Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock

[Alex Shi]

在 2020/11/5 下午12:57, Matthew Wilcox 写道:
> On Thu, Nov 05, 2020 at 12:52:05PM +0800, Alex Shi wrote:
>> @@ -1054,8 +1054,27 @@ static void __page_set_anon_rmap(struct page *page,
>>  	if (!exclusive)
>>  		anon_vma = anon_vma->root;
>>  
>> +	/*
>> +	 * w/o the WRITE_ONCE here the following scenario may happens due to
>> +	 * store reordering.
>> +	 *
>> +	 *      CPU 0                                          CPU 1
>> +	 *
>> +	 * do_anonymous_page				page_idle_clear_pte_refs
>> +	 *   __page_set_anon_rmap
>> +	 *     page->mapping = anon_vma + PAGE_MAPPING_ANON
>> +	 *   lru_cache_add_inactive_or_unevictable()
>> +	 *     SetPageLRU(page)
>> +	 *                                               rmap_walk
>> +	 *                                                if PageAnon(page)
>> +	 *
>> +	 *  The 'SetPageLRU' may reordered before page->mapping setting, and
>> +	 *  page->mapping may set with anon_vma, w/o anon bit, then rmap_walk
>> +	 *  may goes to rmap_walk_file() for a anon page.
>> +	 */
>> +
>>  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
>> -	page->mapping = (struct address_space *) anon_vma;
>> +	WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
>>  	page->index = linear_page_index(vma, address);
>>  }
> 
> I don't like these verbose comments with detailed descriptions in
> the source code.  They're fine in changelogs, but they clutter the
> code, and they get outdated really quickly.  My preference is for
> something more brief:
> 
> 	/*
> 	 * Prevent page->mapping from pointing to an anon_vma without
> 	 * the PAGE_MAPPING_ANON bit set.  This could happen if the
> 	 * compiler stores anon_vma and then adds PAGE_MAPPING_ANON to it.
> 	 */
> 

Yes, it's reansonble. So is the following fine?