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SubjectRe: copy_page_range() with VM_LOCKED
On Mon, Mar 10, 2008 at 5:40 PM, Hugh Dickins <hugh@veritas.com> wrote:

> > mmap with MAP_LOCKED only promises the page will not be paged out, not
> > that the page will never fault but I would like to get that behaviour.
> >
> > Would it be possible to add VM_LOCKED to the above conditional so
> > copy_page_range would always copy VM_LOCKED vma page tables or would
> > that be considered insane and broken?
>
> It would be possible, but I don't think it would be justified - for
> everybody who wants your behaviour, there'll be others who want the
> current behaviour (and that condition reflects those cases on which
> correctness demands we do the copy).
>
> If you really want that, I think you'll have to patch your own kernel.
>
> But much much better, mlock that area in your child after the fork,
> since it isn't actually locked at present: that mlock will bring in
> the pages - not quite as efficiently as copy_page_range would have done,
> but as or more efficiently than the original mlock in the parent.

The reason I want to do this is to work around a silicon bug so I'd
rather keep the whole workaround in the kernel rather than hacking the
C library etc. I wasn't aware that the VM_LOCKED flag would be lost in
the child, it looks like I'll have to come up with a more creative
solution...

Thanks for taking the time to explain how this stuff fits together,


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