linux-kernel - Re: [PATCH RESEND] sh: sq: Use the bitmap API when applicable

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Date:   Tue, 18 Apr 2023 10:30:01 +0300
From:   Dan Carpenter <error27@...il.com>
To:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-sh@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH RESEND] sh: sq: Use the bitmap API when applicable

On Tue, Apr 18, 2023 at 08:36:40AM +0200, John Paul Adrian Glaubitz wrote:
> Hi Christophe!
> 
> Thanks for your patch. The changes look good to me. However, I have
> one question, see below.
> 
> On Sun, 2023-04-16 at 21:05 +0200, Christophe JAILLET wrote:
> > Using the bitmap API is less verbose than hand writing them.
> > It also improves the semantic.
> > 
> > Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> > ---
> > This is a resend of [1].
> > 
> > Now cross-compile tested with CONFIG_CPU_SUBTYPE_SH7770=y
> > 
> > [1]: https://lore.kernel.org/all/521788e22ad8f7a5058c154f068b061525321841.1656142814.git.christophe.jaillet@wanadoo.fr/
> > ---
> >  arch/sh/kernel/cpu/sh4/sq.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/sh/kernel/cpu/sh4/sq.c b/arch/sh/kernel/cpu/sh4/sq.c
> > index 27f2e3da5aa2..d289e99dc118 100644
> > --- a/arch/sh/kernel/cpu/sh4/sq.c
> > +++ b/arch/sh/kernel/cpu/sh4/sq.c
> > @@ -372,7 +372,6 @@ static struct subsys_interface sq_interface = {
> >  static int __init sq_api_init(void)
> >  {
> >  	unsigned int nr_pages = 0x04000000 >> PAGE_SHIFT;
> > -	unsigned int size = (nr_pages + (BITS_PER_LONG - 1)) / BITS_PER_LONG;
> >  	int ret = -ENOMEM;
> >  
> >  	printk(KERN_NOTICE "sq: Registering store queue API.\n");
> > @@ -382,7 +381,7 @@ static int __init sq_api_init(void)
> >  	if (unlikely(!sq_cache))
> >  		return ret;
> >  
> > -	sq_bitmap = kzalloc(size, GFP_KERNEL);
> > +	sq_bitmap = bitmap_zalloc(nr_pages, GFP_KERNEL);
> >  	if (unlikely(!sq_bitmap))
> >  		goto out;
> > 
> 
> I have look through other patches where k{z,c,m}alloc() were replaced with
> bitmap_zalloc() and I noticed that in the other cases such as [1], kcalloc()
> was used instead of kzalloc() in our cases with the element size set to
> sizeof(long) while kzalloc() is using an element size equal to a byte.
> 
> Wouldn't that mean that the current code in sq is allocating a buffer that is
> too small by a factor of 1/sizeof(long) or am I missing something?

Yes.  You are correct.  The original code is buggy.

size is the number of longs we need so kzalloc() is allocating a too
small buffer.  It should be kzalloc(size * sizeof(long), ... etc as you
say.

I have some unpublished Smatch stuff which tries to track "variable x
is in terms of bit units or byte units etc."  I will try to make a
static checker rule for this.

regards,
dan carpenter