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Date: Wed, 10 Apr 2024 18:40:55 +0200
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Subject: Re: [PATCH] mm/shmem: Inline shmem_is_huge() for disabled transparent
 hugepages
To: Peter Xu <peterx@redhat.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>, akpm@linux-foundation.org,
 linux-mm@kvack.org, hughd@google.com, hca@linux.ibm.com, gor@linux.ibm.com,
 agordeev@linux.ibm.com, iii@linux.ibm.com, linux-s390@vger.kernel.org,
 linux-kernel@vger.kernel.org, Jason Gunthorpe <jgg@nvidia.com>
References: <20240409155407.2322714-1-sumanthk@linux.ibm.com>
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 <1ce2fe6c-b56a-4582-a5d8-babc8fccef52@redhat.com> <Zha_auw6yBx0YRWQ@x1n>
From: David Hildenbrand <david@redhat.com>
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On 10.04.24 18:33, Peter Xu wrote:
> On Wed, Apr 10, 2024 at 06:12:34PM +0200, David Hildenbrand wrote:
>> On 10.04.24 18:07, Sumanth Korikkar wrote:
>>> On Wed, Apr 10, 2024 at 05:51:28PM +0200, David Hildenbrand wrote:
>>>> On 10.04.24 17:26, Sumanth Korikkar wrote:
>>>>> On Wed, Apr 10, 2024 at 02:34:35PM +0200, David Hildenbrand wrote:
>>>>>> On 09.04.24 17:54, Sumanth Korikkar wrote:
>>>>>>> In order to  minimize code size (CONFIG_CC_OPTIMIZE_FOR_SIZE=y),
>>>>>>> compiler might choose to make a regular function call (out-of-line) for
>>>>>>> shmem_is_huge() instead of inlining it. When transparent hugepages are
>>>>>>> disabled (CONFIG_TRANSPARENT_HUGEPAGE=n), it can cause compilation
>>>>>>> error.
>>>>>>>
>>>>>>> mm/shmem.c: In function ‘shmem_getattr’:
>>>>>>> ./include/linux/huge_mm.h:383:27: note: in expansion of macro ‘BUILD_BUG’
>>>>>>>       383 | #define HPAGE_PMD_SIZE ({ BUILD_BUG(); 0; })
>>>>>>>           |                           ^~~~~~~~~
>>>>>>> mm/shmem.c:1148:33: note: in expansion of macro ‘HPAGE_PMD_SIZE’
>>>>>>>      1148 |                 stat->blksize = HPAGE_PMD_SIZE;
>>>>>>>
>>>>>>> To prevent the possible error, always inline shmem_is_huge() when
>>>>>>> transparent hugepages are disabled.
>>>>>>>
>>>>>>
>>>>>> Do you know which commit introduced that?
>>>>> Hi David,
>>>>>
>>>>> Currently with CONFIG_CC_OPTIMIZE_FOR_SIZE=y and expirementing with
>>>>> -fPIC kernel compiler option, I could see this error on s390.
>>>>
>>>> Got it. I assume on Linus' tree, not mm/unstable?
>>>
>>> It's not yet upstream.
>>>>
>>>>>
>>>>> However, default kernel compiler options doesnt end up with the above
>>>>> pattern right now.
>>>>
>>>> Okay, just asking if this is related to recent HPAGE_PMD_SIZE changes:
>>>>
>>>> commit c1a1e497a3d5711dbf8fa6d7432d6b83ec18c26f
>>>> Author: Peter Xu <peterx@redhat.com>
>>>> Date:   Wed Mar 27 11:23:22 2024 -0400
>>>>
>>>>       mm: make HPAGE_PXD_* macros even if !THP
>>>>
>>>> Which is still in mm-unstable and not upstream.
>>>
>>> Not related to this commit. I tried on master branch.
>>
>> Thanks! Can you try with Peters patch? (ccing Peter)
>>
>> If I am not wrong, that should also resolve the issue you are seeing.
> 
> David,
> 
> Do you mean this one?
> 
> https://lore.kernel.org/all/20240403013249.1418299-4-peterx@redhat.com/
> 

No, I meant:

https://lore.kernel.org/all/20240327152332.950956-4-peterx@redhat.com/

which removes the "#define HPAGE_PMD_SIZE ({ BUILD_BUG(); 0; })" that we 
seem to trigger here.


.. but it's been a long day, so maybe I'm all wrong :)

> That's indeed similar but that was for pud_pfn() not HPAGE_* stuff.
> 
> I just had a quick look, Sumanth's fix looks valid, and IIUC the goal is
> also that we should keep these build checks around for the long term goal
> (Jason definitely preferred that [1] too, which I agree).
> 
> I removed that build check there for pud_pfn just to avoid other build
> fallouts for other archs as a temporary measure.  For this one if it's in
> common code for a long time and if it's the single spot maybe it's nice to
> have this patch as proposed, as it means it optimizes the if check too
> besides fixing the build error.  After all referencing HPAGE_* with
> !THP+!HUGETLB shouldn't happen logically.
> 
> [1] https://lore.kernel.org/r/20240404112404.GG1723999@nvidia.com
> 
> Thanks,
> 

-- 
Cheers,

David / dhildenb