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    SubjectRE: [patch 2/8] x86 PAT: set VM_PFNMAP flag in vm_insert_pfn


    >-----Original Message-----
    >From: Nick Piggin [mailto:npiggin@suse.de]
    >Sent: Wednesday, November 12, 2008 7:44 PM
    >To: Pallipadi, Venkatesh
    >Cc: Ingo Molnar; Thomas Gleixner; H Peter Anvin; Hugh Dickins;
    >Roland Dreier; Jesse Barnes; Jeremy Fitzhardinge; Arjan van de
    >Ven; linux-kernel@vger.kernel.org; Siddha, Suresh B
    >Subject: Re: [patch 2/8] x86 PAT: set VM_PFNMAP flag in vm_insert_pfn
    >
    >On Wed, Nov 12, 2008 at 04:02:47PM -0800, Pallipadi, Venkatesh wrote:
    >>
    >>
    >> >-----Original Message-----
    >> >From: Nick Piggin [mailto:npiggin@suse.de]
    >> >Sent: Wednesday, November 12, 2008 3:23 PM
    >> >To: Pallipadi, Venkatesh
    >> >Cc: Ingo Molnar; Thomas Gleixner; H.Peter Anvin; Hugh Dickins;
    >> >Roland Dreier; Jesse Barnes; Jeremy Fitzhardinge; Arjan van de
    >> >Ven; linux-kernel@vger.kernel.org; Siddha, Suresh B
    >> >Subject: Re: [patch 2/8] x86 PAT: set VM_PFNMAP flag in
    >vm_insert_pfn
    >> >
    >> >You have to be careful of this, because it can be called
    >with mmap_sem
    >> >held for read only. Hmm, I guess vm_insert_page is doing the
    >> >same thing.
    >> >Probably mostly works because all other modifiers of vm_flags
    >> >are holding
    >> >mmap_sem.
    >>
    >> Yes. I did the patch looking at vm_insert_page doing similar thing.
    >>
    >> >
    >> >However, in some cases, code can do vm_insert_pfn and vm_insert_page
    >> >(actually hmm, no vm_insert_mixed actually should cover
    >most of those
    >> >cases).
    >> >
    >> >Still, I'd be much happier if we could make these into
    >BUG_ON, and then
    >> >teach callers to set it in their .mmap routines.
    >>
    >> Actually, vm_insert_pfn() already has a BUG_ON() at the
    >start for cases
    >> where neither (or both) MIXEDMAP and PFNMAP is not set. So,
    >that should
    >> cover the case we are worried about it here and we can eliminate this
    >> patch altogether. Only part I am not sure about is why we are looking
    >> for MIXEDMAP here. Shouldn't they be using vm_insert_mixed instead?
    >
    >They should, but it will do an inesrt_pfn in some cases, won't it?
    >

    Yes. It does. But, it calls a lower level insert_pfn() function. The lower
    level insert_pfn() does not have any bug checks. But the higher level
    vm_insert_pfn() checks for PFNMAP or MIXEDMAP.

    Thanks,
    Venki


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