Re: [PATCH v6 20/37] mm: fix non-compound multi-order memory accounting in __free_pages

Matthew Wilcox
2024-03-21T17:04:33Z

On Thu, Mar 21, 2024 at 04:48:53PM +0000, Matthew Wilcox wrote:
> On Thu, Mar 21, 2024 at 09:36:42AM -0700, Suren Baghdasaryan wrote:
> > +++ b/mm/page_alloc.c
> > @@ -4700,12 +4700,15 @@ void __free_pages(struct page *page, unsigned int order)
> > {
> > /* get PageHead before we drop reference */
> > int head = PageHead(page);
> > + struct alloc_tag *tag = pgalloc_tag_get(page);
> >
> > if (put_page_testzero(page))
> > free_the_page(page, order);
> > - else if (!head)
> > + else if (!head) {
> > + pgalloc_tag_sub_pages(tag, (1 << order) - 1);
> > while (order-- > 0)
> > free_the_page(page + (1 << order), order);
> > + }
>
> Why do you need these new functions instead of just:
>
> + else if (!head) {
> + pgalloc_tag_sub(page, (1 << order) - 1);
> while (order-- > 0)
> free_the_page(page + (1 << order), order);
> + }

Actually, I'm not sure this is safe (I don't fully understand codetags,
so it may be safe). What can happen is that the put_page() can come in
before the pgalloc_tag_sub(), and then that page can be allocated again.
Will that cause confusion?