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