Re: [PATCH v4 5/7] mm: thp: split huge page to any lower order pages (except order-1).

[Zi Yan <ziy@nvidia.com>]

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On 14 Feb 2024, at 5:38, Ryan Roberts wrote:

> On 13/02/2024 21:55, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> To split a THP to any lower order (except order-1) pages, we need to
>> reform THPs on subpages at given order and add page refcount based on the
>> new page order. Also we need to reinitialize page_deferred_list after
>> removing the page from the split_queue, otherwise a subsequent split will
>> see list corruption when checking the page_deferred_list again.
>>
>> It has many uses, like minimizing the number of pages after
>> truncating a huge pagecache page. For anonymous THPs, we can only split
>> them to order-0 like before until we add support for any size anonymous
>> THPs.
>
> multi-size THP is now upstream. Not sure if this comment still makes sense.
Will change it to reflect the fact that multi-size THP is already upstream.

> Still its not completely clear to me how you would integrate this new machinery
> and decide what non-zero order to split anon THP to?

Originally, it was developed along with my 1GB THP support. So it was intended
to split order-18 to order-9. But for now, like you and David said in the cover
letter email thread, we might not want to use it for anonymous large folios
until we find a necessary use case.

>>
>> Order-1 folio is not supported because _deferred_list, which is used by
>> partially mapped folios, is stored in subpage 2 and an order-1 folio only
>> has subpage 0 and 1.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>  include/linux/huge_mm.h |  21 +++++---
>>  mm/huge_memory.c        | 114 +++++++++++++++++++++++++++++++---------
>>  2 files changed, 101 insertions(+), 34 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 5adb86af35fc..de0c89105076 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -265,10 +265,11 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
>>
>>  void folio_prep_large_rmappable(struct folio *folio);
>>  bool can_split_folio(struct folio *folio, int *pextra_pins);
>> -int split_huge_page_to_list(struct page *page, struct list_head *list);
>> +int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>> +		unsigned int new_order);
>>  static inline int split_huge_page(struct page *page)
>>  {
>> -	return split_huge_page_to_list(page, NULL);
>> +	return split_huge_page_to_list_to_order(page, NULL, 0);
>>  }
>>  void deferred_split_folio(struct folio *folio);
>>
>> @@ -422,7 +423,8 @@ can_split_folio(struct folio *folio, int *pextra_pins)
>>  	return false;
>>  }
>>  static inline int
>> -split_huge_page_to_list(struct page *page, struct list_head *list)
>> +split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>> +		unsigned int new_order)
>>  {
>>  	return 0;
>>  }
>> @@ -519,17 +521,20 @@ static inline bool thp_migration_supported(void)
>>  }
>>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>
>> -static inline int split_folio_to_list(struct folio *folio,
>> -		struct list_head *list)
>> +static inline int split_folio_to_list_to_order(struct folio *folio,
>> +		struct list_head *list, int new_order)
>>  {
>> -	return split_huge_page_to_list(&folio->page, list);
>> +	return split_huge_page_to_list_to_order(&folio->page, list, new_order);
>>  }
>>
>> -static inline int split_folio(struct folio *folio)
>> +static inline int split_folio_to_order(struct folio *folio, int new_order)
>>  {
>> -	return split_folio_to_list(folio, NULL);
>> +	return split_folio_to_list_to_order(folio, NULL, new_order);
>>  }
>>
>> +#define split_folio_to_list(f, l) split_folio_to_list_to_order(f, l, 0)
>> +#define split_folio(f) split_folio_to_order(f, 0)
>> +
>>  /*
>>   * archs that select ARCH_WANTS_THP_SWAP but don't support THP_SWP due to
>>   * limitations in the implementation like arm64 MTE can override this to
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index ad7133c97428..d0e555a8ea98 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2718,11 +2718,14 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
>>
>>  static void unmap_folio(struct folio *folio)
>>  {
>> -	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD |
>> -		TTU_SYNC | TTU_BATCH_FLUSH;
>> +	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SYNC |
>> +		TTU_BATCH_FLUSH;
>>
>>  	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>
>> +	if (folio_test_pmd_mappable(folio))
>> +		ttu_flags |= TTU_SPLIT_HUGE_PMD;
>
> Should we split this change out? I think it makes sense independent of this series?
>

Sure. Since multi-size THP is upstream, this avoid unnecessary code path if
the THP is not PMD-mapped.

