diff mbox series

mm: kmalloc_index: remove case when size is more than 32MB

Message ID 20210508221328.7338-1-42.hyeyoo@gmail.com (mailing list archive)
State New
Headers show
Series mm: kmalloc_index: remove case when size is more than 32MB | expand

Commit Message

Hyeonggon Yoo May 8, 2021, 10:13 p.m. UTC
the return value of kmalloc_index is used as index of kmalloc_caches,
which is defined as:
  struct kmem_cache *
  kmalloc_caches[NR_KMALLOC_TYPES][KMALLOC_SHIFT_HIGH + 1]

and KMALLOC_SHIFT_HIGH is defined as:
  #define KMALLOC_SHIFT_HIGH    ((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
                              (MAX_ORDER + PAGE_SHIFT - 1) : 25)

KMALLOC_SHIFT_HIGH is maximum 25 by its definition. thus index of
kmalloc_caches cannot be 26. so this case should be removed.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
---
 include/linux/slab.h | 1 -
 1 file changed, 1 deletion(-)

Comments

Matthew Wilcox May 8, 2021, 11:19 p.m. UTC | #1
On Sun, May 09, 2021 at 07:13:28AM +0900, Hyeonggon Yoo wrote:
> the return value of kmalloc_index is used as index of kmalloc_caches,

it doesn't matter.  every few weeks somebody posts a patch to "optimise"
kmalloc_index, failing to appreciate that it's only ever run at compile
time because it's all under __builtin_constant_p().
Hyeonggon Yoo May 9, 2021, 5:33 a.m. UTC | #2
On Sun, May 09, 2021 at 12:19:40AM +0100, Matthew Wilcox wrote:
> On Sun, May 09, 2021 at 07:13:28AM +0900, Hyeonggon Yoo wrote:
> > the return value of kmalloc_index is used as index of kmalloc_caches,
>
> it doesn't matter.  every few weeks somebody posts a patch to "optimise"
> kmalloc_index, failing to appreciate that it's only ever run at compile
> time because it's all under __builtin_constant_p().

Oh thanks, I didn't know about __builtin_constant_p.

But I was not optimizing kmalloc_index. isn't it confusing that
kmalloc_caches alllows maximum size of 32MB, and kmalloc_index allows
maximum size of 64MB?

and even if the code I removed is never reached because 64MB is always
bigger than KMALLOC_MAX_CACHE_SIZE, it will cause an error if reached.
Vlastimil Babka May 10, 2021, 10:09 a.m. UTC | #3
On 5/9/21 7:33 AM, Hyeonggon Yoo wrote:
> On Sun, May 09, 2021 at 12:19:40AM +0100, Matthew Wilcox wrote:
>> On Sun, May 09, 2021 at 07:13:28AM +0900, Hyeonggon Yoo wrote:
>> > the return value of kmalloc_index is used as index of kmalloc_caches,
>>
>> it doesn't matter.  every few weeks somebody posts a patch to "optimise"
>> kmalloc_index, failing to appreciate that it's only ever run at compile
>> time because it's all under __builtin_constant_p().
> 
> Oh thanks, I didn't know about __builtin_constant_p.
> 
> But I was not optimizing kmalloc_index. isn't it confusing that
> kmalloc_caches alllows maximum size of 32MB, and kmalloc_index allows
> maximum size of 64MB?
> 
> and even if the code I removed is never reached because 64MB is always
> bigger than KMALLOC_MAX_CACHE_SIZE, it will cause an error if reached.

KMALLOC_MAX_CACHE_SIZE depends on KMALLOC_SHIFT_HIGH
size of kmalloc_caches array depends on KMALLOC_SHIFT_HIGH

So I don't an easy way how it could become reachable while causing the index to
overflow - if someone increased KMALLOC_SHIFT_HIGH from 25 to 26, all should be
fine, AFAICS.

The problem would be if someone increased it to 27, then we might suddenly get a
BUG() in kmalloc_index(). We should probably replace that BUG() with
BUILD_BUG_ON(1) to catch that at compile time. Hopefully no supported compiler
will break because it's not able to do the proper compile-time evaluation - but
if it does, at least we would know.

