Re: [PATCH v5 7/8] mm: thp: split huge page to any lower order pages

Ryan Roberts
2024-02-28T15:44:53Z

On 28/02/2024 15:42, Zi Yan wrote:
> On 28 Feb 2024, at 3:23, Ryan Roberts wrote:
>
>> Hi Zi,
>>
>>
>> On 26/02/2024 20:55, Zi Yan wrote:
>>> From: Zi Yan <ziy@nvidia.com>
>>>
>>> To split a THP to any lower order 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.
>>>
>>> Note: Anonymous 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. File-backed order-1 folios are
>>> fine, since they do not use _deferred_list.
>>>
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> ---
>>> include/linux/huge_mm.h | 21 +++++---
>>> mm/huge_memory.c | 110 +++++++++++++++++++++++++++++++---------
>>> 2 files changed, 99 insertions(+), 32 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 b2df788c11fa..8b47a96a28f9 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2770,7 +2770,6 @@ 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);
>>>
>>> @@ -2791,7 +2790,8 @@ 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;
>>> @@ -2861,10 +2861,15 @@ 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(new_folio);
>>> + }
>>>
>>> /* 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(new_folio) : 0));
>>>
>>> if (folio_test_young(folio))
>>> folio_set_young(new_folio);
>>> @@ -2882,7 +2887,7 @@ 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;
>>> @@ -2890,11 +2895,12 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>> struct address_space *swap_cache = NULL;
>>> unsigned long offset = 0;
>>> int i, nr_dropped = 0;
>>> + unsigned int new_nr = 1 << new_order;
>>> int order = folio_order(folio);
>>> unsigned int nr = 1 << order;
>>>
>>> /* 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);
>>> @@ -2907,8 +2913,8 @@ 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);
>>> @@ -2929,24 +2935,30 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>> }
>>> }
>>>
>>> - ClearPageCompound(head);
>>> + if (!new_order)
>>> + ClearPageCompound(head);
>>> + else {
>>> + struct folio *new_folio = (struct folio *)head;
>>> +
>>> + folio_set_order(new_folio, new_order);
>>> + }
>>> unlock_page_lruvec(lruvec);
>>> /* Caller disabled irqs, so they are still disabled here */
>>>
>>> - split_page_owner(head, order, 0);
>>> + split_page_owner(head, order, new_order);
>>>
>>> /* See comment in __split_huge_page_tail() */
>>> if (PageAnon(head)) {
>>> /* Additional pin to swap cache */
>>> if (PageSwapCache(head)) {
>>> - page_ref_add(head, 2);
>>> + page_ref_add(head, 1 + new_nr);
>>> xa_unlock(&swap_cache->i_pages);
>>> } else {
>>> page_ref_inc(head);
>>> }
>>> } else {
>>> /* Additional pin to page cache */
>>> - page_ref_add(head, 2);
>>> + page_ref_add(head, 1 + new_nr);
>>> xa_unlock(&head->mapping->i_pages);
>>> }
>>> local_irq_enable();
>>> @@ -2958,7 +2970,15 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>> if (folio_test_swapcache(folio))
>>> split_swap_cluster(folio->swap);
>>>
>>> - for (i = 0; i < nr; i++) {
>>> + /*
>>> + * set page to its compound_head when split to non order-0 pages, so
>>> + * we can skip unlocking it below, since PG_locked is transferred to
>>> + * the compound_head of the page and the caller will unlock it.
>>> + */
>>> + if (new_order)
>>> + page = compound_head(page);
>>> +
>>> + for (i = 0; i < nr; i += new_nr) {
>>> struct page *subpage = head + i;
>>> if (subpage == page)
>>> continue;
>>> @@ -2992,29 +3012,36 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
>>> }
>>>
>>> /*
>>> - * This function splits huge page into normal pages. @page can point to any
>>> - * subpage of huge page to split. Split doesn't change the position of @page.
>>> + * This function splits huge page into pages in @new_order. @page can point to
>>> + * any subpage of huge page to split. Split doesn't change the position of
>>> + * @page.
>>> + *
>>> + * NOTE: order-1 anonymous 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. File-backed order-1 folios are OK,
>>> + * since they do not use _deferred_list.
>>> *
>>> * Only caller must hold pin on the @page, otherwise split fails with -EBUSY.
>>> * The huge page must be locked.
>>> *
>>> * If @list is null, tail pages will be added to LRU list, otherwise, to @list.
>>> *
>>> - * Both head page and tail pages will inherit mapping, flags, and so on from
>>> - * the hugepage.
>>> + * Pages in new_order will inherit mapping, flags, and so on from the hugepage.
>>> *
>>> - * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if
>>> - * they are not mapped.
>>> + * GUP pin and PG_locked transferred to @page or the compound page @page belongs
>>> + * to. Rest subpages can be freed if they are not mapped.
>>> *
>>> * Returns 0 if the hugepage is split successfully.
>>> * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
>>> * us.
>>> */
>>> -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)
>>> {
>>> struct folio *folio = page_folio(page);
>>> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>> - XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>>> + /* reset xarray order to new order after split */
>>> + XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
>>> struct anon_vma *anon_vma = NULL;
>>> struct address_space *mapping = NULL;
>>> int extra_pins, ret;
>>> @@ -3024,6 +3051,34 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>> VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>>> VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>>
>>> + /* Cannot split anonymous THP to order-1 */
>>> + if (new_order == 1 && folio_test_anon(folio)) {
>>> + VM_WARN_ONCE(1, "Cannot split to order-1 folio");
>>> + return -EINVAL;
>>> + }
>>> +
>>> + if (new_order) {
>>> + /* Only swapping a whole PMD-mapped folio is supported */
>>> + if (folio_test_swapcache(folio)) {
>>> + VM_WARN_ONCE(1,
>>> + "Cannot split swap-cached folio to non-0 order");
>>
>> My understanding may be wrong here, but can't the folio be moved to swapcache
>> asynchronously? How does the caller guarrantee that the folio is not in
>> swapcache and will not be moved between the call to
>> split_huge_page_to_list_to_order() and this test? If the caller can't prevent
>> it, then isn't it wrong to raise a warning here? Perhaps you just have to fail
>> to split?
>
> Right. That is why I only use VM_WARN_ONCE here. You mean it is better to
> get rid of the warning. I have no strong preference about it.

