linux-kernel - [PATCH v1 v6.12.y] nfsd: Replace clamp_t in nfsd4_get_drc

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Message-ID: <20251114211922.6312-1-cel@kernel.org>
Date: Fri, 14 Nov 2025 16:19:22 -0500
From: Chuck Lever <cel@...nel.org>
To: Andrew Morten <akpm@...ux-foundation.org>,
	david.laight.linux@...il.com,
	NeilBrown <neil@...wn.name>,
	Jeff Layton <jlayton@...nel.org>,
	Olga Kornievskaia <okorniev@...hat.com>,
	Dai Ngo <dai.ngo@...cle.com>,
	Tom Talpey <tom@...pey.com>
Cc: <linux-nfs@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<stable@...r.kernel.org>,
	speedcracker@...mail.com
Subject: [PATCH v1 v6.12.y] nfsd: Replace clamp_t in nfsd4_get_drc_mem()

From: NeilBrown <neil@...wn.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@...wn.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@...cle.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;
-- 
2.51.0