linux-kernel - Re: [RFC PATCH 00/15] Make MAX_ORDER adjustable as a kernel boot time parameter.

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Message-ID: <c2a4dd1d-c1ac-876b-4a3-c5ea7555498@google.com>
Date:   Sun, 8 Aug 2021 21:29:29 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Hugh Dickins <hughd@...gle.com>, Zi Yan <ziy@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...nel.org>,
        John Hubbard <jhubbard@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/15] Make MAX_ORDER adjustable as a kernel boot
 time parameter.

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.

Hugh