linux-kernel - Re: linux-next: Tree for May 13 (mm/page_alloc.c, <linux/mm.h>: sizeof(struct page))

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Date:   Mon, 17 May 2021 21:59:54 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: linux-next: Tree for May 13 (mm/page_alloc.c, <linux/mm.h>:
 sizeof(struct page))

On 5/17/21 9:03 PM, Anshuman Khandual wrote:
> 
> 
> On 5/14/21 2:57 AM, Matthew Wilcox wrote:
>> On Thu, May 13, 2021 at 10:29:49AM -0700, Randy Dunlap wrote:
>>> On 5/12/21 10:44 PM, Stephen Rothwell wrote:
>>>> Hi all,
>>>>
>>>> Changes since 20210512:
>>>>
>>>
>>> on x86_64:
>>>
>>> In function ‘__mm_zero_struct_page.isra.75’,
>>>     inlined from ‘__init_single_page.isra.76’ at ../mm/page_alloc.c:1494:2:
>>> ./../include/linux/compiler_types.h:328:38: error: call to ‘__compiletime_assert_162’ declared with attribute error: BUILD_BUG_ON failed: sizeof(struct page) > 80
>>
>> Hmm.
>>
>>                 struct {
>>                         long unsigned int _pt_pad_1;     /*     8     8 */
>>                         pgtable_t  pmd_huge_pte;         /*    16     8 */
>>                         long unsigned int _pt_pad_2;     /*    24     8 */
>>                         union {
>>                                 struct mm_struct * pt_mm; /*    32     8 */
>>                                 atomic_t pt_frag_refcount; /*    32     4 */
>>                         };                               /*    32     8 */
>>                         spinlock_t ptl;                  /*    40    72 */
>>                 };                                       /*     8   104 */
>>
>> #if ALLOC_SPLIT_PTLOCKS
>>                         spinlock_t *ptl;
>> #else
>>                         spinlock_t ptl;
>> #endif
>>
>> something has disabled ALLOC_SPLIT_PTLOCKS when it ought to be enabled.
>>
>> #if USE_SPLIT_PTE_PTLOCKS
>> #define ALLOC_SPLIT_PTLOCKS     (SPINLOCK_SIZE > BITS_PER_LONG/8)
>> #else
>> #define ALLOC_SPLIT_PTLOCKS     0
>> #endif
>>
>> Oh.  This is Anshuman's fault.
>>
>> commit 9b8a39056e2472592a5e5897987387f43038b8ba
>> Author: Anshuman Khandual <anshuman.khandual@....com>
>> Date:   Tue May 11 15:06:01 2021 +1000
>>
>>     mm/thp: make ALLOC_SPLIT_PTLOCKS dependent on USE_SPLIT_PTE_PTLOCKS
>>
>>
> 
> Previously ALLOC_SPLIT_PTLOCKS was evaluated and the spin lock element
> in struct page was getting created independent of whether split pte
> locks are being used or not. AFAICS without USE_SPLIT_PTE_PTLOCKS, it
> does not really matter whether struct page has spinlock_t *ptl or ptl
> element because that is not going to be used. Should the BUILD_BUG_ON()
> evaluation be changed when USE_SPLIT_PTE_PTLOCKS is not enabled or we
> could something like this which drops the ptl element in such cases ?
> 
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -152,11 +152,13 @@ struct page {
>                                 struct mm_struct *pt_mm; /* x86 pgds only */
>                                 atomic_t pt_frag_refcount; /* powerpc */
>                         };
> +#if USE_SPLIT_PTE_PTLOCKS
>  #if ALLOC_SPLIT_PTLOCKS
>                         spinlock_t *ptl;
>  #else
>                         spinlock_t ptl;
>  #endif
> +#endif
>                 };
>                 struct {        /* ZONE_DEVICE pages */
>                         /** @pgmap: Points to the hosting device page map. */
> 

OK, that works.  Thanks.

Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested


I guess you should send a proper patch to Andrew.  The code above
is whitespace-damaged (cut'n'paste).

-- 
~Randy