Re: [PATCH] mm/folio: Avoid special handling for order value 0 in folio_set_order

[Tarun Sahu]

Hi Sidhartha,

Thanks for your inputs, please find my comments inline

> On 4/14/23 12:48 PM, Tarun Sahu wrote:
>> folio_set_order(folio, 0); which is an abuse of folio_set_order as 0-order
>> folio does not have any tail page to set order. folio->_folio_nr_pages is
>> set to 0 for order 0 in folio_set_order. It is required because
>
> In the previous discussion of this function, Mike mentioned having 
> folio_set_order() be used for non-zero orders and adding a 
> folio_clear_order() that is used to set order to 0. This could be done 
> to reduce confusion.
>
Yes, I agree, I replied to Mathew reply to this thread, Lemme know your
thought on this. In this patch, I proposed that there won't be need of
folio_clear_order if folio_set_order(folio, 0) is not needed with minor
changes in code path.

>> _folio_nr_pages overlapped with page->mapping and leaving it non zero
>> caused "bad page" error while freeing gigantic hugepages. This was fixed in
>> Commit ba9c1201beaa ("mm/hugetlb: clear compound_nr before freeing gigantic
>> pages"). Also commit a01f43901cfb ("hugetlb: be sure to free demoted CMA
>> pages to CMA") now explicitly clear page->mapping and hence we won't see
>> the bad page error even if _folio_nr_pages remains unset. Also the order 0
>> folios are not supposed to call folio_set_order, So now we can get rid of
>> folio_set_order(folio, 0) from hugetlb code path to clear the confusion.
>> 
>> The patch also moves _folio_set_head and folio_set_order calls in
>> __prep_compound_gigantic_folio() such that we avoid clearing them in the
>> error path.
>> 
>> Testing: I have run LTP tests, which all passes. and also I have written
>> the test in LTP which tests the bug caused by compound_nr and page->mapping
>> overlapping.
>> 
>> https://lore.kernel.org/all/20230413090753.883953-1-tsahu@xxxxxxxxxxxxx/
>> 
>> Running on older kernel ( < 5.10-rc7) with the above bug this fails while
>> on newer kernel and, also with this patch it passes.
>> 
>> Signed-off-by: Tarun Sahu <tsahu@xxxxxxxxxxxxx>
>> ---
>>   mm/hugetlb.c  | 9 +++------
>>   mm/internal.h | 8 ++------
>>   2 files changed, 5 insertions(+), 12 deletions(-)
>> 
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index f16b25b1a6b9..e2540269c1dc 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1489,7 +1489,6 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
>>   			set_page_refcounted(p);
>>   	}
>>   
>> -	folio_set_order(folio, 0);
>>   	__folio_clear_head(folio);
>>   }
>>   
>> @@ -1951,9 +1950,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>>   	struct page *p;
>>   
>>   	__folio_clear_reserved(folio);
>> -	__folio_set_head(folio);
>> -	/* we rely on prep_new_hugetlb_folio to set the destructor */
>> -	folio_set_order(folio, order);
>>   	for (i = 0; i < nr_pages; i++) {
>>   		p = folio_page(folio, i);
>>   
>> @@ -1999,6 +1995,9 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>>   		if (i != 0)
>>   			set_compound_head(p, &folio->page);
>>   	}
>
> calling set_compound_head() for the tail page before the folio has the 
> head flag set could seem misleading. At this point order is not set as 
> well so it is not clear that the folio is a compound page folio.
>
Yeah, I agree, But they are part of same call. I can avoid moving
__folio_set_head. And I think, It wont mislead if I avoid moving
__folio_set_head. Below function has the similar path.

void prep_compound_page(struct page *page, unsigned int order)
{
	int i;
	int nr_pages = 1 << order;

	__SetPageHead(page);
	for (i = 1; i < nr_pages; i++)
		prep_compound_tail(page, i);

	prep_compound_head(page, order);
}

Lemme know you thoughts.


~Tarun

>> +	__folio_set_head(folio);
>> +	/* we rely on prep_new_hugetlb_folio to set the destructor */
>> +	folio_set_order(folio, order);
>>   	atomic_set(&folio->_entire_mapcount, -1);
>>   	atomic_set(&folio->_nr_pages_mapped, 0);
>>   	atomic_set(&folio->_pincount, 0);
>> @@ -2017,8 +2016,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>>   		p = folio_page(folio, j);
>>   		__ClearPageReserved(p);
>>   	}
>> -	folio_set_order(folio, 0);
>> -	__folio_clear_head(folio);
>>   	return false;
>>   }
>>   
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 18cda26b8a92..0d96a3bc1d58 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -425,16 +425,12 @@ int split_free_page(struct page *free_page,
>>    */
>>   static inline void folio_set_order(struct folio *folio, unsigned int order)
>>   {
>> -	if (WARN_ON_ONCE(!folio_test_large(folio)))
>> +	if (WARN_ON_ONCE(!order || !folio_test_large(folio)))
>>   		return;
>>   
>>   	folio->_folio_order = order;
>>   #ifdef CONFIG_64BIT
>> -	/*
>> -	 * When hugetlb dissolves a folio, we need to clear the tail
>> -	 * page, rather than setting nr_pages to 1.
>> -	 */
>> -	folio->_folio_nr_pages = order ? 1U << order : 0;
>> +	folio->_folio_nr_pages = 1U << order;
>>   #endif
>>   }
>>