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    SubjectRe: why does the macro "ZERO_PAGE" take an argument?
    Ralf Baechle wrote:
    > Cache aliases. When the same page of physical memory is mapped twice to
    > user space, let's say at address addr and addr + PAGE_SIZE this is normally
    > harmless although wasteful on processors with virtually indexed caches as
    > long as the page is mapped read-only such as in case of ZERO_PAGE.
    >
    > If the same thing happens with a writable page there is the chance of
    > severe data corruption. Some members of the R4000 family are now trying
    > to be helpful by throwing the kernel a "virtual coherency" exception. The
    > bad news about this exception is there might be thousands (the theoretical
    > worst case would be millions) of it in a single second, so servicing can be
    > very expensive. For the ZERO page this can be avoided by using several
    > pages mapped in a way such that their addresses don't conflict.

    Note that it can be a *very expensive* waste even on machines that do
    this in hardware. Colouring the zeropage can have sizable performance
    advantages for virtually no cost.

    -hpa

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