linux-ext4 - Re: [PATCH 07/13] huge_memory: Allow mappings of PUD sized pages

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Message-ID: <20240702115151.GA16313@lst.de>
Date: Tue, 2 Jul 2024 13:51:51 +0200
From: Christoph Hellwig <hch@....de>
To: Alistair Popple <apopple@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, dan.j.williams@...el.com,
	vishal.l.verma@...el.com, dave.jiang@...el.com, logang@...tatee.com,
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	catalin.marinas@....com, will@...nel.org, mpe@...erman.id.au,
	npiggin@...il.com, dave.hansen@...ux.intel.com, ira.weiny@...el.com,
	willy@...radead.org, djwong@...nel.org, tytso@....edu,
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	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
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	linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org,
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	linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
	david@...morbit.com
Subject: Re: [PATCH 07/13] huge_memory: Allow mappings of PUD sized pages

On Tue, Jul 02, 2024 at 08:19:01PM +1000, Alistair Popple wrote:
> > (B) As long as we have subpage mapcounts, this prevents vmemmap
> >     optimizations [1]. Is that only used for device-dax for now and are
> >     there no plans to make use of that for fs-dax?
> 
> I don't have any plans to. This is purely focussed on refcounting pages
> "like normal" so we can get rid of all the DAX special casing.
> 
> > (C) We managed without so far :)
> 
> Indeed, although Christoph has asked repeatedly ([1], [2] and likely
> others) that this gets fixed and I finally got sick of it coming up
> everytime I need to touch something with ZONE_DEVICE pages :)
> 
> Also it removes the need for people to understand the special DAX page
> recounting scheme and ends up removing a bunch of cruft as a bonus:
> 
>  59 files changed, 485 insertions(+), 869 deletions(-)
> 
> And that's before I clean up all the pgmap reference handling. It also
> removes the pXX_trans_huge and pXX_leaf distinction. So we managed, but
> things could be better IMHO.

Yes.  I can't wait for this series making the finish line.  There might
be more chance for cleanups and optimizations around ZONE_DEVICE, but
this alone is a huge step forward.