Messages in this thread | | | Subject | Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments | From | Lee Schermerhorn <> | Date | Thu, 29 Jan 2009 23:43:40 -0500 |
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On Thu, 2009-01-29 at 15:02 -0800, Linus Torvalds wrote: > > On Thu, 29 Jan 2009, Hugh Dickins wrote: > > > > The problem has always been that file->f_op->mmap(file, vma) on a > > special file can do various strange things, changing surprising > > fields of the struct vma passed to it, changing its mergeability. > > Ahh, you're right. That's somewhat bogus, but it does explain why we have > those two separate merge_vma() calls. > > And as you also point out: > > > Now it may well be that every driver which does something strange > > there already sets one of the VM_SPECIAL flags which prevent merging, > > or can easily be fixed to do so, or otherwise clearly cannot pose a > > problem. > > .. and I think this is the right answer. If a device driver really does > something as odd as play games with the offset that would make merging > wrong, it had better set some of the VM_SPECIAL bits (presumably VM_IO) in > order to never merge at all.
Just want to note that install_special_mappings()--used for, e.g., the vdso--sets VM_DONTEXPAND [one of the VM_SPECIAL flags] which, if we want to prevent merging, makes a lot of sense to me. It appears that get_user_pages() will balk at addresses in vmas with VM_IO [and VM_PFNMAP] which might not be what one wants. For example, you can't pre-populate the ptes via make_pages_present() with this flag.
Lee
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