Re: [PATCH v1 v6.12.y] nfsd: Replace clamp_t in nfsd4_get_drc_mem()

[David Laight <david.laight.linux@gmail.com>]

On Fri, 14 Nov 2025 16:19:22 -0500
Chuck Lever <cel@kernel.org> wrote:

> From: NeilBrown <neil@brown.name>
> 
> A recent change to clamp_t() in 6.1.y caused fs/nfsd/nfs4state.c to fail
> to compile with gcc-9. The code in nfsd4_get_drc_mem() was written with
> the assumption that when "max < min",
> 
>    clamp(val, min, max)
> 
> would return max.  This assumption is not documented as an API promise
> and the change caused a compile failure if it could be statically
> determined that "max < min".
> 
> The relevant code was no longer present upstream when commit 1519fbc8832b
> ("minmax.h: use BUILD_BUG_ON_MSG() for the lo < hi test in clamp()")
> landed there, so there is no upstream change to nfsd4_get_drc_mem() to
> backport.
> 
> There is no clear case that the existing code in nfsd4_get_drc_mem()
> is functioning incorrectly. The goal of this patch is to permit the clean
> application of commit 1519fbc8832b ("minmax.h: use BUILD_BUG_ON_MSG() for
> the lo < hi test in clamp()"), and any commits that depend on it, to LTS
> kernels without affecting the ability to compile those kernels. This is
> done by open-coding the __clamp() macro sans the built-in type checking.
> 
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220745#c0
> Signed-off-by: NeilBrown <neil@brown.name>
> Stable-dep-of: 1519fbc8832b ("minmax.h: use BUILD_BUG_ON_MSG() for the lo < hi test in clamp()")
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Reviewed_by: David Laight <david.laight.linux@gmail.com>
> ---
>  fs/nfsd/nfs4state.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> Changes since Neil's post:
> * Editorial changes to the commit message
> * Attempt to address David's review comments
> * Applied to linux-6.12.y, passed NFSD upstream CI suite
> 
> This patch is intended to be applied to linux-6.12.y, and should
> apply cleanly to other LTS kernels since nfsd4_get_drc_mem hasn't
> changed since v5.4.
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 7b0fabf8c657..41545933dd18 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1983,8 +1983,10 @@ static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca, struct nfsd_net *nn
>  	 */
>  	scale_factor = max_t(unsigned int, 8, nn->nfsd_serv->sv_nrthreads);
>  
> -	avail = clamp_t(unsigned long, avail, slotsize,
> -			total_avail/scale_factor);
> +	if (avail > total_avail / scale_factor)
> +		avail = total_avail / scale_factor;
> +	else if (avail < slotsize)
> +		avail = slotsize;
>  	num = min_t(int, num, avail / slotsize);
>  	num = max_t(int, num, 1);
>  	nfsd_drc_mem_used += num * slotsize;