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Date: Wed, 23 Mar 2022 18:10:30 -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 22 Mar 2022, at 22:31, Miaohe Lin wrote:
> On 2022/3/22 22:30, Zi Yan wrote:
>> 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.
>>
>
> Many thanks for clarifying. I'm sorry but I haven't followed Matthew's patches. I am
> wondering could huge page cache folio be treated as THP ? If so, how to ensure the
> correctness of huge page cache ?
You are right. All these HPAGE_PMD_ORDRE locations should be replaced by thp_nr_pages().
I will look into it.
Thanks a lot.
--
Best Regards,
Yan, Zi
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