linux-kernel - Re: [PATCH v5 00/25] fs/dax: Fix ZONE

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Message-Id: <20250107222643.80d5509219d6b66c15b1b8af@linux-foundation.org>
Date: Tue, 7 Jan 2025 22:26:43 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Alistair Popple <apopple@...dia.com>
Cc: dan.j.williams@...el.com, linux-mm@...ck.org, lina@...hilina.net,
 zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com,
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Subject: Re: [PATCH v5 00/25] fs/dax: Fix ZONE_DEVICE page reference counts

On Tue,  7 Jan 2025 14:42:16 +1100 Alistair Popple <apopple@...dia.com> wrote:

> Device and FS DAX pages have always maintained their own page
> reference counts without following the normal rules for page reference
> counting. In particular pages are considered free when the refcount
> hits one rather than zero and refcounts are not added when mapping the
> page.
> 
> Tracking this requires special PTE bits (PTE_DEVMAP) and a secondary
> mechanism for allowing GUP to hold references on the page (see
> get_dev_pagemap). However there doesn't seem to be any reason why FS
> DAX pages need their own reference counting scheme.
> 
> By treating the refcounts on these pages the same way as normal pages
> we can remove a lot of special checks. In particular pXd_trans_huge()
> becomes the same as pXd_leaf(), although I haven't made that change
> here. It also frees up a valuable SW define PTE bit on architectures
> that have devmap PTE bits defined.
> 
> It also almost certainly allows further clean-up of the devmap managed
> functions, but I have left that as a future improvment. It also
> enables support for compound ZONE_DEVICE pages which is one of my
> primary motivators for doing this work.
> 

https://lkml.kernel.org/r/wysuus23bqmjtwkfu3zutqtmkse3ki3erf45x32yezlrl24qto@xlqt7qducyld
made me expect merge/build/runtime issues, however this series merges
and builds OK on mm-unstable.  Did something change?  What's the story
here?

Oh well, it built so I'll ship it!