Re: [PATCH v2] mm/folio: Avoid special handling for order value 0 in folio_set_order

Matthew Wilcox
2023-05-15T17:18:35Z

On Mon, May 15, 2023 at 10:38:09PM +0530, Tarun Sahu wrote:
> @@ -1951,9 +1950,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
> struct page *p;
>
> __folio_clear_reserved(folio);
> - __folio_set_head(folio);
> - /* we rely on prep_new_hugetlb_folio to set the destructor */
> - folio_set_order(folio, order);
> for (i = 0; i < nr_pages; i++) {
> p = folio_page(folio, i);
>
> @@ -1999,6 +1995,9 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
> if (i != 0)
> set_compound_head(p, &folio->page);
> }
> + __folio_set_head(folio);
> + /* we rely on prep_new_hugetlb_folio to set the destructor */
> + folio_set_order(folio, order);

This makes me nervous, as I said before. This means that
compound_head(tail) can temporarily point to a page which is not marked
as a head page. That's different from prep_compound_page(). You need to
come up with some good argumentation for why this is safe, and no amount
of testing you do can replace it -- any race in this area will be subtle.