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

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Message-ID: <2025120202-appetizer-pushy-6b96@gregkh>
Date: Tue, 2 Dec 2025 16:51:16 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Chuck Lever <cel@...nel.org>
Cc: Sasha Levin <sashal@...nel.org>, linux-nfs@...r.kernel.org,
	Olga Kornievskaia <okorniev@...hat.com>,
	linux-kernel@...r.kernel.org, speedcracker@...mail.com,
	Dai Ngo <dai.ngo@...cle.com>, Tom Talpey <tom@...pey.com>,
	Jeff Layton <jlayton@...nel.org>, NeilBrown <neil@...wn.name>,
	david.laight.linux@...il.com,
	Andrew Morten <akpm@...ux-foundation.org>, stable@...r.kernel.org
Subject: Re: [PATCH v1 v6.12.y] nfsd: Replace clamp_t in nfsd4_get_drc_mem()

On Mon, Dec 01, 2025 at 02:28:50PM -0500, Chuck Lever wrote:
> On 11/14/25 4:19 PM, Chuck Lever wrote:
> > 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;
> 
> Is something else needed from me to get this applied to v6.12 and older
> LTS kernels?

Oops, sorry about that, now queued up.

greg k-h