linux-kernel - Re: [PATCH] z3fold: use %z modifier for format string

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Date:   Fri, 25 Nov 2016 09:41:53 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Vitaly Wool <vitalywool@...il.com>
Cc:     Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Streetman <ddstreet@...e.org>,
        zhong jiang <zhongjiang@...wei.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] z3fold: use %z modifier for format string

On Friday, November 25, 2016 8:38:25 AM CET Vitaly Wool wrote:
> >> diff --git a/mm/z3fold.c b/mm/z3fold.c
> >> index e282ba073e77..66ac7a7dc934 100644
> >> --- a/mm/z3fold.c
> >> +++ b/mm/z3fold.c
> >> @@ -884,7 +884,7 @@ static int __init init_z3fold(void)
> >>  {
> >>       /* Fail the initialization if z3fold header won't fit in one chunk */
> >>       if (sizeof(struct z3fold_header) > ZHDR_SIZE_ALIGNED) {
> >> -             pr_err("z3fold: z3fold_header size (%d) is bigger than "
> >> +             pr_err("z3fold: z3fold_header size (%zd) is bigger than "
> >>                       "the chunk size (%d), can't proceed\n",
> >>                       sizeof(struct z3fold_header) , ZHDR_SIZE_ALIGNED);
> >>               return -E2BIG;
> >
> > The embedded "z3fold: " prefix here should be removed
> > as there's a pr_fmt that also adds it.
> >
> > The test looks like it should be a BUILD_BUG_ON rather
> > than any runtime test too.
> 
> It used to be BUILD_BUG_ON but we deliberately changed that because
> sizeof(spinlock_t) gets bloated in debug builds, so it just won't
> build with default CHUNK_SIZE.

Could this be improved by making the CHUNK_SIZE bigger depending on
the debug options?

Alternatively, how about using a bit_spin_lock instead of raw_spin_lock?
That would guarantee a fixed size for the lock and make z3fold_header
always 24 bytes (on 32-bit architectures) or 40 bytes
(on 64-bit architectures). You could even play some tricks with the
first_num field to make it fit in the same word as the lock and make the
structure fit into 32 bytes if you care about that.

	Arnd