linux-kernel - Re: [PATCH -next 2/2] bcache: Use max_t() when comparing different types

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Date:	Thu, 6 Feb 2014 12:50:58 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Joe Perches <joe@...ches.com>, Kent Overstreet <kmo@...erainc.com>,
	Neil Brown <neilb@...e.de>, linux-bcache@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-Next <linux-next@...r.kernel.org>
Subject: Re: [PATCH -next 2/2] bcache: Use max_t() when comparing different
 types

On Thu, 6 Feb 2014 21:45:36 +0100 Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> > --- a/drivers/md/bcache/btree.c~bcache-drop-l-suffix-when-comparing-ssize_t-with-0-fix
> > +++ a/drivers/md/bcache/btree.c
> > @@ -1805,7 +1805,7 @@ static bool btree_insert_key(struct btre
> >
> >  static size_t insert_u64s_remaining(struct btree *b)
> >  {
> > -       ssize_t ret = bch_btree_keys_u64s_remaining(&b->keys);
> > +       size_t ret = bch_btree_keys_u64s_remaining(&b->keys);
> >
> >         /*
> >          * Might land in the middle of an existing extent and have to split it
> > @@ -1813,7 +1813,7 @@ static size_t insert_u64s_remaining(stru
> >         if (b->keys.ops->is_extents)
> >                 ret -= KEY_MAX_U64S;
> 
> I think the reason is the line above: with size_t, ret may become a big
> positive number when the subtraction wraps below zero.

Well, I assumed that case would be a bug - otherwise the programmer
would have commented such a subtlety.  Wouldn't he?

> >
> > -       return max(ret, 0);
> > +       return max_t(size_t, ret, 0);
> 
> That part is OK, cfr. my v1 (which I had planned to send out as v3 again).

It needs to be ssize_t.
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