Date: Wed, 19 Oct 2016 19:01:27 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Hugh Dickins <hughd@google.com>,
        Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        adi-buildroot-devel@lists.sourceforge.net, ceph-devel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
        kvm@vger.kernel.org, linux-alpha@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-cris-kernel@axis.com,
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        linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org,
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        linux-samsung-soc@vger.kernel.org, linux-scsi@vger.kernel.org,
        linux-security-module@vger.kernel.org, linux-sh@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org,
        sparclinux@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH 00/10] mm: adjust get_user_pages* functions to explicitly
 pass FOLL_* flags
Message-ID: <20161019170127.GN24393@dhcp22.suse.cz>
References: <20161013002020.3062-1-lstoakes@gmail.com>
 <20161018153050.GC13117@dhcp22.suse.cz>
 <20161019085815.GA22239@lucifer>
 <20161019090727.GE7517@dhcp22.suse.cz>
 <5807A427.7010200@linux.intel.com>
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On Wed 19-10-16 09:49:43, Dave Hansen wrote:
> On 10/19/2016 02:07 AM, Michal Hocko wrote:
> > On Wed 19-10-16 09:58:15, Lorenzo Stoakes wrote:
> >> On Tue, Oct 18, 2016 at 05:30:50PM +0200, Michal Hocko wrote:
> >>> I am wondering whether we can go further. E.g. it is not really clear to
> >>> me whether we need an explicit FOLL_REMOTE when we can in fact check
> >>> mm != current->mm and imply that. Maybe there are some contexts which
> >>> wouldn't work, I haven't checked.
> >>
> >> This flag is set even when /proc/self/mem is used. I've not looked deeply into
> >> this flag but perhaps accessing your own memory this way can be considered
> >> 'remote' since you're not accessing it directly. On the other hand, perhaps this
> >> is just mistaken in this case?
> > 
> > My understanding of the flag is quite limited as well. All I know it is
> > related to protection keys and it is needed to bypass protection check.
> > See arch_vma_access_permitted. See also 1b2ee1266ea6 ("mm/core: Do not
> > enforce PKEY permissions on remote mm access").
> 
> Yeah, we need the flag to tell us when PKEYs should be applied or not.
> The current task's PKRU (pkey rights register) should really only be
> used to impact access to the task's memory, but has no bearing on how a
> given task should access remote memory.

The question I had earlier was whether this has to be an explicit FOLL
flag used by g-u-p users or we can just use it internally when mm !=
current->mm

-- 
Michal Hocko
SUSE Labs