Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 26 Jan 2021 22:10:23 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Joao Martins <joao.m.martins@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording
Message-ID: <20210127021023.GC4605@ziepe.ca>
References: <20210125205744.10203-1-joao.m.martins@oracle.com>
 <20210125205744.10203-3-joao.m.martins@oracle.com>
 <3d34111f-8365-ab95-af11-aaf4825204be@oracle.com>
 <1ae0313d-de9b-4553-1f68-04c4f5a3f7eb@oracle.com>
 <4d3914e9-f448-8a86-9fc6-e71cec581115@oracle.com>
 <20210127000730.GB4605@ziepe.ca>
 <f1fc1543-72fa-44f2-50fc-0220abfdc484@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <f1fc1543-72fa-44f2-50fc-0220abfdc484@oracle.com>
Precedence: bulk

On Tue, Jan 26, 2021 at 05:58:53PM -0800, Mike Kravetz wrote:

> As pointed out by Joao, you can also see the differences in pfn_to_page
> for CONFIG_SPARSE_VMEMMAP and CONFIG_SPARSEMEM.  The only time we might
> have issues is with CONFIG_SPARSEMEM.  I would bet CONFIG_SPARSE_VMEMMAP
> is far more common.

I think it is fine to have a different pfn_to_page, it should just be
illegal to combine pages into a compound if their tail pages are not
linear in the map.

Matt's folio work might present an option to audit the whole mm for
this pattern and provide some folio_next_tail_page() accessor that
does the fast thing - but I question the value of such a project for a
2008 era PPC platform with 16GB pages (seriously?) that may be using
VMEMMAP today anyhow??

Maybe others know of more modern use cases

Jason