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Date:   Fri, 14 Dec 2018 12:29:27 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        John Hubbard <jhubbard@nvidia.com>,
        david <david@fromorbit.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Jan Kara <jack@suse.cz>, John Hubbard <john.hubbard@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>, tom@talpey.com,
        Al Viro <viro@zeniv.linux.org.uk>, benve@cisco.com,
        Christoph Hellwig <hch@infradead.org>,
        Christopher Lameter <cl@linux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
        Doug Ledford <dledford@redhat.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Marciniszyn <mike.marciniszyn@intel.com>,
        rcampbell@nvidia.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Message-ID: <20181214202927.GI10600@bombadil.infradead.org>
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On Fri, Dec 14, 2018 at 12:17:08PM -0800, Dan Williams wrote:
> On Fri, Dec 14, 2018 at 12:03 PM Matthew Wilcox <willy@infradead.org> wrote:
> > Yes; working on the pfn-to-page structure right now as it happens ...
> > in the meantime, an XArray for it probably wouldn't be _too_ bad.
> 
> It might... see the recent patch from Ketih responding to complaints
> about get_dev_pagemap() lookup overhead:

Yeah, I saw.  I called xa_dump() on the pgmap_array() running under
QEmu and it's truly awful because the NVDIMMs presented by QEmu are
very misaligned.  If we can make the NVDIMMs better aligned, we won't
hit such a bad case in the XArray data structure.