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SubjectRe: [PATCH 1/2] mm: introduce MAP_FIXED_SAFE
On Wed 13-12-17 04:50:53, Matthew Wilcox wrote:
> On Wed, Dec 13, 2017 at 10:25:49AM +0100, Michal Hocko wrote:
> > +++ b/mm/mmap.c
> > @@ -1342,6 +1342,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> > if (!(file && path_noexec(&file->f_path)))
> > prot |= PROT_EXEC;
> >
> > + /* force arch specific MAP_FIXED handling in get_unmapped_area */
> > + if (flags & MAP_FIXED_SAFE)
> > + flags |= MAP_FIXED;
> > +
> > if (!(flags & MAP_FIXED))
> > addr = round_hint_to_min(addr);
> >
>
> We're up to 22 MAP_ flags now. We'll run out soon. Let's preserve half
> of a flag by giving userspace the definition:
>
> #define MAP_FIXED_SAFE (MAP_FIXED | _MAP_NOT_HINT)

I've already tried to explain why this cannot be a modifier for
MAP_FIXED. Read about the backward compatibility note...
Or do I misunderstand what you are saying here?

> then in here:
>
> if ((flags & _MAP_NOT_HINT) && !(flags & MAP_FIXED))
> return -EINVAL;
>
> Now we can use _MAP_NOT_HINT all by itself in the future to mean
> something else.

--
Michal Hocko
SUSE Labs

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