linux-kernel - Re: [PATCH v5 1/8] dax: alloc_dax() return ERR_PTR(-EOPNOTSUPP) for CONFIG

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Date: Tue, 13 Feb 2024 11:07:42 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Lukas Wunner <lukas@...ner.de>, Mathieu Desnoyers
	<mathieu.desnoyers@...icios.com>
CC: Dan Williams <dan.j.williams@...el.com>, Arnd Bergmann <arnd@...db.de>,
	Dave Chinner <david@...morbit.com>, <linux-kernel@...r.kernel.org>, "Andrew
 Morton" <akpm@...ux-foundation.org>, Linus Torvalds
	<torvalds@...ux-foundation.org>, Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>, Matthew Wilcox <willy@...radead.org>,
	Russell King <linux@...linux.org.uk>, <linux-arch@...r.kernel.org>,
	<linux-cxl@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-mm@...ck.org>, <linux-xfs@...r.kernel.org>,
	<dm-devel@...ts.linux.dev>, <nvdimm@...ts.linux.dev>,
	<linux-s390@...r.kernel.org>
Subject: Re: [PATCH v5 1/8] dax: alloc_dax() return ERR_PTR(-EOPNOTSUPP) for
 CONFIG_DAX=n

Lukas Wunner wrote:
> On Mon, Feb 12, 2024 at 11:30:54AM -0500, Mathieu Desnoyers wrote:
> > Change the return value from NULL to PTR_ERR(-EOPNOTSUPP) for
> > CONFIG_DAX=n to be consistent with the fact that CONFIG_DAX=y
> > never returns NULL.
> 
> All the callers of alloc_dax() only check for IS_ERR().
> 
> Doesn't this result in a change of behavior in all the callers?
> Previously they'd ignore the NULL return value and continue,
> now they'll error out.
> 
> Given that, seems dangerous to add a Fixes tag with a v4.0 commit
> and thus risk regressing all stable kernels.

Oh, good catch, yes that Fixes tag should be:

4e4ced93794a dax: Move mandatory ->zero_page_range() check in alloc_dax()

..as that was the one that updated the alloc_dax() calling convention
without fixing up the CONFIG_DAX=n case.

This is a pre-requisite for restoring DAX operation on ARM32.