Re: Nick's core remove PageReserved broke vmware...

From: Nick Piggin (nickpiggin_at_yahoo.com.au)
Date: 11/02/05

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    Date:	Wed, 02 Nov 2005 11:34:02 +1100
To: Petr Vandrovec <vandrove@vc.cvut.cz>

    Petr Vandrovec wrote:
    > Hello Nick,
    > what's the reason behind disallowing get_user_pages() on VM_RESERVED
    > regions? vmmon uses VM_RESERVED on its 'vma' as otherwise some kernels
    > used by SUSE complained loudly about mismatch between PageReserved() and
    > VM_RESERVED flags.
    >

    Hi Petr,

    The reason is that VM_RESERVED indicates that the core vm is not allowed
    to touch any 'struct page' through this mapping, which get_user_pages
    would do.

    > I'll remove it from vmmon for >= 2.6.14 kernels as that bogus test
    > never made to Linux kernel, but I cannot find any reason why
    > get_user_pages() should not work on VM_RESERVED (or VM_IO for that
    > matter) user pages. Can you show me reasoning behind that decision ?

    The reasoning behind the decision was so VM_RESERVED is usable for a
    complete replacement to PageReserved. For example mappings through
    /dev/mem should not touch the page count.

    You may be able to go a step further and clear PageReserved from your
    pages as well, and thus have a working driver without special casing
    for both kernels.

    Thanks,
    Nick 

    -- 
SUSE Labs, Novell Inc.
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