linux-ext4 - Re: [PATCH v1 2/2] mm: remove extra ZONE

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Date:   Tue, 19 Oct 2021 12:21:09 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Joao Martins <joao.m.martins@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alex Sierra <alex.sierra@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kuehling, Felix" <Felix.Kuehling@....com>,
        Linux MM <linux-mm@...ck.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>, Christoph Hellwig <hch@....de>,
        Jérôme Glisse <jglisse@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v1 2/2] mm: remove extra ZONE_DEVICE struct page refcount

On Tue, Oct 19, 2021 at 9:02 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> 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.

Yeah, I forgot that 7af75561e171 eliminated any possibility of
longterm-gup-fast for device-dax, let's not disturb that status quo.

> > 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..

Yes, agree with the principle of reducing type-implied special casing.