From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1754042AbcJYXqh (ORCPT <rfc822;w@1wt.eu>);
        Tue, 25 Oct 2016 19:46:37 -0400
Received: from mail-wm0-f42.google.com ([74.125.82.42]:36554 "EHLO
        mail-wm0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751643AbcJYXqf (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 25 Oct 2016 19:46:35 -0400
Date: Wed, 26 Oct 2016 00:46:31 +0100
From: Lorenzo Stoakes <lstoakes@gmail.com>
To: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>, Jan Kara <jack@suse.cz>,
        Hugh Dickins <hughd@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: remove unnecessary __get_user_pages_unlocked() calls
Message-ID: <20161025234631.GA5946@lucifer>
References: <20161025233609.5601-1-lstoakes@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161025233609.5601-1-lstoakes@gmail.com>
User-Agent: Mutt/1.7.1 (2016-10-04)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

The holdout for unexporting __get_user_pages_unlocked() is its invocation in
mm/process_vm_access.c: process_vm_rw_single_vec(), as this definitely _does_
seem to invoke VM_FAULT_RETRY behaviour which get_user_pages_remote() will not
trigger if we were to replace it with the latter.

I'm not sure how to proceed in this case - get_user_pages_remote() invocations
assume mmap_sem is held so can't offer VM_FAULT_RETRY behaviour as the lock
can't be assumed to be safe to release, and get_user_pages_unlocked() assumes
tsk, mm are set to current, current->mm respectively so we can't use that here
either.

Is it important to retain VM_FAULT_RETRY behaviour here, does it matter? If it
isn't so important then we can just go ahead and replace with
get_user_pages_remote() and unexport away.

Of course the whole idea of unexporting __get_user_pages_unlocked() might be
bogus so let me know in that case also :)