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Subject: Re: [PATCH -V10 00/15] THP support for PPC64
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Date: Wed, 19 Jun 2013 08:03:24 +1000
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On Wed, 2013-06-19 at 00:16 +0530, Aneesh Kumar K.V wrote:

> But will that by anonymous memory ? ie, will we find them suitable for
> THP allocation ?

The 4k pages themselves with 4k_PFN no, but the segment yes. A single of
these will demote the whole segment, ie 256M or 1T.

> >   * If you find a THP in hash_page and the segment size is 4k, fault
> >
> >   * In do_page_fault, re-check for that condition (or maybe we can make
> > hash_page return a specific bit that gets ORed into the error_code into
> > do_page_fault ?) and split huge pages there.
> >
> > But that's just an idea off the top of my mind, there might be a better
> > way. Of course this needs to be tested.
> >
> > BTW. For the subpage protection, similarily, you need to make sure you
> > properly map the entire segment as "no THP", not just the range
> > passed-in by the user.
> 
> Can you explain that more, why should the entire segment be marked no THP ?
> The segment can work with 4K base page size and we still be able to
> allocate a hugepage in that segment.

Will we be able to track all the possible hashings of the huge page on a
4k segment ?

Ben.