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Date:      Tue, 21 Jul 2026 11:57:41 -0700
From:      Ryan Libby <rlibby@freebsd.org>
To:        Ed Maste <emaste@freebsd.org>, Mark Johnston <markj@freebsd.org>
Cc:        src-committers@freebsd.org, dev-commits-src-all@freebsd.org,  dev-commits-src-main@freebsd.org
Subject:   Re: git: caabdb3aefdc - main - contigmalloc.9: Note that M_WAITOK may still return NULL
Message-ID:  <CAHgpiFwq_m3cS80e-WrBcftvE-M-1zb-0_9L6p5qD_vw139K8w@mail.gmail.com>
In-Reply-To: <6a5fba36.3d93d.694b6c5c@gitrepo.freebsd.org>
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On Tue, Jul 21, 2026 at 11:28 AM Ed Maste <emaste@freebsd.org> wrote:
>
> The branch main has been updated by emaste:
>
> URL: https://cgit.FreeBSD.org/src/commit/?id=caabdb3aefdc45cae90203210034086801fa9005
>
> commit caabdb3aefdc45cae90203210034086801fa9005
> Author:     Ed Maste <emaste@FreeBSD.org>
> AuthorDate: 2026-07-21 18:10:00 +0000
> Commit:     Ed Maste <emaste@FreeBSD.org>
> CommitDate: 2026-07-21 18:27:47 +0000
>
>     contigmalloc.9: Note that M_WAITOK may still return NULL
>
>     Reviewed by:    markj, bapt
>     Sponsored by:   The FreeBSD Foundation
>     Differential Revision: https://reviews.freebsd.org/D58382
> ---
>  share/man/man9/contigmalloc.9 | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/share/man/man9/contigmalloc.9 b/share/man/man9/contigmalloc.9
> index 2e5d55ae8ba1..a9ebaf100eb9 100644
> --- a/share/man/man9/contigmalloc.9
> +++ b/share/man/man9/contigmalloc.9
> @@ -23,7 +23,7 @@
>  .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
>  .\" POSSIBILITY OF SUCH DAMAGE.
>  .\"
> -.Dd July 26, 2024
> +.Dd July 21, 2026
>  .Dt CONTIGMALLOC 9
>  .Os
>  .Sh NAME
> @@ -124,6 +124,15 @@ function returns a kernel virtual address if allocation succeeds,
>  or
>  .Dv NULL
>  otherwise.
> +Note that in contrast with
> +.Xr malloc 9 ,
> +.Fn contigmalloc
> +may return
> +.Dv NULL
> +even if
> +.Dv M_WAITOK
> +is specified, if no physically congiguous range is available that meets the
> +specified constraints.
>  .Sh EXAMPLES
>  .Bd -literal
>  void *p;
>

But is this desired and intentional behavior, or simply what the code does?

Paging through grep of contigmalloc with M_WAITOK, it appears to be a
mix of callers that handle and do not handle a NULL return.  Some of
them just handle it by panicking.  Is someone planning to audit
M_WAITOK callers?

> +is specified, if no physically congiguous range is available that meets the

Typo: "congiguous".

Ryan
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