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

[Ryan Roberts <ryan.roberts@arm.com>]

On 14/02/2024 16:11, Zi Yan wrote:
> 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.

No not yet. I have to respin it. Its on my todo list.

I'm not sure I understand the dependency though?

> 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);
>>>  		}
>>>  	}
>>>
> 
> 
> --
> Best Regards,
> Yan, Zi