iov_iter: Handle compound highmem pages in copy_page_from_iter_atomic()

copy_page_from_iter_atomic() already handles !highmem compound
pages correctly, but if we are passed a highmem compound page,
each base page needs to be mapped & unmapped individually.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Kent Overstreet <kent.overstreet@linux.dev>

lib/iov_iter.c

@@ -566,24 +566,37 @@ size_t iov_iter_zero(size_t bytes, struct iov_iter *i)
 }
 EXPORT_SYMBOL(iov_iter_zero);
 
-size_t copy_page_from_iter_atomic(struct page *page, unsigned offset, size_t bytes,
-				  struct iov_iter *i)
+size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
+		size_t bytes, struct iov_iter *i)
 {
-	char *p;
+	size_t n, copied = 0;
 
 	if (!page_copy_sane(page, offset, bytes))
 		return 0;
 	if (WARN_ON_ONCE(!i->data_source))
 		return 0;
 
-	p = kmap_atomic(page) + offset;
-	iterate_and_advance(i, bytes, base, len, off,
-		copyin(p + off, base, len),
-		memcpy_from_iter(i, p + off, base, len)
-	)
-	kunmap_atomic(p);
+	do {
+		char *p;
 
-	return bytes;
+		n = bytes - copied;
+		if (PageHighMem(page)) {
+			page += offset / PAGE_SIZE;
+			offset %= PAGE_SIZE;
+			n = min_t(size_t, n, PAGE_SIZE - offset);
+		}
+
+		p = kmap_atomic(page) + offset;
+		iterate_and_advance(i, n, base, len, off,
+			copyin(p + off, base, len),
+			memcpy_from_iter(i, p + off, base, len)
+		)
+		kunmap_atomic(p);
+		copied += n;
+		offset += n;
+	} while (PageHighMem(page) && copied != bytes && n > 0);
+
+	return copied;
 }
 EXPORT_SYMBOL(copy_page_from_iter_atomic);