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Date:   Mon, 9 Aug 2021 12:22:09 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Zi Yan <ziy@nvidia.com>, Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Michal Hocko <mhocko@kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 00/15] Make MAX_ORDER adjustable as a kernel boot
 time parameter.
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On Sun, Aug 08, 2021 at 09:29:29PM -0700, Hugh Dickins wrote:
> On Sat, 7 Aug 2021, Matthew Wilcox wrote:
> > 
> > I am, however, of the opinion that 2MB pages give us so much trouble
> > because they're so very special.  Few people exercise those code paths and
> > it's easy to break them without noticing.  This is partly why I want to
> > do arbitrary-order pages.  If everybody is running with compound pages
> > all the time, we'll see the corner cases often, and people other than
> > Hugh, Kirill and Mike will be able to work on them.
> 
> I don't entirely agree.  I'm all for your use of compound pages in page
> cache, but don't think its problems are representative of the problems
> in aiming for a PMD (or PUD) bar, with the weird page table transitions
> we expect of "THP" there.
> 
> I haven't checked: is your use of compound pages in page cache still
> limited to the HAVE_ARCH_TRANSPARENT_HUGEPAGE architectures?  When
> the others could just as well use compound pages in page cache too.

It is no longer gated by whether TRANSPARENT_HUGEPAGE is enabled or not.
That's a relatively recent change, and I can't say that I've tested it.
I'll give it a try today on a PA-RISC system I've booted recently.

One of the followup pieces of work that I hope somebody other than
myself will undertake is using 64KB PTEs on a 4KB PAGE_SIZE ARM/POWER
machine if the stars align.