Re: [PATCH 07/11] unpaged: COW on VM_UNPAGED

From: David S. Miller
Date: Thu Nov 17 2005 - 18:52:52 EST

From: Hugh Dickins <hugh@xxxxxxxxxxx>
Date: Thu, 17 Nov 2005 19:37:23 +0000 (GMT)

> Remove the BUG_ON(vma->vm_flags & VM_UNPAGED) from do_wp_page, and let
> it do Copy-On-Write without touching the VM_UNPAGED's page counts - but
> this is incomplete, because the anonymous page it inserts will itself
> need to be handled, here and in other functions - next patch.
> We still don't copy the page if the pfn is invalid, because the
> copy_user_highpage interface does not allow it. But that's not been
> a problem in the past: can be added in later if the need arises.
> Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>

Do we even need this? It is a very serious question...

The cases I've seen, and am aware of, I documented in a previous
mail and those involve MAP_PRIVATE maps of /dev/mem or other
similar fixed page mapping devices.

Which case truly needs COW faults on VM_RESERVED memory which
isn't an application bug of some sort?
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