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With aliasing VIPT cache support, the ARM implementation of clear_user_page() and copy_user_page() sets up a temporary kernel space mapping such that we have the same cache colour as the userspace page. This avoids having to consider any userspace aliases from this operation. However, when highmem is enabled, kmap_atomic() have to setup mappings. The copy_user_highpage() and clear_user_highpage() call these functions before delegating the copies to copy_user_page() and clear_user_page(). The effect of this is that each of the *_user_highpage() functions setup their own kmap mapping, followed by the *_user_page() functions setting up another mapping. This is rather wasteful. Thankfully, copy_user_highpage() can be overriden by architectures by defining __HAVE_ARCH_COPY_USER_HIGHPAGE. However, replacement of clear_user_highpage() is more difficult because its inline definition is not conditional. It seems that you're expected to define __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE and provide a replacement __alloc_zeroed_user_highpage() implementation instead. The allocation itself is fine, so we don't want to override that. What we really want to do is to override clear_user_highpage() with our own version which doesn't kmap_atomic() unnecessarily. Other VIPT architectures (PARISC and SH) would also like to override this function as well. Acked-by: Hugh Dickins <hugh@veritas.com> Acked-by: James Bottomley <James.Bottomley@HansenPartnership.com> Acked-by: Paul Mundt <lethal@linux-sh.org> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk> |
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