linux-kernel - Re: [PATCH v2 4/4] mm/gup: remove get_user_pages

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Date:   Tue, 1 Feb 2022 08:31:34 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>, Jan Kara <jack@...e.cz>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH v2 4/4] mm/gup: remove get_user_pages_locked()

On Tue, Feb 01, 2022 at 02:11:08AM -0800, John Hubbard wrote:
> There is only one caller of get_user_pages_locked(). The purpose of
> get_user_pages_locked() is to allow for unlocking the mmap_lock when
> reading a page from the disk during a page fault (hidden behind
> VM_FAULT_RETRY). The idea is to reduce contention on the heavily-used
> mmap_lock. (Thanks to Jan Kara for clearly pointing that out, and in
> fact I've used some of his wording here.)
> 
> However, it is unlikely for lookup_node() to take a page fault. With
> that in mind, change over to calling get_user_pages_fast(). This
> simplifies the code, runs a little faster in the expected case, and
> allows removing get_user_pages_locked() entirely.
> 
> Cc: Jan Kara <jack@...e.cz>
> Suggested-by: Jason Gunthorpe <jgg@...pe.ca>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  include/linux/mm.h |  2 --
>  mm/gup.c           | 59 ----------------------------------------------
>  mm/mempolicy.c     | 21 +++++++----------
>  3 files changed, 9 insertions(+), 73 deletions(-)

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

Jason