On 10/12/18 8:55 PM, Dave Chinner wrote:
> On Thu, Oct 11, 2018 at 11:00:12PM -0700, john.hubbard@xxxxxxxxx wrote:
>> From: John Hubbard <jhubbard@xxxxxxxxxx>
[...]
>> 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
