Re: [PATCH RFC v2 0/3] mm: Properly document tail pages for a folio

[Peter Xu <peterx@redhat.com>]

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