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