Re: [PATCH v2] mm: fix is_pinnable_page against on cma page

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  4 May 2022 23:44:29 -0700 Minchan Kim <minchan@kernel.org> wrote:

> Pages on CMA area could have MIGRATE_ISOLATE as well as MIGRATE_CMA
> so current is_pinnable_page could miss CMA pages which has MIGRATE_
> ISOLATE. It ends up putting CMA pages longterm pinning possible on
> pin_user_pages APIs so CMA allocation fails.
> 
> The CMA allocation path protects the migration type change race
> using zone->lock but what GUP path need to know is just whether the
> page is on CMA area or not rather than exact type. Thus, we don't
> need zone->lock but just checks the migratype in either of
> (MIGRATE_ISOLATE and MIGRATE_CMA).
> 
> Adding the MIGRATE_ISOLATE check in is_pinnable_page could cause
> rejecting of pinning the page on MIGRATE_ISOLATE pageblock even
> thouth it's neither CMA nor movable zone if the page is temporarily

"though"

> unmovable. However, the migration failure is general issue, not
> only come from MIGRATE_ISOLATE and the MIGRATE_ISOLATE is also
> transient state like other temporal refcount holding of pages.
> 
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1625,8 +1625,18 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
>  #ifdef CONFIG_MIGRATION
>  static inline bool is_pinnable_page(struct page *page)
>  {
> -	return !(is_zone_movable_page(page) || is_migrate_cma_page(page)) ||
> -		is_zero_pfn(page_to_pfn(page));
> +#ifdef CONFIG_CMA
> +	/*
> +	 * use volatile to use local variable mt instead of
> +	 * refetching mt value.
> +	 */
> +	volatile int mt = get_pageblock_migratetype(page);
> +
> +	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> +		return false;
> +#endif

Open-coded use of `volatile' draws unwelcome attention.

What are we trying to do here?  Prevent the compiler from rerunning all
of get_pageblock_migratetype() (really __get_pfnblock_flags_mask())
twice?  That would be pretty dumb of it?

Would a suitably-commented something like

	int __mt = get_pageblock_migratetype(page);
	int mt = __READ_ONCE(__mt);

express this better?
	
> +
> +	return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
>  }
>  #else
>  static inline bool is_pinnable_page(struct page *page)