>> +
>>  	/*
>>  	 * Anon pages need migration entries to preserve them, but file
>>  	 * pages can simply be left unmapped, then faulted back on demand.
>> @@ -2756,7 +2759,6 @@ static void lru_add_page_tail(struct page *head, struct page *tail,
>>  		struct lruvec *lruvec, struct list_head *list)
>>  {
>>  	VM_BUG_ON_PAGE(!PageHead(head), head);
>> -	VM_BUG_ON_PAGE(PageCompound(tail), head);
>>  	VM_BUG_ON_PAGE(PageLRU(tail), head);
>>  	lockdep_assert_held(&lruvec->lru_lock);
>>
>> @@ -2777,7 +2779,8 @@ static void lru_add_page_tail(struct page *head, struct page *tail,
>>  }
>>
>>  static void __split_huge_page_tail(struct folio *folio, int tail,
>> -		struct lruvec *lruvec, struct list_head *list)
>> +		struct lruvec *lruvec, struct list_head *list,
>> +		unsigned int new_order)
>>  {
>>  	struct page *head = &folio->page;
>>  	struct page *page_tail = head + tail;
>> @@ -2847,10 +2850,15 @@ static void __split_huge_page_tail(struct folio *folio, int tail,
>>  	 * which needs correct compound_head().
>>  	 */
>>  	clear_compound_head(page_tail);
>> +	if (new_order) {
>> +		prep_compound_page(page_tail, new_order);
>> +		folio_prep_large_rmappable(page_folio(page_tail));
>> +	}
>>
>>  	/* Finally unfreeze refcount. Additional reference from page cache. */
>> -	page_ref_unfreeze(page_tail, 1 + (!folio_test_anon(folio) ||
>> -					  folio_test_swapcache(folio)));
>> +	page_ref_unfreeze(page_tail,
>> +		1 + ((!folio_test_anon(folio) || folio_test_swapcache(folio)) ?
>> +			     folio_nr_pages(page_folio(page_tail)) : 0));
>>
>>  	if (folio_test_young(folio))
>>  		folio_set_young(new_folio);
>> @@ -2868,7 +2876,7 @@ static void __split_huge_page_tail(struct folio *folio, int tail,
>>  }
>>
>>  static void __split_huge_page(struct page *page, struct list_head *list,
>> -		pgoff_t end)
>> +		pgoff_t end, unsigned int new_order)
>>  {
>>  	struct folio *folio = page_folio(page);
>>  	struct page *head = &folio->page;
>> @@ -2877,10 +2885,11 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  	unsigned long offset = 0;
>>  	unsigned int nr = thp_nr_pages(head);
>>  	int i, nr_dropped = 0;
>> +	unsigned int new_nr = 1 << new_order;
>>  	int order = folio_order(folio);
>>
>>  	/* complete memcg works before add pages to LRU */
>> -	split_page_memcg(head, order, 0);
>> +	split_page_memcg(head, order, new_order);
>>
>>  	if (folio_test_anon(folio) && folio_test_swapcache(folio)) {
>>  		offset = swp_offset(folio->swap);
>> @@ -2893,8 +2902,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>
>>  	ClearPageHasHWPoisoned(head);
>>
>> -	for (i = nr - 1; i >= 1; i--) {
>> -		__split_huge_page_tail(folio, i, lruvec, list);
>> +	for (i = nr - new_nr; i >= new_nr; i -= new_nr) {
>> +		__split_huge_page_tail(folio, i, lruvec, list, new_order);
>>  		/* Some pages can be beyond EOF: drop them from page cache */
>>  		if (head[i].index >= end) {
>>  			struct folio *tail = page_folio(head + i);
>> @@ -2910,29 +2919,41 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>  			__xa_store(&head->mapping->i_pages, head[i].index,
>>  					head + i, 0);
>>  		} else if (swap_cache) {
>> +			/*
>> +			 * split anonymous THPs (including swapped out ones) to
>> +			 * non-zero order not supported
>> +			 */
>> +			VM_WARN_ONCE(new_order,
>> +				"Split swap-cached anon folio to non-0 order not supported");
>
> Why isn't it supported? Even if it's not supported, is this level the right
> place to enforce these kinds of policy decisions? I wonder if we should be
> leaving that to the higher level to decide?

Is the swap-out small-size THP without splitting merged? This needs that patchset.
You are right that a warning here is not appropriate. I will fail the splitting
if the folio is swapcached and going to be split into >0 order.

>>  			__xa_store(&swap_cache->i_pages, offset + i,
>>  					head + i, 0);
>>  		}
>>  	}
>>


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Best Regards,
Yan, Zi

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