Re: [PATCH] mm/mremap: unmap full fixed target for multi-VMA moves

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, May 11, 2026 at 06:02:37PM +0200, Jann Horn wrote:
> On Mon, May 11, 2026 at 6:00 PM Lorenzo Stoakes <ljs@kernel.org> wrote:
> > On Mon, May 11, 2026 at 05:40:24PM +0200, Jann Horn wrote:
> > > On Mon, May 11, 2026 at 5:32 PM Lorenzo Stoakes <ljs@kernel.org> wrote:
> > > > On Mon, May 11, 2026 at 05:19:50PM +0200, Jann Horn wrote:
> > > > > On Mon, May 11, 2026 at 5:05 PM Lorenzo Stoakes <ljs@kernel.org> wrote:
> > > > > > Hmmm I think it's a bit debateable honestly. The ability to handle there being
> > > > > > gaps is a _new thing_, so there are no semantics to speak of. Prevoiusly
> > > > > > mremap() simply required that you only span across a single VMA.
> > > > >
> > > > > FWIW, I think mremap() on a source region with gaps is such a
> > > > > hazardous operation that nearly no userspace code should be doing it -
> > > > > gaps are areas in which any mmap() call without a fixed address could
> > > > > place unrelated mappings (unless stack VMAs are involved, which would
> > > > > also be a weird scenario), so to use it safely, you have to, among
> > > > > other things, make sure not to use libc malloc() at a time when that
> > > > > could place an allocation in the gap (which means you also can't use
> > > > > printf(), and so on, unless you have swapped out the memory
> > > > > allocator), and make sure that you have no other threads that could be
> > > > > doing that, and so on. There are rare circumstances under which it
> > > > > could be safe, but I think it is almost always better to have a
> > > > > PROT_NONE anonymous VMA or such as a placeholder.
> > > >
> > > > Well, we're holding the mmap write lock so none of that could happen
> > > > _during_ the operation right?
> > >
> > > Not during the operation, but right before the operation. So from the
> > > userspace perspective, you have to know that there are no concurrent
> > > threads that could be creating memory mappings at non-fixed addresses,
> > > and you have to know that no mappings can have been created in the
> > > memory range between when you checked that it's empty and when you
> > > make the syscall.
> >
> > That's a very good point :)
> >
> > But I guess applies to any operations that operate over a range of mappings
> > anyway (madvise() lets you also do this, though it'll give an error code
> > _at the end_ _after having done the operations_ if there are gaps).
> >
> > So madvise() can have the exact same thing happen right? which is... fun :)
> >
> > I actually wonder if we shouldn't just change this to disallow gaps. It'd
>
> That would be nice...

Yeah let's go with that then I think :)

>
> > simplify the code and we could even do the check upfront in one pass. It's
> > doubtful anybody is relying on the gaps behaviour for anything real.
>
> (I remember that in the past, there were concerns that MM operations
> with multiple passes are slow, but I guess mremap() is probably not
> such a hot operation that that's a concern here.)

Yeah not at all, it's rather an expensive operation and if it's on your hot path
then you did something wrong :)

Cheers, Lorenzo