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Message-ID: <D03D6945-8BFE-4137-BDB6-BD884656B65B@nvidia.com>
Date: Tue, 22 Mar 2022 10:30:14 -0400
From: Zi Yan <ziy@...dia.com>
To: Miaohe Lin <linmiaohe@...wei.com>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>,
Shuah Khan <shuah@...nel.org>, Yang Shi <shy828301@...il.com>,
Hugh Dickins <hughd@...gle.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
linux-kselftest@...r.kernel.org,
Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
Yu Zhao <yuzhao@...gle.com>
Subject: Re: [RFC PATCH 3/5] mm: thp: split huge page to any lower order pages.
On 21 Mar 2022, at 23:21, Miaohe Lin wrote:
> On 2022/3/21 22:21, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.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.
>>
>> It has many uses, like minimizing the number of pages after
>> truncating a pagecache THP. For anonymous THPs, we can only split them
>> to order-0 like before until we add support for any size anonymous THPs.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>> include/linux/huge_mm.h | 8 +++
>> mm/huge_memory.c | 111 ++++++++++++++++++++++++++++++----------
>> 2 files changed, 91 insertions(+), 28 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 2999190adc22..c7153cd7e9e4 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -186,6 +186,8 @@ void free_transhuge_page(struct page *page);
>>
>> 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);
>> @@ -355,6 +357,12 @@ split_huge_page_to_list(struct page *page, struct list_head *list)
>> {
>> return 0;
>> }
>> +static inline int
>> +split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>> + unsigned int new_order)
>> +{
>> + return 0;
>> +}
>> static inline int split_huge_page(struct page *page)
>> {
>> return 0;
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index fcfa46af6c4c..3617aa3ad0b1 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2236,11 +2236,13 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
>> static void unmap_page(struct page *page)
>> {
>> struct folio *folio = page_folio(page);
>> - enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD |
>> - TTU_SYNC;
>> + enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SYNC;
>>
>> VM_BUG_ON_PAGE(!PageHead(page), page);
>>
>> + if (folio_order(folio) >= HPAGE_PMD_ORDER)
>> + ttu_flags |= TTU_SPLIT_HUGE_PMD;
>> +
>> /*
>> * Anon pages need migration entries to preserve them, but file
>> * pages can simply be left unmapped, then faulted back on demand.
>> @@ -2254,9 +2256,9 @@ static void unmap_page(struct page *page)
>> VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
>> }
>>
>> -static void remap_page(struct folio *folio, unsigned long nr)
>> +static void remap_page(struct folio *folio, unsigned short nr)
>> {
>> - int i = 0;
>> + unsigned int i;
>>
>> /* If unmap_page() uses try_to_migrate() on file, remove this check */
>> if (!folio_test_anon(folio))
>> @@ -2274,7 +2276,6 @@ 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);
>>
>> @@ -2295,9 +2296,10 @@ static void lru_add_page_tail(struct page *head, struct page *tail,
>> }
>>
>> static void __split_huge_page_tail(struct page *head, int tail,
>> - struct lruvec *lruvec, struct list_head *list)
>> + struct lruvec *lruvec, struct list_head *list, unsigned int new_order)
>> {
>> struct page *page_tail = head + tail;
>> + unsigned long compound_head_flag = new_order ? (1L << PG_head) : 0;
>>
>> VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail);
>>
>> @@ -2321,6 +2323,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
>> #ifdef CONFIG_64BIT
>> (1L << PG_arch_2) |
>> #endif
>> + compound_head_flag |
>> (1L << PG_dirty)));
>>
>> /* ->mapping in first tail page is compound_mapcount */
>> @@ -2329,7 +2332,10 @@ static void __split_huge_page_tail(struct page *head, int tail,
>> page_tail->mapping = head->mapping;
>> page_tail->index = head->index + tail;
>>
>> - /* Page flags must be visible before we make the page non-compound. */
>> + /*
>> + * Page flags must be visible before we make the page non-compound or
>> + * a compound page in new_order.
>> + */
>> smp_wmb();
>>
>> /*
>> @@ -2339,10 +2345,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
>> * which needs correct compound_head().
>> */
>> clear_compound_head(page_tail);
>> + if (new_order) {
>> + prep_compound_page(page_tail, new_order);
>> + prep_transhuge_page(page_tail);
>> + }
>
> Many thanks for your series. It looks really good. One question:
> IIUC, It seems there has assumption that LRU compound_pages should
> be PageTransHuge. So PageTransHuge just checks PageHead:
>
> static inline int PageTransHuge(struct page *page)
> {
> VM_BUG_ON_PAGE(PageTail(page), page);
> return PageHead(page);
> }
>
> So LRU pages with any order( > 0) will might be wrongly treated as THP which
> has order = HPAGE_PMD_ORDER. We should ensure thp_nr_pages is used instead of
> hard coded HPAGE_PMD_ORDER.
>
> Looks at the below code snippet:
> mm/mempolicy.c:
> static struct page *new_page(struct page *page, unsigned long start)
> {
> ...
> } else if (PageTransHuge(page)) {
> struct page *thp;
>
> thp = alloc_hugepage_vma(GFP_TRANSHUGE, vma, address,
> HPAGE_PMD_ORDER);
> ^^^^^^^^^^^^^^^^
> if (!thp)
> return NULL;
> prep_transhuge_page(thp);
> return thp;
> }
> ...
> }
>
> HPAGE_PMD_ORDER is used instead of thp_nr_pages. So the lower order pages might be
> used as if its order is HPAGE_PMD_ORDER. All of such usage might need to be fixed.