Yes; I don't think we should be issuing warnings when the caller has done
nothing wrong?

>
>>
>> I'm guessing this restriction is because swap only supports order-0 and
>> pmd-order folios currently? (And you only have split_swap_cluster() to downgrade
>> from pmd-order to order-0). Perhaps you need my series that allows swapping out
>> any order THP? Current version at [1] but I'm working on a new version.
>>
>> [1] https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@arm.com/
>
> Right. Once your patchset is in, the above check can be removed.
>
>>> + return -EINVAL;
>>> + }
>>> + /* Split shmem folio to non-zero order not supported */
>>> + if (shmem_mapping(folio->mapping)) {
>>> + VM_WARN_ONCE(1,
>>> + "Cannot split shmem folio to non-0 order");
>>> + return -EINVAL;
>>> + }
>>> + /* No split if the file system does not support large folio */
>>> + if (!mapping_large_folio_support(folio->mapping)) {
>>> + VM_WARN_ONCE(1,
>>> + "Cannot split file folio to non-0 order");
>>> + return -EINVAL;
>>> + }
>>> + }
>>> +
>>> +
>>> is_hzp = is_huge_zero_page(&folio->page);
>>> if (is_hzp) {
>>> pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
>>> @@ -3120,14 +3175,21 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>> if (folio_order(folio) > 1 &&
>>> !list_empty(&folio->_deferred_list)) {
>>> ds_queue->split_queue_len--;
>>> - list_del(&folio->_deferred_list);
>>> + /*
>>> + * 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.
>>> + */
>>> + list_del_init(&folio->_deferred_list);
>>> }
>>> spin_unlock(&ds_queue->split_queue_lock);
>>> if (mapping) {
>>> int nr = folio_nr_pages(folio);
>>>
>>> xas_split(&xas, folio, folio_order(folio));
>>> - if (folio_test_pmd_mappable(folio)) {
>>> + if (folio_test_pmd_mappable(folio) &&
>>> + new_order < HPAGE_PMD_ORDER) {
>>> if (folio_test_swapbacked(folio)) {
>>> __lruvec_stat_mod_folio(folio,
>>> NR_SHMEM_THPS, -nr);
>>> @@ -3139,7 +3201,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>> }
>>> }
>>>
>>> - __split_huge_page(page, list, end);
>>> + __split_huge_page(page, list, end, new_order);
>>> ret = 0;
>>> } else {
>>> spin_unlock(&ds_queue->split_queue_lock);
>
>
> --
> Best Regards,
> Yan, Zi