Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count

[John Hubbard <jhubbard@nvidia.com>]

On 10/12/18 8:55 PM, Dave Chinner wrote:
> On Thu, Oct 11, 2018 at 11:00:12PM -0700, john.hubbard@gmail.com wrote:
>> From: John Hubbard <jhubbard@nvidia.com>
[...]
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 5ed8f6292a53..017ab82e36ca 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -78,12 +78,22 @@ struct page {
>>  	 */
>>  	union {
>>  		struct {	/* Page cache and anonymous pages */
>> -			/**
>> -			 * @lru: Pageout list, eg. active_list protected by
>> -			 * zone_lru_lock.  Sometimes used as a generic list
>> -			 * by the page owner.
>> -			 */
>> -			struct list_head lru;
>> +			union {
>> +				/**
>> +				 * @lru: Pageout list, eg. active_list protected
>> +				 * by zone_lru_lock.  Sometimes used as a
>> +				 * generic list by the page owner.
>> +				 */
>> +				struct list_head lru;
>> +				/* Used by get_user_pages*(). Pages may not be
>> +				 * on an LRU while these dma_pinned_* fields
>> +				 * are in use.
>> +				 */
>> +				struct {
>> +					unsigned long dma_pinned_flags;
>> +					atomic_t      dma_pinned_count;
>> +				};
>> +			};
> 
> Isn't this broken for mapped file-backed pages? i.e. they may be
> passed as the user buffer to read/write direct IO and so the pages
> passed to gup will be on the active/inactive LRUs. hence I can't see
> how you can have dual use of the LRU list head like this....
> 
> What am I missing here?

Hi Dave,

In patch 6/6, pin_page_for_dma(), which is called at the end of get_user_pages(),
unceremoniously rips the pages out of the LRU, as a prerequisite to using
either of the page->dma_pinned_* fields. 

The idea is that LRU is not especially useful for this situation anyway,
so we'll just make it one or the other: either a page is dma-pinned, and
just hanging out doing RDMA most likely (and LRU is less meaningful during that
time), or it's possibly on an LRU list.


-- 
thanks,
John Hubbard
NVIDIA