Re: [PATCH 08/10] mm: replace __access_remote_vm() write parameter with gup_flags

Michal Hocko
2016-10-19T08:13:52Z

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@gmail.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
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