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SubjectRe: [PATCH] mm/page_alloc: make calling prep_compound_head more reliable
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On 2022/6/14 21:13, Matthew Wilcox wrote:
> On Wed, Jun 08, 2022 at 08:17:35PM +0800, Miaohe Lin wrote:
>> +++ b/mm/page_alloc.c
>> @@ -6771,13 +6771,18 @@ static void __ref memmap_init_compound(struct page *head,
>> set_page_count(page, 0);
>>
>> /*
>> - * The first tail page stores compound_mapcount_ptr() and
>> - * compound_order() and the second tail page stores
>> - * compound_pincount_ptr(). Call prep_compound_head() after
>> - * the first and second tail pages have been initialized to
>> - * not have the data overwritten.
>> + * The first tail page stores compound_mapcount_ptr(),
>> + * compound_order() and compound_pincount_ptr(). Call
>> + * prep_compound_head() after the first tail page have
>> + * been initialized to not have the data overwritten.
>> + *
>> + * Note the idea to make this right after we initialize
>> + * the offending tail pages is trying to take advantage
>> + * of the likelihood of those tail struct pages being
>> + * cached given that we will read them right after in
>> + * prep_compound_head().
>
> It's not that we'll read them again, it's that the cacheline will still
> be in cache, and therefore dirty.

Thanks for pointing this out.

>
> Honestly, I don't think we need this extra explanation in a comment.
> Just change the first paragraph to reflect reality and leave it at that.

Will do it in next version if prep_compound_head is not moved outside loop.

>
>> */
>> - if (pfn == head_pfn + 2)
>> + if (unlikely(pfn == head_pfn + 1))
>
> We definitely don't need the unlikely here.

Could you please give me a more detailed explanation? IIUC, the above if condition
will only meet at a probability of 1/512. So unlikely tells the compiler to do some
optimization around it. Or am I miss something?

Thanks!

>
>> prep_compound_head(head, order);
>> }
>> }
>>
>> Or am I miss something?
>>
>> Thanks!
>>
>>> .
>>>
>>
>
> .
>

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