On 31 Mar 2021, at 12:44, Matthew Wilcox wrote:

On Mon, Mar 29, 2021 at 11:39:32AM -0400, Zi Yan wrote:

quoted

+	for (off_cur = off_start; off_cur < off_end;) {
+		struct page *fpage = pagecache_get_page(mapping, off_cur,
+						FGP_ENTRY | FGP_HEAD, 0);
+
+		if (xa_is_value(fpage) || !fpage) {
+			off_cur += PAGE_SIZE;
+			continue;
+		}
+
+		if (!is_transparent_hugepage(fpage)) {
+			off_cur += PAGE_SIZE;
+			goto next;
+		}
+		total++;
+		off_cur = fpage->index + thp_size(fpage);

That can't be right.  fpage->index is in units of pages and thp_size is
in units of bytes.  I wish C had a better type system.
I think you meant:

		off_cur = fpage->index + thp_nr_pages(fpage);

Also, I think this loop would read better as ...

	for (index = off_start; index < off_end; index += nr_pages) {
		struct page *fpage = pagecache_get_page(mapping, index,
						FGP_ENTRY | FGP_HEAD, 0);
		nr_pages = 1;
		if (xa_is_value(fpage) || !fpage)
			continue;
		if (!is_transparent_hugepage(fpage))
			goto next;
		total++;
		nr_pages = thp_nr_pages(fpage);
...

Thanks for catching this! I mixed this with looping through VMA, which
is in units of bytes. I will fix this and use your suggested loop code.

—
Best Regards,
Yan Zi