Re: [PATCH v2 3/7] fs/proc/page: respect folio head-page flag placement

[Matthew Wilcox <willy@infradead.org>]

On Fri, Nov 10, 2023 at 11:33:20AM +0800, Kefeng Wang wrote:
> kpageflags reads page-flags directly from the page, even when the
> respective flag is only updated on the headpage of a folio.
> 
> Since most flags are stored in head flags, make k = folio->flags,
> and add new p = page->flags used for per-page flags.

You'd do better to steal Greg's commit message.

> Originally-from: Gregory Price <gregory.price@memverge.com>
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>

> @@ -202,7 +202,7 @@ u64 stable_page_flags(struct page *page)
>  	u |= kpf_copy_bit(k, KPF_MLOCKED,	PG_mlocked);
>  
>  #ifdef CONFIG_MEMORY_FAILURE
> -	u |= kpf_copy_bit(k, KPF_HWPOISON,	PG_hwpoison);
> +	u |= kpf_copy_bit(p, KPF_HWPOISON,	PG_hwpoison);

This is correct.

> @@ -211,13 +211,13 @@ u64 stable_page_flags(struct page *page)
>  
>  	u |= kpf_copy_bit(k, KPF_RESERVED,	PG_reserved);
>  	u |= kpf_copy_bit(k, KPF_MAPPEDTODISK,	PG_mappedtodisk);
> -	u |= kpf_copy_bit(k, KPF_PRIVATE,	PG_private);
> -	u |= kpf_copy_bit(k, KPF_PRIVATE_2,	PG_private_2);
> -	u |= kpf_copy_bit(k, KPF_OWNER_PRIVATE,	PG_owner_priv_1);
> +	u |= kpf_copy_bit(p, KPF_PRIVATE,	PG_private);
> +	u |= kpf_copy_bit(p, KPF_PRIVATE_2,	PG_private_2);
> +	u |= kpf_copy_bit(p, KPF_OWNER_PRIVATE,	PG_owner_priv_1);

This is not.  PG_private is not, I believe, set on tail pages.
Ditto the other two.  If you know differently ... ?

>  #ifdef CONFIG_ARCH_USES_PG_ARCH_X
> -	u |= kpf_copy_bit(k, KPF_ARCH_2,	PG_arch_2);
> -	u |= kpf_copy_bit(k, KPF_ARCH_3,	PG_arch_3);
> +	u |= kpf_copy_bit(p, KPF_ARCH_2,	PG_arch_2);
> +	u |= kpf_copy_bit(p, KPF_ARCH_3,	PG_arch_3);
>  #endif

I also don't think this is correct, but there are many uses of
PG_arch* and I may have missed something.