Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20221129063358.3012362-1-feng.tang@intel.com> <20221129063358.3012362-2-feng.tang@intel.com>
 <CANpmjNNkLWvNYUf-bPPDBcqFWegp3_NpYdhygvYU1dtT52m29g@mail.gmail.com>
 <67e6ebce-f8cc-7d28-5e85-8a3909c2d180@suse.cz> <CANpmjNNZhXX830jqPn9eaQZHwKhBb4b_PEuUdH6O69ELqW470w@mail.gmail.com>
 <d6c614fe-cc80-7a19-1fa6-2ed6cc75b705@suse.cz> <Y4YAC9UTxQLg4Y5q@feng-clx>
In-Reply-To: <Y4YAC9UTxQLg4Y5q@feng-clx>
From:   Marco Elver <elver@google.com>
Date:   Tue, 29 Nov 2022 13:56:15 +0100
Message-ID: <CANpmjNPm1JUB6O-xBOT_Ab0ztKnjX1kgnr=81AzTg=Tkt6yCBA@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] mm/slub, kunit: Add a test case for kmalloc
 redzone check
To:     Feng Tang <feng.tang@intel.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oliver Glitta <glittao@gmail.com>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, 29 Nov 2022 at 13:53, Feng Tang <feng.tang@intel.com> wrote:
>
> On Tue, Nov 29, 2022 at 08:02:51PM +0800, Vlastimil Babka wrote:
> > On 11/29/22 12:48, Marco Elver wrote:
> > > On Tue, 29 Nov 2022 at 12:01, Vlastimil Babka <vbabka@suse.cz> wrote:
> > >>
> > >> On 11/29/22 10:31, Marco Elver wrote:
> > >> > On Tue, 29 Nov 2022 at 07:37, Feng Tang <feng.tang@intel.com> wrote:
> > >
> > >> For SLAB_SKIP_KFENCE, we could also add the flag after creation to avoid
> > >> this trouble? After all there is a sysfs file to control it at runtime
> > >> anyway (via skip_kfence_store()).
> > >> In that case patch 1 would have to wrap kmem_cache_create() and the flag
> > >> addition with a new function to avoid repeating. That function could also be
> > >> adding SLAB_NO_USER_FLAGS to kmem_cache_create(), instead of the #define
> > >> DEFAULT_FLAGS.
> > >
> > > I wouldn't overcomplicate it, all we need is a way to say "this flag
> > > should not be used directly" - and only have it available via an
> > > indirect step. Availability via sysfs is one such step.
> > >
> > > And for tests, there are 2 options:
> > >
> > > 1. we could provide a function "kmem_cache_set_test_flags(cache,
> > > gfp_flags)" and define SLAB_TEST_FLAGS (which would include
> > > SLAB_SKIP_KFENCE). This still allows to set it generally, but should
> > > make abuse less likely due to the "test" in the name of that function.
> > >
> > > 2. just set it directly, s->flags |= SLAB_SKIP_KFENCE.
> > >
> > > If you're fine with #2, that seems simplest and would be my preference.
> >
> > Yeah, that's what I meant. But slub_kunit.c could still have own internal
> > cache creation function so the "|SLAB_NO_USER_FLAGS" and "s->flags |=
> > SLAB_SKIP_KFENCE" is not repeated X times.
>
> I just quickly tried adding a new wrapper, like
>
>   struct kmem_cache *debug_kmem_cache_create(const char *name, unsigned int size,
>                         unsigned int align, slab_flags_t flags,
>                         void (*ctor)(void *), slab_flags_t debug_flags);
>
> and found that, IIUC, both SLAB_KMALLOC and SLAB_NO_USER are creation
> time flag, while SLAB_SKIP_KFENCE is an allocation runtime flag which
> could be set after creation.
>
> So how about use the initial suggestion from Vlastimil to set the
> SKIP_KFENCE flag through an internal wrapper in slub_kunit.c?
>
>         /* Only for debug and test use, to skip kfence allocation */
>         static inline void kmem_cache_skip_kfence(struct kmem_cache *s)
>         {
>                 s->flags |= SLAB_SKIP_KFENCE;
>         }

Yes, that's fine - as long as it's local to slub_kunit.c, this seems
very reasonable.

Thanks,
-- Marco