linux-kernel - Re: [PATCH v3 1/3] mm: document zone device struct page field usage

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Message-ID: <8acce6b0-7e84-9dc6-9268-eaf0e814d994@nvidia.com>
Date:   Thu, 25 Jul 2019 11:19:07 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Christoph Hellwig <hch@....de>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <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>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 1/3] mm: document zone device struct page field usage


On 7/24/19 10:38 PM, Christoph Hellwig wrote:
> On Wed, Jul 24, 2019 at 04:26:58PM -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.
>>
>> Add comments to struct page and remove the unused "_zd_pad_1" field
>> to make this more clear.
> 
> I still think we should also fix up the layout, and I haven't seen
> a reply from Matthew justifying his curses for your patch that makes
> the struct page layout actually match how it is used.
> 

Well, I can kind of see this both ways since ZONE_DEVICE
MEMORY_DEVICE_DEVDAX and MEMORY_DEVICE_PCI_P2PDMA don't
seem to use the 3 words like MEMORY_DEVICE_PRIVATE and
MEMORY_DEVICE_FS_DAX.

I like v3 because not all of the ZONE_DEVICE types are handled
the same in regards to using the 3 words and there may be future
ZONE_DEVICE types that use the 3 words differently which might
require a union.

I agree, I would like to hear from Matthew on his thoughts.