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

[Christoph Hellwig]

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.