linux-kernel - Re: [PATCH 08/10] mm: replace __access_remote_vm() write parameter with gup

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Date:   Wed, 19 Oct 2016 10:13:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        adi-buildroot-devel@...ts.sourceforge.net,
        ceph-devel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-cris-kernel@...s.com, linux-fbdev@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-mips@...ux-mips.org, linux-rdma@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-sh@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        netdev@...r.kernel.org, sparclinux@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 08/10] mm: replace __access_remote_vm() write parameter
 with gup_flags

On Wed 19-10-16 09:59:03, Jan Kara wrote:
> On Thu 13-10-16 01:20:18, Lorenzo Stoakes wrote:
> > This patch removes the write parameter from __access_remote_vm() and replaces it
> > with a gup_flags parameter as use of this function previously _implied_
> > FOLL_FORCE, whereas after this patch callers explicitly pass this flag.
> > 
> > We make this explicit as use of FOLL_FORCE can result in surprising behaviour
> > (and hence bugs) within the mm subsystem.
> > 
> > Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> 
> So I'm not convinced this (and the following two patches) is actually
> helping much. By grepping for FOLL_FORCE we will easily see that any caller
> of access_remote_vm() gets that semantics and can thus continue search

I am really wondering. Is there anything inherent that would require
FOLL_FORCE for access_remote_vm? I mean FOLL_FORCE is a really
non-trivial thing. It doesn't obey vma permissions so we should really
minimize its usage. Do all of those users really need FOLL_FORCE?

Anyway I would rather see the flag explicit and used at more places than
hidden behind a helper function.
-- 
Michal Hocko
SUSE Labs