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

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Date:   Wed, 17 Jul 2019 06:22:33 +0200
From:   Christoph Hellwig <hch@....de>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Ralph Campbell <rcampbell@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, willy@...radead.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Pekka Enberg <penberg@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/3] mm: document zone device struct page reserved
 fields

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.