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@xxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
---
 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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