Re: [PATCH v1 05/15] mm/rmap: convert RMAP flags to a proper distinct rmap_t type

[Nadav Amit <namit@vmware.com>]

> On Mar 8, 2022, at 6:14 AM, David Hildenbrand <david@redhat.com> wrote:
> 
> We want to pass the flags to more than one anon rmap function, getting
> rid of special "do_page_add_anon_rmap()". So let's pass around a distinct
> __bitwise type and refine documentation.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> include/linux/rmap.h | 22 ++++++++++++++++++----
> mm/memory.c          |  6 +++---
> mm/rmap.c            |  7 ++++---
> 3 files changed, 25 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 92c3585b8c6a..49f6b208938c 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -158,9 +158,23 @@ static inline void anon_vma_merge(struct vm_area_struct *vma,
> 
> struct anon_vma *page_get_anon_vma(struct page *page);
> 
> -/* bitflags for do_page_add_anon_rmap() */
> -#define RMAP_EXCLUSIVE 0x01
> -#define RMAP_COMPOUND 0x02
> +/* RMAP flags, currently only relevant for some anon rmap operations. */
> +typedef int __bitwise rmap_t;
> +
> +/*
> + * No special request: if the page is a subpage of a compound page, it is
> + * mapped via a PTE. The mapped (sub)page is possibly shared between processes.
> + */
> +#define RMAP_NONE		((__force rmap_t)0)
> +
> +/* The (sub)page is exclusive to a single process. */
> +#define RMAP_EXCLUSIVE		((__force rmap_t)BIT(0))
> +
> +/*
> + * The compound page is not mapped via PTEs, but instead via a single PMD and
> + * should be accounted accordingly.
> + */
> +#define RMAP_COMPOUND		((__force rmap_t)BIT(1))

I was once shouted at for a similar suggestion, but I am going to try
once more… If you already define a new type, why not to use bitfields?

It would be much easier to read. The last time I made such a suggestion,
Ingo said "I personally like bitfields in theory … [but] older versions of
GCC did a really poor job of optimizing them.” At the time (2014), I looked
at GCC-4.4 and GCC-4.8 and there were some differences in the quality of
the generated code. Is it still the case?