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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: "Dan Williams" <dan.j.williams@intel.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Alex Sierra" <alex.sierra@amd.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Kuehling, Felix" <Felix.Kuehling@amd.com>,
	"Linux MM" <linux-mm@kvack.org>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	"amd-gfx list" <amd-gfx@lists.freedesktop.org>,
	"Maling list - DRI developers" <dri-devel@lists.freedesktop.org>,
	"Christoph Hellwig" <hch@lst.de>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Alistair Popple" <apopple@nvidia.com>,
	"Vishal Verma" <vishal.l.verma@intel.com>,
	"Dave Jiang" <dave.jiang@intel.com>,
	"Linux NVDIMM" <nvdimm@lists.linux.dev>,
	"David Hildenbrand" <david@redhat.com>
Subject: Re: [PATCH v1 2/2] mm: remove extra ZONE_DEVICE struct page refcount
Date: Tue, 19 Oct 2021 13:01:36 -0300	[thread overview]
Message-ID: <20211019160136.GH3686969@ziepe.ca> (raw)
In-Reply-To: <499043a0-b3d8-7a42-4aee-84b81f5b633f@oracle.com>

On Tue, Oct 19, 2021 at 04:13:34PM +0100, Joao Martins wrote:
> On 10/19/21 00:06, Jason Gunthorpe wrote:
> > On Mon, Oct 18, 2021 at 12:37:30PM -0700, Dan Williams wrote:
> > 
> >>> device-dax uses PUD, along with TTM, they are the only places. I'm not
> >>> sure TTM is a real place though.
> >>
> >> I was setting device-dax aside because it can use Joao's changes to
> >> get compound-page support.
> > 
> > Ideally, but that ideas in that patch series have been floating around
> > for a long time now..
> >  
> The current status of the series misses a Rb on patches 6,7,10,12-14.
> Well, patch 8 too should now drop its tag, considering the latest
> discussion.
> 
> If it helps moving things forward I could split my series further into:
> 
> 1) the compound page introduction (patches 1-7) of my aforementioned series
> 2) vmemmap deduplication for memory gains (patches 9-14)
> 3) gup improvements (patch 8 and gup-slow improvements)

I would split it, yes..

I think we can see a general consensus that making compound_head/etc
work consistently with how THP uses it will provide value and
opportunity for optimization going forward.

> Whats the benefit between preventing longterm at start
> versus only after mounting the filesystem? Or is the intended future purpose
> to pass more context into an holder potential future callback e.g. nack longterm
> pins on a page basis?

I understood Dan's remark that the device-dax path allows
FOLL_LONGTERM and the FSDAX path does not ?

Which, IIRC, today is signaled basd on vma properties and in all cases
fast-gup is denied.

> Maybe we can start by at least not add any flags and just prevent
> FOLL_LONGTERM on fsdax -- which I guess was the original purpose of
> commit 7af75561e171 ("mm/gup: add FOLL_LONGTERM capability to GUP fast").
> This patch (which I can formally send) has a sketch of that (below scissors mark):
> 
> https://lore.kernel.org/linux-mm/6a18179e-65f7-367d-89a9-d5162f10fef0@oracle.com/

Yes, basically, whatever test we want for 'deny fast gup foll
longterm' is fine. 

Personally I'd like to see us move toward a set of flag specifying
each special behavior and not a collection of types that imply special
behaviors.

Eg we have at least:
 - Block gup fast on foll_longterm
 - Capture the refcount ==1 and use the pgmap free hook
   (confusingly called page_is_devmap_managed())
 - Always use a swap entry
 - page->index/mapping are used in the usual file based way?

Probably more things..

Jason


  reply	other threads:[~2021-10-19 16:01 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-14 15:39 [PATCH v1 0/2] " Alex Sierra
2021-10-14 15:39 ` [PATCH v1 1/2] ext4/xfs: add page refcount helper Alex Sierra
2021-10-14 16:25   ` Jason Gunthorpe
2021-10-14 16:40   ` Matthew Wilcox
2021-10-14 15:39 ` [PATCH v1 2/2] mm: remove extra ZONE_DEVICE struct page refcount Alex Sierra
2021-10-14 16:52   ` Matthew Wilcox
2021-10-14 17:06   ` Jason Gunthorpe
2021-10-14 17:35     ` Ralph Campbell
2021-10-14 18:01       ` Jason Gunthorpe
2021-10-14 20:57         ` Ralph Campbell
2021-10-15  3:45           ` Sierra Guiza, Alejandro (Alex)
2021-10-15 11:06             ` Jason Gunthorpe
2021-10-14 18:43     ` Matthew Wilcox
2021-10-14 19:01       ` Dan Williams
2021-10-14 23:06         ` Jason Gunthorpe
2021-10-15  1:37           ` Dan Williams
2021-10-16 15:44             ` Jason Gunthorpe
2021-10-16 16:39               ` Matthew Wilcox
2021-10-17 18:20                 ` Dan Williams
2021-10-17 18:35               ` Dan Williams
2021-10-18 18:25                 ` Jason Gunthorpe
2021-10-18 19:37                   ` Dan Williams
2021-10-18 23:06                     ` Jason Gunthorpe
2021-10-19 15:13                       ` Joao Martins
2021-10-19 16:01                         ` Jason Gunthorpe [this message]
2021-10-19 19:21                           ` Dan Williams
2021-10-20 17:06                             ` Joao Martins
2021-10-20 17:12                               ` Dan Williams
2021-10-20 18:51                                 ` Joao Martins
2021-11-15 19:33 [PATCH v1 0/2] Remove extra ZONE_DEVICE " Alex Sierra
2021-11-15 19:33 ` [PATCH v1 2/2] mm: remove extra ZONE_DEVICE struct " Alex Sierra

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