mm/highmem: delete memmove_page()

Matthew Wilcox reported that, while he was looking at memmove_page(), he
realized that it can't actually work.

The reasons are hidden in its implementation, which makes use of memmove()
on logical addresses provided by kmap_local_page().  memmove() does the
wrong thing when it tests "if (dest <= src)".

Therefore, delete memmove_page().

No need to change any other code because we have no call sites of
memmove_page() across the whole kernel.

Link: https://lkml.kernel.org/r/20220606141533.555-1-fmdefrancesco@gmail.com
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reported-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: xp4ns3

include/linux/highmem.h

@@ -336,19 +336,6 @@ static inline void memcpy_page(struct page *dst_page, size_t dst_off,
 	kunmap_local(dst);
 }
 
-static inline void memmove_page(struct page *dst_page, size_t dst_off,
-			       struct page *src_page, size_t src_off,
-			       size_t len)
-{
-	char *dst = kmap_local_page(dst_page);
-	char *src = kmap_local_page(src_page);
-
-	VM_BUG_ON(dst_off + len > PAGE_SIZE || src_off + len > PAGE_SIZE);
-	memmove(dst + dst_off, src + src_off, len);
-	kunmap_local(src);
-	kunmap_local(dst);
-}
-
 static inline void memset_page(struct page *page, size_t offset, int val,
 			       size_t len)
 {