On Tue, Aug 15, 2023 at 04:45:52AM +0100, Matthew Wilcox wrote:
> I always forget about THP_SWAP using tail->private. That actually needs
> to be asserted by the compiler, not just documented. Something along
> these lines.
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 659c7b84726c..3880b3f2e321 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -340,8 +340,11 @@ struct folio {
> atomic_t _pincount;
> #ifdef CONFIG_64BIT
> unsigned int _folio_nr_pages;
> -#endif
> + /* 4 byte gap here */
> /* private: the union with struct page is transitional */
> + /* Fix THP_SWAP to not use tail->private */
> + unsigned long _private_1;
> +#endif
> };
> struct page __page_1;
> };
> @@ -362,6 +365,9 @@ struct folio {
> /* public: */
> struct list_head _deferred_list;
> /* private: the union with struct page is transitional */
> + unsigned long _avail_2a;
> + /* Fix THP_SWAP to not use tail->private */
> + unsigned long _private_2a;
> };
> struct page __page_2;
> };
> @@ -386,12 +392,18 @@ FOLIO_MATCH(memcg_data, memcg_data);
> offsetof(struct page, pg) + sizeof(struct page))
> FOLIO_MATCH(flags, _flags_1);
> FOLIO_MATCH(compound_head, _head_1);
> +#ifdef CONFIG_64BIT
> +FOLIO_MATCH(private, _private_1);
> +#endif
> #undef FOLIO_MATCH
> #define FOLIO_MATCH(pg, fl) \
> static_assert(offsetof(struct folio, fl) == \
> offsetof(struct page, pg) + 2 * sizeof(struct page))
> FOLIO_MATCH(flags, _flags_2);
> FOLIO_MATCH(compound_head, _head_2);
> +FOLIO_MATCH(flags, _flags_2a);
> +FOLIO_MATCH(compound_head, _head_2a);
> +FOLIO_MATCH(private, _private_2a);
> #undef FOLIO_MATCH
>
> /*
>
> This is against the patchset I just posted which frees up a word in the
> first tail page.
Okay, I assume you meant to suggest leverage FOLIO_MATCH(), which I can
definitely try. But then I'd hope it covers not only private field but all
the fields that the tail pages reuses; the goal is to document everything
no matter in what form. I'll see what I can get.. Thanks.
--
Peter Xu