So I would accept the patch if it also changed BUG() to e.g. BUILD_BUG_ON_MSG(1,
"unexpected size in kmalloc_index()");
and expanded the function's comment that this is always compile-time evaluated
and thus no attempts at "optimizing" the code should be made.
Hyeonggon Yoo May 10, 2021, 1:58 p.m. UTC | #4
On Mon, May 10, 2021 at 12:09:55PM +0200, Vlastimil Babka wrote:
> On 5/9/21 7:33 AM, Hyeonggon Yoo wrote:
> > On Sun, May 09, 2021 at 12:19:40AM +0100, Matthew Wilcox wrote:
> >> On Sun, May 09, 2021 at 07:13:28AM +0900, Hyeonggon Yoo wrote:
> >> > the return value of kmalloc_index is used as index of kmalloc_caches,
> >>
> >> it doesn't matter.  every few weeks somebody posts a patch to "optimise"
> >> kmalloc_index, failing to appreciate that it's only ever run at compile
> >> time because it's all under __builtin_constant_p().
> > 
> > Oh thanks, I didn't know about __builtin_constant_p.
> > 
> > But I was not optimizing kmalloc_index. isn't it confusing that
> > kmalloc_caches alllows maximum size of 32MB, and kmalloc_index allows
> > maximum size of 64MB?
> > 
> > and even if the code I removed is never reached because 64MB is always
> > bigger than KMALLOC_MAX_CACHE_SIZE, it will cause an error if reached.
> 
> KMALLOC_MAX_CACHE_SIZE depends on KMALLOC_SHIFT_HIGH
> size of kmalloc_caches array depends on KMALLOC_SHIFT_HIGH
> 
> So I don't an easy way how it could become reachable while causing the index to
> overflow - if someone increased KMALLOC_SHIFT_HIGH from 25 to 26, all should be
> fine, AFAICS.
> 
> The problem would be if someone increased it to 27, then we might suddenly get a
> BUG() in kmalloc_index(). We should probably replace that BUG() with
> BUILD_BUG_ON(1) to catch that at compile time. Hopefully no supported compiler
> will break because it's not able to do the proper compile-time evaluation - but
> if it does, at least we would know.
> 
> So I would accept the patch if it also changed BUG() to e.g. BUILD_BUG_ON_MSG(1,
> "unexpected size in kmalloc_index()");
> and expanded the function's comment that this is always compile-time evaluated
> and thus no attempts at "optimizing" the code should be made.
> 

Thank you so much reviewing and replying to my patch.
plecase check if I understood well.

Okay, I'll do that work. then the following patch will:
	- remove case when size is more than 32MB
	- change "BUG to BUILD_BUG_ON to let compiler know when the size is not supported"
	- add comment that there's no need to optimize it

is it what you mean. right?

and I have a question. in the lin 751 of mm/slab_common.c,
thre's struct kmalloc_info_struct kmalloc_info. and it initializes kmalloc info
up to 64MB, which is currently not supported. should I change it too? in a separate patch?
Vlastimil Babka May 10, 2021, 2:04 p.m. UTC | #5
On 5/10/21 3:58 PM, Hyeonggon Yoo wrote:
> On Mon, May 10, 2021 at 12:09:55PM +0200, Vlastimil Babka wrote:
>> On 5/9/21 7:33 AM, Hyeonggon Yoo wrote:
>> > On Sun, May 09, 2021 at 12:19:40AM +0100, Matthew Wilcox wrote:
>> >> On Sun, May 09, 2021 at 07:13:28AM +0900, Hyeonggon Yoo wrote:
>> >> > the return value of kmalloc_index is used as index of kmalloc_caches,
>> >>
>> >> it doesn't matter.  every few weeks somebody posts a patch to "optimise"
>> >> kmalloc_index, failing to appreciate that it's only ever run at compile
>> >> time because it's all under __builtin_constant_p().
>> > 
>> > Oh thanks, I didn't know about __builtin_constant_p.
>> > 
>> > But I was not optimizing kmalloc_index. isn't it confusing that
>> > kmalloc_caches alllows maximum size of 32MB, and kmalloc_index allows
>> > maximum size of 64MB?
>> > 
>> > and even if the code I removed is never reached because 64MB is always
>> > bigger than KMALLOC_MAX_CACHE_SIZE, it will cause an error if reached.
>> 
>> KMALLOC_MAX_CACHE_SIZE depends on KMALLOC_SHIFT_HIGH
>> size of kmalloc_caches array depends on KMALLOC_SHIFT_HIGH
>> 
>> So I don't an easy way how it could become reachable while causing the index to
>> overflow - if someone increased KMALLOC_SHIFT_HIGH from 25 to 26, all should be
>> fine, AFAICS.
>> 
>> The problem would be if someone increased it to 27, then we might suddenly get a
>> BUG() in kmalloc_index(). We should probably replace that BUG() with
>> BUILD_BUG_ON(1) to catch that at compile time. Hopefully no supported compiler
>> will break because it's not able to do the proper compile-time evaluation - but
>> if it does, at least we would know.
>> 
>> So I would accept the patch if it also changed BUG() to e.g. BUILD_BUG_ON_MSG(1,
>> "unexpected size in kmalloc_index()");
>> and expanded the function's comment that this is always compile-time evaluated
>> and thus no attempts at "optimizing" the code should be made.
>> 
> 
> Thank you so much reviewing and replying to my patch.
> plecase check if I understood well.
> 
> Okay, I'll do that work. then the following patch will:
> 	- remove case when size is more than 32MB
> 	- change "BUG to BUILD_BUG_ON to let compiler know when the size is not supported"
> 	- add comment that there's no need to optimize it
> 
> is it what you mean. right?

Exactly.

> and I have a question. in the lin 751 of mm/slab_common.c,
> thre's struct kmalloc_info_struct kmalloc_info. and it initializes kmalloc info
> up to 64MB, which is currently not supported. should I change it too? in a separate patch?

Yeah that could be also changed, in the same patch.
diff mbox series

Patch

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 0c97d788762c..4694b1db4cb2 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -382,7 +382,6 @@  static __always_inline unsigned int kmalloc_index(size_t size)
 	if (size <=  8 * 1024 * 1024) return 23;
 	if (size <=  16 * 1024 * 1024) return 24;
 	if (size <=  32 * 1024 * 1024) return 25;
-	if (size <=  64 * 1024 * 1024) return 26;
 	BUG();
 
 	/* Will never be reached. Needed because the compiler may complain */