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?
> 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
