btrfs: refactor main loop in memcpy_extent_buffer()

[BACKGROUND]
Currently memcpy_extent_buffer() does a loop where it would stop at
any page boundary inside [dst_offset, dst_offset + len) or [src_offset,
src_offset + len).

This is mostly allowing us to do copy_pages(), but if we're going to use
folios we will need to handle multi-page (the old behavior) or single
folio (the new optimization).

The current code would be a burden for future changes.

[ENHANCEMENT]
There is a hidden pitfall of the naming memcpy_extent_buffer(), unlike
regular memcpy(), this function can handle overlapping ranges.

So here we extract write_extent_buffer() into a new internal helper,
__write_extent_buffer(), and add a new parameter @use_memmove, to
indicate whether we should use memmove() or regular memcpy().

Now we can go __write_extent_buffer() to handle writing into the dst
range, with proper overlapping detection.

This has a tiny change to the chance of calling memmove().
As the split only happens at the source range page boundaries, the
memcpy/memmove() range would be slightly larger than the old code,
thus slightly increase the chance we call memmove() other than memcopy().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4129,8 +4129,9 @@ static void assert_eb_page_uptodate(const struct extent_buffer *eb,
 	}
 }
 
-void write_extent_buffer(const struct extent_buffer *eb, const void *srcv,
-			 unsigned long start, unsigned long len)
+static void __write_extent_buffer(const struct extent_buffer *eb,
+				  const void *srcv, unsigned long start,
+				  unsigned long len, bool use_memmove)
 {
 	size_t cur;
 	size_t offset;
@@ -4138,6 +4139,8 @@ void write_extent_buffer(const struct extent_buffer *eb, const void *srcv,
 	char *kaddr;
 	char *src = (char *)srcv;
 	unsigned long i = get_eb_page_index(start);
+	/* For unmapped (dummy) ebs, no need to check their uptodate status. */
+	const bool check_uptodate = !test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags);
 
 	WARN_ON(test_bit(EXTENT_BUFFER_NO_CHECK, &eb->bflags));
 
@@ -4148,11 +4151,15 @@ void write_extent_buffer(const struct extent_buffer *eb, const void *srcv,
 
 	while (len > 0) {
 		page = eb->pages[i];
-		assert_eb_page_uptodate(eb, page);
+		if (check_uptodate)
+			assert_eb_page_uptodate(eb, page);
 
 		cur = min(len, PAGE_SIZE - offset);
 		kaddr = page_address(page);
-		memcpy(kaddr + offset, src, cur);
+		if (use_memmove)
+			memmove(kaddr + offset, src, cur);
+		else
+			memcpy(kaddr + offset, src, cur);
 
 		src += cur;
 		len -= cur;
@@ -4161,6 +4168,12 @@ void write_extent_buffer(const struct extent_buffer *eb, const void *srcv,
 	}
 }
 
+void write_extent_buffer(const struct extent_buffer *eb, const void *srcv,
+			 unsigned long start, unsigned long len)
+{
+	return __write_extent_buffer(eb, srcv, start, len, false);
+}
+
 static void memset_extent_buffer(const struct extent_buffer *eb, int c,
 				 unsigned long start, unsigned long len)
 {
@@ -4409,34 +4422,25 @@ void memcpy_extent_buffer(const struct extent_buffer *dst,
 			  unsigned long dst_offset, unsigned long src_offset,
 			  unsigned long len)
 {
-	size_t cur;
-	size_t dst_off_in_page;
-	size_t src_off_in_page;
-	unsigned long dst_i;
-	unsigned long src_i;
+	unsigned long cur_off = 0;
 
 	if (check_eb_range(dst, dst_offset, len) ||
 	    check_eb_range(dst, src_offset, len))
 		return;
 
-	while (len > 0) {
-		dst_off_in_page = get_eb_offset_in_page(dst, dst_offset);
-		src_off_in_page = get_eb_offset_in_page(dst, src_offset);
+	while (cur_off < len) {
+		unsigned long cur_src = cur_off + src_offset;
+		unsigned long pg_index = get_eb_page_index(cur_src);
+		unsigned long pg_off = get_eb_offset_in_page(dst, cur_src);
+		unsigned long cur_len = min(src_offset + len - cur_src,
+					    PAGE_SIZE - pg_off);
+		void *src_addr = page_address(dst->pages[pg_index]) + pg_off;
+		const bool use_memmove = areas_overlap(src_offset + cur_off,
+						       dst_offset + cur_off, cur_len);
 
-		dst_i = get_eb_page_index(dst_offset);
-		src_i = get_eb_page_index(src_offset);
-
-		cur = min(len, (unsigned long)(PAGE_SIZE -
-					       src_off_in_page));
-		cur = min_t(unsigned long, cur,
-			(unsigned long)(PAGE_SIZE - dst_off_in_page));
-
-		copy_pages(dst->pages[dst_i], dst->pages[src_i],
-			   dst_off_in_page, src_off_in_page, cur);
-
-		src_offset += cur;
-		dst_offset += cur;
-		len -= cur;
+		__write_extent_buffer(dst, src_addr, dst_offset + cur_off, cur_len,
+				      use_memmove);
+		cur_off += cur_len;
 	}
 }