Re: [PATCH] mm: Handle compound pages better in __dump_page()

[Matthew Wilcox]

On Sun, Nov 17, 2024 at 08:46:45PM -0800, Kees Cook wrote:
> On Mon, Nov 18, 2024 at 04:10:52AM +0000, Matthew Wilcox wrote:
> > folio_test_large() does not touch page[1].  Look:
> 
> It does, though. :( It's via the PageTail(), which calls page_is_fake_head():

Oh.  It shouldn't; that's unnecessary.

+++ b/include/linux/page-flags.h
@@ -306,7 +306,7 @@ static const unsigned long *const_folio_flags(const struct folio *folio,
 {
        const struct page *page = &folio->page;

-       VM_BUG_ON_PGFLAGS(PageTail(page), page);
+       VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
        VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
        return &page[n].flags;
 }
@@ -315,7 +315,7 @@ static unsigned long *folio_flags(struct folio *folio, unsigned n)
 {
        struct page *page = &folio->page;

-       VM_BUG_ON_PGFLAGS(PageTail(page), page);
+       VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
        VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
        return &page[n].flags;
 }

should fix that.