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Date:   Thu, 3 Feb 2022 06:01:36 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        David Hildenbrand <david@redhat.com>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>,
        Jan Kara <jack@suse.cz>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Jason Gunthorpe <jgg@nvidia.com>
Subject: Re: [PATCH v3 4/4] mm/gup: remove get_user_pages_locked()
Message-ID: <YfvgQHewEbN3lR33@infradead.org>
References: <20220203093232.572380-1-jhubbard@nvidia.com>
 <20220203093232.572380-5-jhubbard@nvidia.com>
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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.