linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Date:   Fri, 14 Dec 2018 12:29:27 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        david <david@...morbit.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, tom@...pey.com,
        Al Viro <viro@...iv.linux.org.uk>, benve@...co.com,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        rcampbell@...dia.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

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@...radead.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.