Re: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 03, 2019 at 11:34:04AM -0600, Khalid Aziz wrote:
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 2c471a2c43fa..d17d33f36a01 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -204,6 +204,14 @@ struct page {
>  #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
>  	int _last_cpupid;
>  #endif
> +
> +#ifdef CONFIG_XPFO
> +	/* Counts the number of times this page has been kmapped. */
> +	atomic_t xpfo_mapcount;
> +
> +	/* Serialize kmap/kunmap of this page */
> +	spinlock_t xpfo_lock;

NAK, see ALLOC_SPLIT_PTLOCKS

spinlock_t can be _huge_ (CONFIG_PROVE_LOCKING=y), also are you _really_
sure you want spinlock_t and not raw_spinlock_t ? For
CONFIG_PREEMPT_FULL spinlock_t turns into a rtmutex.

> +#endif

Growing the page-frame by 8 bytes (in the good case) is really sad,
that's a _lot_ of memory.

>  } _struct_page_alignment;