> Or am I miss something ?
>
> Thanks again for your work. :)
THP will still only have HPAGE_PMD_ORDER and will not be split into any order
other than 0. This series only allows to split huge page cache folio (added by Matthew)
into any lower order. I have an explicit VM_BUG_ON() to ensure new_order
is only 0 when non page cache page is the input. Since there is still non-trivial
amount of work to add any order THP support in the kernel. IIRC, Yu Zhao (cc’d) was
planning to work on that.
Thanks for checking the patches.
>>
>> /* Finally unfreeze refcount. Additional reference from page cache. */
>> - page_ref_unfreeze(page_tail, 1 + (!PageAnon(head) ||
>> - PageSwapCache(head)));
>> + page_ref_unfreeze(page_tail, 1 + ((!PageAnon(head) ||
>> + PageSwapCache(head)) ?
>> + thp_nr_pages(page_tail) : 0));
>>
>> if (page_is_young(head))
>> set_page_young(page_tail);
>> @@ -2360,7 +2371,7 @@ static void __split_huge_page_tail(struct page *head, 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;
>> @@ -2369,10 +2380,11 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>> unsigned long offset = 0;
>> unsigned int order = thp_order(head);
>> unsigned int nr = thp_nr_pages(head);
>> + unsigned int new_nr = 1 << new_order;
>> int i;
>>
>> /* complete memcg works before add pages to LRU */
>> - split_page_memcg(head, nr, 0);
>> + split_page_memcg(head, nr, new_order);
>>
>> if (PageAnon(head) && PageSwapCache(head)) {
>> swp_entry_t entry = { .val = page_private(head) };
>> @@ -2387,42 +2399,50 @@ 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(head, i, lruvec, list);
>> + for (i = nr - new_nr; i >= new_nr; i -= new_nr) {
>> + __split_huge_page_tail(head, i, lruvec, list, new_order);
>> /* Some pages can be beyond EOF: drop them from page cache */
>> if (head[i].index >= end) {
>> ClearPageDirty(head + i);
>> __delete_from_page_cache(head + i, NULL);
>> if (shmem_mapping(head->mapping))
>> - shmem_uncharge(head->mapping->host, 1);
>> + shmem_uncharge(head->mapping->host, new_nr);
>> put_page(head + i);
>> } else if (!PageAnon(page)) {
>> __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_BUG_ON(new_order);
>> __xa_store(&swap_cache->i_pages, offset + i,
>> head + i, 0);
>> }
>> }
>>
>> - ClearPageCompound(head);
>> + if (!new_order)
>> + ClearPageCompound(head);
>> + else
>> + set_compound_order(head, 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();
>> @@ -2435,7 +2455,14 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>> split_swap_cluster(entry);
>> }
>>
>> - for (i = 0; i < nr; i++) {
>> + /*
>> + * set page to its compound_head when split to THPs, so that GUP pin and
>> + * PG_locked are transferred to the right after-split page
>> + */
>> + if (new_order)
>> + page = compound_head(page);
>> +
>> + for (i = 0; i < nr; i += new_nr) {
>> struct page *subpage = head + i;
>> if (subpage == page)
>> continue;
>> @@ -2472,36 +2499,60 @@ 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.
>> *
>> + * See split_huge_page_to_list_to_order() for more details.
>> + *
>> + * 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)
>> +{
>> + return split_huge_page_to_list_to_order(page, list, 0);
>> +}
>> +
>> +/*
>> + * 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.
>> + *
>> * 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 page *head = &folio->page;
>> struct deferred_split *ds_queue = get_deferred_split_queue(head);
>> - XA_STATE(xas, &head->mapping->i_pages, head->index);
>> + /* reset xarray order to new order after split */
>> + XA_STATE_ORDER(xas, &head->mapping->i_pages, head->index, new_order);
>> struct anon_vma *anon_vma = NULL;
>> struct address_space *mapping = NULL;
>> int extra_pins, ret;
>> pgoff_t end;
>>
>> + VM_BUG_ON(thp_order(head) <= new_order);
>> VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
>> VM_BUG_ON_PAGE(!PageLocked(head), head);
>> VM_BUG_ON_PAGE(!PageCompound(head), head);
>>
>> + /* Cannot split THP to order-1 (no order-1 THPs) */
>> + VM_BUG_ON(new_order == 1);
>> +
>> + /* Split anonymous THP to non-zero order not support */
>> + VM_BUG_ON(PageAnon(head) && new_order);
>> +
>> if (PageWriteback(head))
>> return -EBUSY;
>>
>> @@ -2582,7 +2633,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>> if (page_ref_freeze(head, 1 + extra_pins)) {
>> if (!list_empty(page_deferred_list(head))) {
>> ds_queue->split_queue_len--;
>> - list_del(page_deferred_list(head));
>> + list_del_init(page_deferred_list(head));
>> }
>> spin_unlock(&ds_queue->split_queue_lock);
>> if (mapping) {
>> @@ -2592,14 +2643,18 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>> if (PageSwapBacked(head)) {
>> __mod_lruvec_page_state(head, NR_SHMEM_THPS,
>> -nr);
>> - } else {
>> + } else if (!new_order) {
>> + /*
>> + * Decrease THP stats only if split to normal
>> + * pages
>> + */
>> __mod_lruvec_page_state(head, NR_FILE_THPS,
>> -nr);
>> filemap_nr_thps_dec(mapping);
>> }
>> }
>>
>> - __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
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