madvise_inject_error skips pages

[Matthew Wilcox <willy@infradead.org>]

I was going to send this patch:

+++ b/mm/madvise.c
@@ -1136,9 +1136,10 @@ static int madvise_inject_error(int behavior,
                /*
                 * When soft offlining hugepages, after migrating the page
                 * we dissolve it, therefore in the second loop "page" will
-                * no longer be a compound page.
+                * no longer be part of a large folio.
                 */
-               size = page_size(compound_head(page));
+               size = folio_size(page_folio(page));
+               start = start & ~(size - 1);

                if (behavior == MADV_SOFT_OFFLINE) {
                        pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",

because right now if you start in the middle of (e.g.) an order-4 folio
followed by an order-0 folio, you'll skip the order-0 folio immediately
following it.

But then I realised that we can come to this path in the middle of
a large file-backed folio that's mapped misaligned and has a COW page
in the middle, and the whole thing is just misguided.  So I gave up.
Anyone want to take a crack at fixing & testing this?