Re: [PATCH -mm] mm: Fix races between swapoff and flush dcache

[Huang\, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Fri,  2 Mar 2018 16:04:26 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:
>
>> From: Huang Ying <ying.huang@xxxxxxxxx>
>> 
>> >From commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB
>> trunks") on, after swapoff, the address_space associated with the swap
>> device will be freed.  So page_mapping() users which may touch the
>> address_space need some kind of mechanism to prevent the address_space
>> from being freed during accessing.
>> 
>> The dcache flushing functions (flush_dcache_page(), etc) in
>> architecture specific code may access the address_space of swap device
>> for anonymous pages in swap cache via page_mapping() function.  But in
>> some cases there are no mechanisms to prevent the swap device from
>> being swapoff, for example,
>> 
>> CPU1					CPU2
>> __get_user_pages()			swapoff()
>>   flush_dcache_page()
>>     mapping = page_mapping()
>>       ...				  exit_swap_address_space()
>>       ...				    kvfree(spaces)
>>       mapping_mapped(mapping)
>> 
>> The address space may be accessed after being freed.
>> 
>> But from cachetlb.txt and Russell King, flush_dcache_page() only care
>> about file cache pages, for anonymous pages, flush_anon_page() should
>> be used.  The implementation of flush_dcache_page() in all
>> architectures follows this too.  They will check whether
>> page_mapping() is NULL and whether mapping_mapped() is true to
>> determine whether to flush the dcache immediately.  And they will use
>> interval tree (mapping->i_mmap) to find all user space mappings.
>> While mapping_mapped() and mapping->i_mmap isn't used by anonymous
>> pages in swap cache at all.
>> 
>> So, to fix the race between swapoff and flush dcache, __page_mapping()
>> is add to return the address_space for file cache pages and NULL
>> otherwise.  All page_mapping() invoking in flush dcache functions are
>> replaced with __page_mapping().
>> 
>> The patch is only build tested, because I have no machine with
>> architecture other than x86.
>> 
>> ...
>>
>> +/*
>> + * For file cache pages, return the address_space, otherwise return NULL
>> + */
>> +struct address_space *__page_mapping(struct page *page)
>> +{
>> +	struct address_space *mapping;
>> +
>> +	page = compound_head(page);
>> +
>> +	/* This happens if someone calls flush_dcache_page on slab page */
>> +	if (unlikely(PageSlab(page)))
>> +		return NULL;
>> +
>> +	mapping = page->mapping;
>> +	if ((unsigned long)mapping & PAGE_MAPPING_ANON)
>> +		return NULL;
>> +
>> +	return (void *)((unsigned long)mapping & ~PAGE_MAPPING_FLAGS);
>> +}
>> +
>
> I think page_mapping_file() would be a better name.

Thanks!  I will use that name.

> And do we really need to duplicate page_mapping()?  Could it be
>
> struct address_space *page_mapping_file(struct page *page)
> {
> 	if (PageSwapCache(page))
> 		return NULL;
> 	return page_mapping(page);
> }

Yes.  This looks better.

> (We don't need to run compound_head() here, do we?)

Yes.  I think so.

Best Regards,
Huang, Ying