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

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Date:   Tue, 23 Jul 2019 21:39:17 +0000
From:   "Weiny, Ira" <ira.weiny@...el.com>
To:     Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.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)?

I'm ok with that...

Ira