Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time

Vlastimil Babka
2021-05-13T11:38:15Z

On 5/13/21 12:31 PM, Marco Elver wrote:
> On Thu, May 13, 2021 at 10:51AM +0200, Vlastimil Babka wrote:
>> On 5/13/21 8:28 AM, Hyeonggon Yoo 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.
>>
>> Agreed.
>
> It's just a test, and performance doesn't matter for it.

Sure. But what if there appear more users where it will matter. Those would get
better performance out of kmalloc_slab().

> The thing is this function lives in <linux/slab.h>, isn't prefixed with
> __ or anything like that, so it really does look like a public function.
>
>> >> Would something like this work?
>>
>> I'd prefer if we kept kmalloc_index() for constant sizes only. The broken build
>> then warns anyone using it the wrong way that they shouldn't.
>
> Agreed. Andrew's size_is_constant would do that. Also see my suggestion
> below to keep the same interface.
>
>> Besides, it really
>> shouldn't be used outside of slab.
>
> It's an allocator test. If we want to facilitate testing, it must be
> allowed to verify or set up test cases that test boundary conditions
> based on internal state.
>
> In the case of kfence_test it wants:  the cache's alignment to create
> accesses that fall on alignment boundaries; and to verify obj_to_index()
> and objs_per_slab_page() are set up correctly.

OK.

> I think the requirements are:
>
> 1. Make the interface hard to abuse. Adding the BUILD_BUG_ON does that.

Yes.

> 2. Facilitate testing.

Right.

>> But if kfence test really needs this, we could perhaps extract the index
>> determining part out of kmalloc_slab().
>
> That would duplicate kmalloc_index()? I don't see the need, let's keep
> things simple.

They are already "duplicated". But one is tailored for constant sizes, the other
for variable sizes.

>> Hmm or I guess the kfence tests could just use kmalloc_slab() directly?
>
> kmalloc_slab() is internal to slab and should not be exported.

So should be kmalloc_index(). However it needs to have the full implementation
in a header accessible to all kmalloc() users to work, so it's there, visible to
anyone.

> It'd
> require exporting because the tests can be built as modules.

That's true.

> kmalloc_index() works perfectly fine, and the test really doesn't care
> about performance of kmalloc_index(). :-)

OK then.

> See my suggestion below that builds on Andrew's size_is_constant but
> would retain the old interface and support testing.

I can accept that, but please also modify/expand the newly added comment. Now
it's *normally* evaluated in compile-time. And there should be warning that
anyone calling it with size_is_constant == false should do that only in context
where performance (and code bloat, most likely too) doesn't matter, such as unit
test.

Thanks, Vlastimil

> Thanks,
> -- Marco
>
> ------ >8 ------
>
> From: Marco Elver <elver@google.com>
> Subject: [PATCH] kfence: test: fix for "mm, slub: change run-time assertion in
> kmalloc_index() to compile-time"
>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> include/linux/slab.h | 9 +++++++--
> mm/kfence/kfence_test.c | 5 +++--
> 2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 27d142564557..7a10bdc4b7a9 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -350,7 +350,8 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
> * 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;
> @@ -386,11 +387,15 @@ static __always_inline unsigned int kmalloc_index(size_t size)
> 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();
>
> /* Will never be reached. Needed because the compiler may complain */
> return -1;
> }
> +#define kmalloc_index(s) __kmalloc_index(s, true)
> #endif /* !CONFIG_SLOB */
>
> void *__kmalloc(size_t size, gfp_t flags) __assume_kmalloc_alignment __malloc;
> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
> index 4acf4251ee04..7f24b9bcb2ec 100644
> --- a/mm/kfence/kfence_test.c
> +++ b/mm/kfence/kfence_test.c
> @@ -197,7 +197,7 @@ static void test_cache_destroy(void)
>
> static inline size_t kmalloc_cache_alignment(size_t size)
> {
> - return kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size)]->align;
> + return kmalloc_caches[kmalloc_type(GFP_KERNEL)][__kmalloc_index(size, false)]->align;
> }
>
> /* Must always inline to match stack trace against caller. */
> @@ -267,7 +267,8 @@ static void *test_alloc(struct kunit *test, size_t size, gfp_t gfp, enum allocat
>
> if (is_kfence_address(alloc)) {
> struct page *page = virt_to_head_page(alloc);
> - struct kmem_cache *s = test_cache ?: kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size)];
> + struct kmem_cache *s = test_cache ?:
> + kmalloc_caches[kmalloc_type(GFP_KERNEL)][__kmalloc_index(size, false)];
>
> /*
> * Verify that various helpers return the right values
>