parisc: Update comment regarding implementation of copy_user_page_asm

The attached patch describes the current implementation of
copy_user_page_asm().  It is possible to implement this routine using
either the kernel page mappings or equivalent aliases.  I tested both
and decided the former was more efficient.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>

1 files changed, 11 insertions, 5 deletions

diff --git a/arch/parisc/kernel/pacache.S b/arch/parisc/kernel/pacache.S
index b743a80eaba0..1250c02201bb 100644
--- a/arch/parisc/kernel/pacache.S
+++ b/arch/parisc/kernel/pacache.S
@@ -573,11 +573,17 @@ ENDPROC(copy_page_asm)
         .endm
 
         /*
-         * We can't do this since copy_user_page is used to bring in
-         * file data that might have instructions. Since the data would
-         * then need to be flushed out so the i-fetch can see it, it
-         * makes more sense to just copy through the kernel translation
-         * and flush it.
+         * copy_user_page_asm() performs a page copy using mappings
+         * equivalent to the user page mappings.  It can be used to
+         * implement copy_user_page() but unfortunately both the `from'
+         * and `to' pages need to be flushed through mappings equivalent
+         * to the user mappings after the copy because the kernel accesses
+         * the `from' page through the kmap kernel mapping and the `to'
+         * page needs to be flushed since code can be copied.  As a
+         * result, this implementation is less efficient than the simpler
+         * copy using the kernel mapping.  It only needs the `from' page
+         * to flushed via the user mapping.  The kunmap routines handle
+         * the flushes needed for the kernel mapping.
          *
          * I'm still keeping this around because it may be possible to
          * use it if more information is passed into copy_user_page().