linux-kernel - Re: [PATCH V2] mm/memremap: fix spurious large folio warning for FS-DAX

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Message-ID: <6945b0c636de7_1cf51005e@dwillia2-mobl4.notmuch>
Date: Fri, 19 Dec 2025 12:08:38 -0800
From: <dan.j.williams@...el.com>
To: John Groves <John@...ves.net>, David Hildenbrand <david@...nel.org>,
	"Oscar Salvador" <osalvador@...e.de>, Andrew Morton
	<akpm@...ux-foundation.org>
CC: John Groves <John@...ves.net>, John Groves <jgroves@...ron.com>, "Darrick
 J . Wong" <djwong@...nel.org>, Dan Williams <dan.j.williams@...el.com>,
	Gregory Price <gourry@...rry.net>, Balbir Singh <bsingharora@...il.com>,
	Alistair Popple <apopple@...dia.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>, Aravind Ramesh <arramesh@...ron.com>, "Ajay
 Joshi" <ajayjoshi@...ron.com>, John Groves <john@...ves.net>
Subject: Re: [PATCH V2] mm/memremap: fix spurious large folio warning for
 FS-DAX

John Groves wrote:
[..]
> The fix is to exempt MEMORY_DEVICE_FS_DAX from the large folio warning,
> allowing FS-DAX to continue using PMD mappings without triggering false
> warnings.

As you note, this patch no longer exempts MEMORY_DEVICE_FS_DAX explicitly, it just
removes the bogus warning, so maybe Andrew can adjust this note on
applying?

> Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
> Signed-off-by: John Groves <john@...ves.net>
> ---
> 
> Change since V1: Deleted the warning altogether, rather than exempting
> fs-dax.
> 
> === How to reproduce ===
> 
> A reproducer is available at:
> 
>     git clone https://github.com/jagalactic/dax-pmd-test.git
>     cd xfs-dax-test
>     make
>     sudo make test

Thanks John, outside of the fixup above, this looks good to me.

Reviewed-by: Dan Williams <dan.j.williams@...el.com>

Now, my first thoughts when seeing this were:

"ooh, I want that test in the regression suite"

...then:

"wait, that sounds exactly like the existing dax.sh test [1]"

[1]: https://github.com/pmem/ndctl/blob/main/test/dax.sh

Alison reports that indeed that existing test triggers the problem which
indicates some process problems to solve.

- Folks touching mm/memremap.c (and anything dax related) do not know
  about / run the regression tests.

- The bespoke nature of the dax testing environment needs some work to
  get it into a kselftest amenable flow, or otherwise need more
  automation to run those tests automatically upon seeing those files
  touched in linux-next so folks see breakage like this earlier.