linux-kernel - Re: [PATCH v5 5/8] virtio: Treat alloc_dax() -EOPNOTSUPP failure as non-fatal

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Message-ID: <65caa3966caa_5a7f294cf@dwillia2-xfh.jf.intel.com.notmuch>
Date: Mon, 12 Feb 2024 15:02:46 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>, Dan Williams
	<dan.j.williams@...el.com>
CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Arnd Bergmann
	<arnd@...db.de>, Dave Chinner <david@...morbit.com>,
	<linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...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>, Alasdair Kergon
	<agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>, Mikulas Patocka
	<mpatocka@...hat.com>, <lukas@...ner.de>
Subject: Re: [PATCH v5 5/8] virtio: Treat alloc_dax() -EOPNOTSUPP failure as
 non-fatal

[ add Lukas ]

Linus Torvalds wrote:
> On Mon, 12 Feb 2024 at 14:04, Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > This works because the internals of virtio_fs_cleanup_dax(), "kill_dax()
> > and put_dax()", know how to handle a NULL @dax_dev. It is still early
> > days with the "cleanup" helpers, but I wonder if anyone else cares that
> > the DEFINE_FREE() above does not check for NULL?
> 
> Well, the main reason for DEFINE_FREE() to check for NULL is not
> correctness, but code generation. See the comment about kfree() in
> <linux/cleanup.h>:
> 
>  * NOTE: the DEFINE_FREE()'s @free expression includes a NULL test even though
>  * kfree() is fine to be called with a NULL value. This is on purpose. This way
>  * the compiler sees the end of our alloc_obj() function as [...]
> 
> with the full explanation there.
> 
> Now, whether the code wants to actually use the cleanup() helpers for
> a single use-case is debatable.
> 
> But yes, if it does, I suspect it should use !IS_ERR_OR_NULL(ptr).o

I am trying to arrive at a common recommendation given Lukas found that
IS_ERR_OR_NULL() resulted in unwanted NULL checks emitted in the
assembly [1].

He is doing something similar:

http://lore.kernel.org/r/4143b15418c4ecf87ddeceb36813943c3ede17aa.1707734526.git.lukas@wunner.de

..and introduced an assume() helper.

However, Lukas, I think Linus is right, your DEFINE_FREE() should use
IS_ERR_OR_NULL(). I.e. the problem is trying to use
__free(x509_free_certificate) in x509_cert_parse().

> --- a/crypto/asymmetric_keys/x509_cert_parser.c
> +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> @@ -60,24 +60,24 @@ void x509_free_certificate(struct x509_certificate *cert)
>   */
>  struct x509_certificate *x509_cert_parse(const void *data, size_t datalen)
>  {
> -       struct x509_certificate *cert;
> -       struct x509_parse_context *ctx;
> +       struct x509_certificate *cert __free(x509_free_certificate);

..make this:

    struct x509_certificate *cert __free(kfree);

..and Mathieu, this should be IS_ERR_OR_NULL() to skip an unnecessary
call to virtio_fs_cleanup_dax() at function exit that the compiler
should elide.