linux-kernel - Re: [PATCH] slub: fix check

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Message-ID: <1312883169.2371.16.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Tue, 09 Aug 2011 11:46:09 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org
Subject: Re: [PATCH] slub: fix check_bytes() for slub debugging

Le mardi 09 août 2011 à 18:38 +0900, Akinobu Mita a écrit :
> 2011/8/9 Eric Dumazet <eric.dumazet@...il.com>:
> 
> >> > diff --git a/mm/slub.c b/mm/slub.c
> >> > index eb5a8f9..5695f92 100644
> >> > --- a/mm/slub.c
> >> > +++ b/mm/slub.c
> >> > @@ -701,7 +701,7 @@ static u8 *check_bytes(u8 *start, u8 value, unsigned int bytes)
> >> >             return check_bytes8(start, value, bytes);
> >> >
> >> >     value64 = value | value << 8 | value << 16 | value << 24;
> >> > -   value64 = value64 | value64 << 32;
> >> > +   value64 = (value64 & 0xffffffff) | value64 << 32;
> >> >     prefix = 8 - ((unsigned long)start) % 8;
> >> >
> >> >     if (prefix) {
> >>
> >> Still buggy I am afraid. Could we use the following ?
> >>
> >>
> >>       value64 = value;
> >>       value64 |= value64 << 8;
> >>       value64 |= value64 << 16;
> >>       value64 |= value64 << 32;
> >>
> >>
> >
> > Well, 'buggy' was not well chosen.
> >
> > Another possibility would be to use a multiply if arch has a fast
> > multiplier...
> >
> >
> >        value64 = value;
> > #if defined(ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
> >        value64 *= 0x0101010101010101;
> > #elif defined(ARCH_HAS_FAST_MULTIPLIER)
> >        value64 *= 0x01010101;
> >        value64 |= value64 << 32;
> > #else
> >        value64 |= value64 << 8;
> >        value64 |= value64 << 16;
> >        value64 |= value64 << 32;
> > #endif
> 
> I don't really care about which one should be used.  So tell me if I need
> to resend it with this improvement.

It would be nice to fix all bugs while we review this code.

Lets push your patch and I'll submit a patch for next kernel.

For example, following code is suboptimal :

        prefix = 8 - ((unsigned long)start) % 8;

        if (prefix) {
                u8 *r = check_bytes8(start, value, prefix);
                if (r)
                        return r;
                start += prefix;
                bytes -= prefix;
        }


Since we always have prefix = 8 if 'start' is longword aligned, so we
call check_bytes8() at least once with 8 bytes to compare...

Also, 32bit arches should be taken into account properly.

Oh well...



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