_head() to report tail pages correctly.
> > > In this context, pages are not pinned or locked, so it's unsafe to
> > > use compound_head() because it can return a dangling pointer.
> > > Maybe it's a thp's/hugetlbfs's (not kpageflags specific) problem,
> > > so going forward with compound_head() expecting that it will be
> > > fixed in the future work can be an option.
> > 
> > It seems that compound_trans_head() solves this problem, so I'll
> > simply use it.
> 
> Naoya, in fact I didn't quite catch your concerns. Why not just test
> 
>         PageTransCompound(page) && PageLRU(page)

If we simply check PageLRU, tail pages in thp only show KPF_COMPOUND_TAIL
and we can't distinguish them from tail pages in non-huge compound pages.

Moreover this behavior is not consistent with that of hugetlbfs tail
pages where tail pages also have KPF_HUGE and are distinct from non-huge
compound pages. I show the output of page-types:

  offset  len     flags
  ...
  2d400   1       ___U_lA____Ma_bH______t____________ (thp head)
  2d401   1ff     ________________T__________________ (thp tail) # no KPF_THP
  ...
  77000   1       ___U_______Ma__H_G_________________ (hugetlbfs head)
  77001   1ff     ________________TG_________________ (hugetlbfs tail)
  ...
  11fb50  1       _______________H___________________ (compound head)
  11fb51  3       ________________T__________________ (compound tail)
  ...
  11fb58  1       _______S_______H___________________ (slab head)
  11fb59  7       ________________T__________________ (slab tail)

    H: KPF_COMPOUND_HEAD   T: KPF_COMPOUND_TAIL
    G: KPF_HUGE            t: KPF_THP

So I think it's better to set KPF_THP on thp tail pages.
  
> The whole page flag report thing is inherently racy and it's fine to
> report wrong values due to races. The "__GFP_COMP reported as THP",
> however, should be avoided because it will make consistent wrong
> reporting of page flags.

Yes, I agree with this point.

Thanks,
Naoya

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