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

[Zi Yan]

On 14 Feb 2024, at 11:22, Ryan Roberts wrote:

> 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@xxxxxxxxxx>
>>>>
>>>> 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@xxxxxxxxxx>
>>>> ---
>>>>  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?

IIUC, swap cache only supports one cluster size, HPAGE_PMD_NR, so splitting
a PMD-size swapcached folio will need to split a cluster to smaller ones, which
needs your patchset support. Let me know if I get it wrong.

>
>> 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


--
Best Regards,
Yan, Zi

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