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

[Tarun Sahu]

Tarun Sahu <tsahu@xxxxxxxxxxxxx> writes:

> 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.

Apologies, Here, I mixed the sentences, I want to say
It won't mislead if we avoid moving __folio_set_head, but move only
folio_set_order. Below function does the same.
>
> 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
>>>   }
>>>