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:   Thu, 25 Mar 2021 11:37:30 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     linux-mm@kvack.org, linux-nvdimm@lists.01.org,
        Christoph Hellwig <hch@lst.de>,
        Shiyang Ruan <ruansy.fnst@fujitsu.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>, david@fromorbit.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the
 get_user_pages_fast() path
Message-ID: <20210325143730.GL2710221@ziepe.ca>
References: <161604048257.1463742.1374527716381197629.stgit@dwillia2-desk3.amr.corp.intel.com>
 <161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com>
Precedence: bulk

On Wed, Mar 17, 2021 at 09:08:28PM -0700, Dan Williams wrote:
> Now that device-dax and filesystem-dax are guaranteed to unmap all user
> mappings of devmap / DAX pages before tearing down the 'struct page'
> array, get_user_pages_fast() can rely on its traditional synchronization
> method "validate_pte(); get_page(); revalidate_pte()" to catch races with
> device shutdown. Specifically the unmap guarantee ensures that gup-fast
> either succeeds in taking a page reference (lock-less), or it detects a
> need to fall back to the slow path where the device presence can be
> revalidated with locks held.
> 
> Reported-by: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> Cc: Vishal Verma <vishal.l.verma@intel.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  mm/gup.c |   38 ++++++++++++++++----------------------
>  1 file changed, 16 insertions(+), 22 deletions(-)

I'm happy to see this, and it is really the right thing that PTEs are
properly removed before anything happens to the pages under them.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason