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    SubjectRe: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)
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    On 11/14/2017 07:44 PM, Matthew Wilcox wrote:
    > On Mon, Nov 13, 2017 at 02:46:25PM -0800, Dave Hansen wrote:
    >> On 11/13/2017 02:20 PM, Dave Hansen wrote:
    >>> On 11/09/2017 05:09 PM, Tycho Andersen wrote:
    >>>> which I guess is from the additional flags in grow_dev_page() somewhere down
    >>>> the stack. Anyway... it seems this is a kernel allocation that's using
    >>>> MIGRATE_MOVABLE, so perhaps we need some more fine tuned heuristic than just
    >>>> all MOVABLE allocations are un-mapped via xpfo, and all the others are mapped.
    >>>>
    >>>> Do you have any ideas?
    >>>
    >>> It still has to do a kmap() or kmap_atomic() to be able to access it. I
    >>> thought you hooked into that. Why isn't that path getting hit for these?
    >>
    >> Oh, this looks to be accessing data mapped by a buffer_head. It
    >> (rudely) accesses data via:
    >>
    >> void set_bh_page(struct buffer_head *bh,
    >> ...
    >> bh->b_data = page_address(page) + offset;
    >
    > We don't need to kmap in order to access MOVABLE allocations. kmap is
    > only needed for HIGHMEM allocations. So there's nothing wrong with ext4
    > or set_bh_page().

    Yeah, it's definitely not _buggy_.

    Although, I do wonder what we should do about these for XPFO. Should we
    just stick a kmap() in there and comment it? What we really need is a
    mechanism to say "use this as a kernel page" and "stop using this as a
    kernel page". kmap() does that... kinda. It's not a perfect fit, but
    it's pretty close.

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