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@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

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@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

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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