linux-kernel - RE: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build breakage

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Message-ID: <01593a9ca971421a39c483819855d41c251da905.camel@ibm.com>
Date: Mon, 10 Nov 2025 19:57:18 +0000
From: Viacheslav Dubeyko <Slava.Dubeyko@....com>
To: Gregory Farnum <gfarnum@...hat.com>
CC: Xiubo Li <xiubli@...hat.com>,
        "justinstitt@...gle.com"
	<justinstitt@...gle.com>,
        "andriy.shevchenko@...ux.intel.com"
	<andriy.shevchenko@...ux.intel.com>,
        "llvm@...ts.linux.dev"
	<llvm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
        "nathan@...nel.org" <nathan@...nel.org>,
        "morbo@...gle.com" <morbo@...gle.com>,
        "idryomov@...il.com"
	<idryomov@...il.com>,
        "nick.desaulniers+lkml@...il.com"
	<nick.desaulniers+lkml@...il.com>,
        "ceph-devel@...r.kernel.org"
	<ceph-devel@...r.kernel.org>
Subject: RE: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build
 breakage

On Mon, 2025-11-10 at 11:48 -0800, Gregory Farnum wrote:
> On Mon, Nov 10, 2025 at 11:42 AM Viacheslav Dubeyko
> <Slava.Dubeyko@....com> wrote:
> > 
> > On Mon, 2025-11-10 at 15:44 +0100, Andy Shevchenko wrote:
> > > In a few cases the code compares 32-bit value to a SIZE_MAX derived
> > > constant which is much higher than that value on 64-bit platforms,
> > > Clang, in particular, is not happy about this
> > > 
> > > fs/ceph/snap.c:377:10: error: result of comparison of constant 2305843009213693948 with expression of type 'u32' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
> > >   377 |         if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
> > >       |             ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > 
> > > Fix this by casting to size_t. Note, that possible replacement of SIZE_MAX
> > > by U32_MAX may lead to the behaviour changes on the corner cases.
> > > 
> > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > > ---
> > >  fs/ceph/snap.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
> > > index c65f2b202b2b..521507ea8260 100644
> > > --- a/fs/ceph/snap.c
> > > +++ b/fs/ceph/snap.c
> > > @@ -374,7 +374,7 @@ static int build_snap_context(struct ceph_mds_client *mdsc,
> > > 
> > >       /* alloc new snap context */
> > >       err = -ENOMEM;
> > > -     if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
> > > +     if ((size_t)num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
> > 
> > The same question is here. Does it makes sense to declare num as size_t? Could
> > it be more clean solution? Or could it introduce another warnings/errors?
> 
> Given that the number of snaps is constrained over the wire as a
> 32-bit integer, you probably want to keep that mapping...(Though I
> guess it's the sum of two 32-bit integers which technically could
> overflow, and I'm not sure what happens if you actually hit those
> boundaries on the server — but nobody generates snapshots on the same
> file in that quantity).
> 
> All that said, it'd be kind of nice if we could just annotate for
> clang that we are perfectly happy for the evaluation to always be true
> on a 64-bit architecture (as snapids are 64 bits, we will always be
> able to count them).

So, are you suggesting to declare num as u64 here? Am I correct?

Thanks,
Slava.

> > 
> > >               goto fail;
> > >       snapc = ceph_create_snap_context(num, GFP_NOFS);
> > >       if (!snapc)