linux-kernel - Re: [PATCH v2 1/3] mm: document zone device struct page field usage

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Date:   Tue, 23 Jul 2019 14:25:43 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        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 v2 1/3] mm: document zone device struct page field usage


On 7/22/19 4:08 AM, Matthew Wilcox wrote:
> On Sun, Jul 21, 2019 at 10:13:45PM -0700, Ira Weiny wrote:
>> On Sun, Jul 21, 2019 at 09:02:04AM -0700, Matthew Wilcox wrote:
>>> On Fri, Jul 19, 2019 at 12:29:53PM -0700, Ralph Campbell wrote:
>>>> Struct page for ZONE_DEVICE private pages uses the page->mapping and
>>>> and page->index fields while the source anonymous pages are migrated to
>>>> device private memory. This is so rmap_walk() can find the page when
>>>> migrating the ZONE_DEVICE private page back to system memory.
>>>> ZONE_DEVICE pmem backed fsdax pages also use the page->mapping and
>>>> page->index fields when files are mapped into a process address space.
>>>>
>>>> Restructure struct page and add comments to make this more clear.
>>>
>>> NAK.  I just got rid of this kind of foolishness from struct page,
>>> and you're making it harder to understand, not easier.  The comments
>>> could be improved, but don't lay it out like this again.
>>
>> Was V1 of Ralphs patch ok?  It seemed ok to me.
> 
> Yes, v1 was fine.  This seems like a regression.
> 

This is about what people find "easiest to understand" and so
I'm not surprised that opinions differ.
What if I post a v3 based on v1 but remove the _zd_pad_* variables
that Christoph found misleading and add some more comments
about how the different ZONE_DEVICE types use the 3 remaining
words (basically the comment from v2)?