linux-kernel - Re: [PATCH v3 4/4] mm/gup: remove get_user_pages

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Message-ID: <YfvgQHewEbN3lR33@infradead.org>
Date:   Thu, 3 Feb 2022 06:01:36 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        David Hildenbrand <david@...hat.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.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,
        Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v3 4/4] mm/gup: remove get_user_pages_locked()

On Thu, Feb 03, 2022 at 01:32:32AM -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.

Maybe split the lookup_node changes into a separate patch, as that
allows to document that change even better.