Re: [PATCH 1/3] mm: document zone device struct page reserved fields

[John Hubbard <jhubbard@nvidia.com>]

On 7/16/19 9:22 PM, Christoph Hellwig wrote:
> On Tue, Jul 16, 2019 at 06:20:23PM -0700, John Hubbard wrote:
>>> -			unsigned long _zd_pad_1;	/* uses mapping */
>>> +			/*
>>> +			 * The following fields are used to hold the source
>>> +			 * page anonymous mapping information while it is
>>> +			 * migrated to device memory. See migrate_page().
>>> +			 */
>>> +			unsigned long _zd_pad_1;	/* aliases mapping */
>>> +			unsigned long _zd_pad_2;	/* aliases index */
>>> +			unsigned long _zd_pad_3;	/* aliases private */
>>
>> Actually, I do think this helps. It's hard to document these fields, and
>> the ZONE_DEVICE pages have a really complicated situation during migration
>> to a device. 
>>
>> Additionally, I'm not sure, but should we go even further, and do this on the 
>> other side of the alias:
> 
> The _zd_pad_* field obviously are NOT used anywhere in the source tree.
> So these comments are very misleading.  If we still keep
> using ->mapping, ->index and ->private we really should clean up the
> definition of struct page to make that obvious instead of trying to
> doctor around it using comments.
> 

OK, so just delete all the _zd_pad_* fields? Works for me. It's misleading to
calling something padding, if it's actually unavailable because it's used
in the other union, so deleting would be even better than commenting.

In that case, it would still be nice to have this new snippet, right?:

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d6ea74e20306..c5ce5989d8a8 100644

--- a/include/linux/mm_types.h

+++ b/include/linux/mm_types.h

@@ -83,7 +83,12 @@

 struct page {

                         * by the page owner. 
                         */ 
                        struct list_head lru; 
-                       /* See page-flags.h for PAGE_MAPPING_FLAGS */ 
+                       /* 
+                        * See page-flags.h for PAGE_MAPPING_FLAGS. 
+                        * 
+                        * Also: the next three fields (mapping, index and 
+                        * private) are all used by ZONE_DEVICE pages. 
+                        */ 
                        struct address_space *mapping; 
                        pgoff_t index;          /* Our offset within mapping. */ 
                        /** 

thanks,
-- 
John Hubbard
NVIDIA