From: Marco Elver <elver@google.com>
To: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
David Rientjes <rientjes@google.com>,
Pekka Enberg <penberg@kernel.org>,
Christoph Lameter <cl@linux.com>,
Linux Memory Management List <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time
Date: Thu, 13 May 2021 10:46:03 +0200 [thread overview]
Message-ID: <CANpmjNPYtWsYv5tcH4wGrTBQx4vU4+LvjX9fG=nC9icDjJXy5g@mail.gmail.com> (raw)
In-Reply-To: <20210513062809.GA319973@hyeyoo>
On Thu, 13 May 2021 at 08:28, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> On Wed, May 12, 2021 at 08:40:24PM -0700, Andrew Morton wrote:
> > On Thu, 13 May 2021 12:12:20 +0900 Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> > > On Wed, May 12, 2021 at 07:52:27PM -0700, Andrew Morton wrote:
> > > > This explodes in mysterious ways. The patch as I have it is appended,
> > > > for reference.
> > > >
> > > > gcc-10.3.0 allmodconfig.
> > > >
> > > > This patch suppresses the error:
> >
> > Ah, yes, of course, your patch changes kmalloc_index() to require that
> > it always is called with a constant `size'. kfence_test doesn't do
> > that.
> >
> > kfence is being a bit naughty here - the other kmalloc_index() callers
> > only comple up the call after verifying that `size' is a compile-time
> > constant.
> >
> > Would something like this work?
> > include/linux/slab.h | 12 ++++++++----
> > mm/kfence/kfence_test.c | 4 ++--
> > 2 files changed, 10 insertions(+), 6 deletions(-)
> >
> > --- a/include/linux/slab.h~b
> > +++ a/include/linux/slab.h
> > @@ -374,7 +374,8 @@ static __always_inline enum kmalloc_cach
> > * Note: there's no need to optimize kmalloc_index because it's evaluated
> > * in compile-time.
> > */
> > -static __always_inline unsigned int kmalloc_index(size_t size)
> > +static __always_inline unsigned int kmalloc_index(size_t size,
> > + bool size_is_constant)
> > {
> > if (!size)
> > return 0;
> > @@ -410,7 +411,10 @@ static __always_inline unsigned int kmal
> > if (size <= 16 * 1024 * 1024) return 24;
> > if (size <= 32 * 1024 * 1024) return 25;
> >
> > - BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
> > + if (size_is_constant)
> > + BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
> > + else
> > + BUG();
>
>
> kfence is randomly generating size. because kfence is using non-constant
> size, we should do run-time assertion or compile-time assertion depending
> on situation.
>
> I think we can use __builtin_constant_p here. we don't need to modify
> kmalloc_index's prototype.
>
> so what about this?
> if you think it makes sense, I'll send patch v4.
>
> I used KMALLOC_MAX_CACHE_SIZE to assure it's safe size.
> it's safer than putting BUILD_BUG_ON_MSG(1, ...) to below if statements
> because KMALLOC_MAX_CACHE_SIZE can be less than 32MB.
I'm actually inclined to say that Andrew's patch with
'size_is_constant' is the better option, because we want to be
explicit about where it's using constant size and where it isn't. I
think in tests like kfence_test, it should be permitted to use
non-constant size, it's a test after all and performance is no
concern. For non-test code, however, we want to ensure size is
constant, and therefore having the distinguishing argument makes
sense. That way non-test code will not compile if our intent does not
match reality.
Thanks,
-- Marco
next prev parent reply other threads:[~2021-05-13 8:46 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-11 17:34 [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time Hyeonggon Yoo
2021-05-11 17:38 ` Hyeonggon Yoo
2021-05-11 18:08 ` Vlastimil Babka
2021-05-13 2:52 ` Andrew Morton
2021-05-13 3:12 ` Hyeonggon Yoo
2021-05-13 3:40 ` Andrew Morton
2021-05-13 6:28 ` Hyeonggon Yoo
2021-05-13 8:46 ` Marco Elver [this message]
2021-05-13 8:51 ` Vlastimil Babka
2021-05-13 10:31 ` Marco Elver
2021-05-13 11:37 ` Vlastimil Babka
2021-05-13 12:08 ` Hyeonggon Yoo
2021-05-13 12:10 ` Hyeonggon Yoo
2021-05-13 12:03 ` Hyeonggon Yoo
2021-05-13 12:29 ` Marco Elver
2021-05-13 12:38 ` Hyeonggon Yoo
2021-05-13 13:08 ` Hyeonggon Yoo
2021-05-13 12:44 ` [PATCH] kfence: test: fix for "mm, slub: change run-time assertion in kmalloc_index() to compile-time" Marco Elver
2021-05-15 21:09 ` [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time Hyeonggon Yoo
2021-05-15 21:24 ` Vlastimil Babka
2021-05-15 21:56 ` Hyeonggon Yoo
2021-05-16 6:34 ` Nathan Chancellor
2021-05-18 0:38 ` Hyeonggon Yoo
2021-05-18 0:43 ` Nathan Chancellor
2021-05-18 1:53 ` Hyeonggon Yoo
2021-05-18 9:28 ` Vlastimil Babka
2021-05-18 11:18 ` Hyeonggon Yoo
2021-05-18 11:34 ` Vlastimil Babka
2021-05-19 5:45 ` Hyeonggon Yoo
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