Re: [patch] mmap bug with drivers that adjust vm_start

Benjamin LaHaise (
Tue, 26 Mar 2002 13:57:03 -0500

On Tue, Mar 26, 2002 at 05:42:36PM +0100, Andrea Arcangeli wrote:
> However if the patch is needed it means the ->mmap also must do the
> do_munmap stuff by hand internally, which is very ugly given we also did
> our own do_munmap in a completly different region (the one requested by
> the user).

At least my own code checks for that and fails if there is a mapping
already placed at the fixed address it needs to use. If we're paranoid,
we could BUG() on getting a vma back from the new find_vma_prepare call.

> Our do_munmap should not happen if we place the mapping
> elsewhere. If possible I would prefer to change those drivers to
> advertise their enforced vm_start with a proper callback, the current
> way is halfway broken still. BTW, which are those drivers, and why they
> needs to enforce a certain vm_start (also despite MAP_FIXED that they
> cannot check within the ->mmap callback)?

Video drivers, others that require specific alignment (4MB pages for
example). Historically, the mmap call has been the hook for doing this,
hence the comment in do_mmap from davem. Unless there's a really good
reason for changing the hook, I don't see doing so as providing much
benefit other than making source compatibility hard.


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