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    SubjectRe: [RFC PATCH 10/47] mm: asi: Support for global non-sensitive direct map allocations
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    On 3/23/22 16:48, Junaid Shahid wrote:
    > Hi Matthew,
    >
    > On 3/23/22 14:06, Matthew Wilcox wrote:
    >> On Tue, Feb 22, 2022 at 09:21:46PM -0800, Junaid Shahid wrote:
    >>> standard ASI instances. A new page flag is also added so that when
    >>> these pages are freed, they can also be unmapped from the ASI page
    >>> tables.
    >>
    >> It's cute how you just throw this in as an aside.  Page flags are
    >> in high demand and just adding them is not to be done lightly.  Is
    >> there any other way of accomplishing what you want?
    >>
    >
    > I suppose we may be able to use page_ext instead. That certainly should be
    > feasible for the PG_local_nonsensitive flag introduced in a later patch,
    > although I am not completely sure about the PG_global_nonsensitive flag. That
    > could get slightly tricky (though likely still possible to do) in case we need
    > to allocate any non-sensitive memory before page_ext is initialized. One concern
    > with using page_ext could be the extra memory usage on large machines.
    >
    > BTW is page flag scarcity an issue on 64-bit systems as well, or only 32-bit
    > systems? ASI is only supported on 64-bit systems (at least currently).
    >

    One other thing that we could do to remove the need for the
    PG_global_nonsensitive flag altogether (though not the PG_local_nonsensitive
    flag) would be to always try to unmap pages from the asi_global_nonsensitive_pgd
    in free_pages(). Basically, that would mean adding a page table walk to every
    free_pages() rather than just non-sensitive free_pages(). Do you think that may
    be a better trade-off in order to avoid the flag?

    Thanks,
    Junaid

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