On Mon, Sep 19, 2022 at 09:57:08PM +0900, Hyeonggon Yoo wrote:
> For now, only SLAB uses _mapcount field as a number of active objects in
> a slab, and other slab allocators do not use it. As 16 bits are enough
> for that, use remaining 16 bits of _mapcount as page_type even when
> SLAB is used. And then move PG_slab flag to page_type!
>
> Note that page_type is always placed in upper 16 bits of _mapcount to
> avoid confusing normal _mapcount as page_type. As underflow (actually
> I mean, yeah, overflow) is not a concern anymore, use more lower bits
> except bit zero.
>
> Add more folio helpers for PAGE_TYPE_OPS() not to break existing
> slab implementations.
>
> Remove PG_slab check from PAGE_FLAGS_CHECK_AT_FREE. buddy will still
> check if _mapcount is properly set at free.
>
> Exclude PG_slab from hwpoison and show_page_flags() for now.
>
> Note that with this patch, page_mapped() and folio_mapped() always return
> false for slab page.
>
[...]
Hi. a silly mistake:
>
> include/linux/mm_types.h | 22 +++++++--
> include/linux/page-flags.h | 83 ++++++++++++++++++++++++++--------
> include/trace/events/mmflags.h | 1 -
> mm/memory-failure.c | 8 ----
> mm/slab.h | 11 ++++-
> 5 files changed, 92 insertions(+), 33 deletions(-)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index cf97f3884fda..4b217c6fbe1f 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -193,12 +193,24 @@ struct page {
> atomic_t _mapcount;
>
> /*
> - * If the page is neither PageSlab nor mappable to userspace,
> - * the value stored here may help determine what this page
> - * is used for. See page-flags.h for a list of page types
> - * which are currently stored here.
> + * If the page is not mappable to userspace, the value
> + * stored here may help determine what this page is used for.
> + * See page-flags.h for a list of page types which are currently
> + * stored here.
> */
> - unsigned int page_type;
> + struct {
> + /*
> + * Always place page_type in
> + * upper 16 bits of _mapcount
> + */
> +#ifdef CPU_BIG_ENDIAN
s/CPU_BIG_ENDIAN/CONFIG_CPU_BIG_ENDIAN/g
> + __u16 page_type;
> + __u16 active;
> +#else
> + __u16 active;
> + __u16 page_type;
> +#endif
> + };
> };
>
> /* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
[...]
> diff --git a/mm/slab.h b/mm/slab.h
> index 985820b9069b..a5273e189265 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -20,7 +20,16 @@ struct slab {
> };
> struct rcu_head rcu_head;
> };
> - unsigned int active;
> + struct {
> + /* always place page_type in upper 16 bits of _mapcount */
> +#ifdef CPU_BIG_ENDIAN
same here.
> + __u16 page_type;
> + __u16 active;
> +#else
> + __u16 active;
> + __u16 page_type;
> +#endif
> + };
>
> #elif defined(CONFIG_SLUB)
>
> --
> 2.32.0
>
--
Thanks,
Hyeonggon
