linux-kernel - Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages

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Date:   Thu, 25 Mar 2021 11:37:30 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        Christoph Hellwig <hch@....de>,
        Shiyang Ruan <ruansy.fnst@...itsu.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>, david@...morbit.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the
 get_user_pages_fast() path

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@...pe.ca>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Shiyang Ruan <ruansy.fnst@...itsu.com>
> Cc: Vishal Verma <vishal.l.verma@...el.com>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Dan Williams <dan.j.williams@...el.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@...dia.com>

Jason