linux-kernel - Re: [PATCH v2] mm: fix is_pinnable

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Date:   Sun, 8 May 2022 02:19:20 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        John Hubbard <jhubbard@...dia.com>,
        John Dias <joaodias@...gle.com>
Subject: Re: [PATCH v2] mm: fix is_pinnable_page against on cma page

On 07.05.22 21:23, Andrew Morton wrote:
> On Wed,  4 May 2022 23:44:29 -0700 Minchan Kim <minchan@...nel.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?

Yes, we want READ_ONCE I think. Apart from that LGTM.


-- 
Thanks,

David / dhildenb