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

[Tarun Sahu]

Hi, Mathew,

I am not sure If I was clear about my intention behind the patch.
Here, I attempt to answer it again. Please lemme know if you agree.

Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:

> On Sat, Apr 15, 2023 at 01:18:32AM +0530, 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.
>
> I think you're missing the point of how folio_set_order() is used.
> When splitting a large folio, we need to zero out the folio_nr_pages
> in the tail, so it does have a tail page, and that tail page needs to
> be zeroed.  We even assert that there is a tail page:
>
>         if (WARN_ON_ONCE(!folio_test_large(folio)))
>                 return;
>
> Or maybe you need to explain yourself better.
>

In the upstream, I don't see folio_set_order(folio, 0) being called in
splitting path. IIUC, we had to zero out _folio_nr_pages during freeing
gigantic folio as described by Commit ba9c1201beaa
			 ("mm/hugetlb: clear compound_nr before freeing gigantic pages").

I agree that folio_set_order(folio, 0) is called with folio having
tail pages. But I meant only that, in general, it is just confusing to
have setting the folio order to 0.

With this patch, I would like to draw attention to the point that there
is no need to call folio_set_order(folio, 0) anymore to zero out
_folio_order and _folio_nr_pages.

In past, it was needed because page->mapping used to overlap with
_folio_nr_pages and _folio_order. So if these fields were left
uncleared during freeing gigantic hugepages, they were causing
"BUG: bad page state" due to non-zero page->mapping. Now, After
Commit a01f43901cfb ("hugetlb: be sure to free demoted CMA pages to
CMA") page->mapping has explicitly been cleared out for tail pages.
Also, _folio_order and _folio_nr_pages no longer overlaps with page->mapping.

         struct page {
         ...
                        struct address_space * mapping;  /*    24     8 */
         ...
         }

         struct folio {
         ...
              union {
                 struct {
                        long unsigned int _flags_1;      /*    64     8 */
                        long unsigned int _head_1;       /*    72     8 */
                        unsigned char _folio_dtor;       /*    80     1 */
                        unsigned char _folio_order;      /*    81     1 */

                        /* XXX 2 bytes hole, try to pack */

                        atomic_t   _entire_mapcount;     /*    84     4 */
                        atomic_t   _nr_pages_mapped;     /*    88     4 */
                        atomic_t   _pincount;            /*    92     4 */
                        unsigned int _folio_nr_pages;    /*    96     4 */
                 };                                       /*    64    40 */
                struct page        __page_1 __attribute__((__aligned__(8))); /*    64    64 */
             }
         ...
         }

So, folio_set_order(folio, 0) can be removed from freeing gigantic
folio path (__destroy_compound_gigantic_folio).

Another place, where folio_set_order(folio, 0) is called inside
__prep_compound_gigantic_folio during error path. Here,
folio_set_order(folio, 0) can also be removed if we move
folio_set_order(folio, order) after for loop. Also, Mike confirmed that
it is safe to move the call.

~Tarun

>> folio->_folio_nr_pages is
>> set to 0 for order 0 in folio_set_order. It is required because
>> _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.
>
> ... this is all very confusing.
>
>> 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.
>
> But don't we need those bits set while we operate on the folio to set it
> up?  It makes me nervous if we don't have those bits set because we can
> end up with speculative references that point to a head page while that
> page is not marked as a head page.  It may not be a problem, but I want
> to see some air-tight analysis of that.
>
>> 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);
>>  	}
>> +	__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
>>  }
>>  
>> -- 
>> 2.31.1
>>