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:
...

quoted

quoted

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+	 * 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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