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Date: Fri, 13 Nov 2020 16:52:49 -0800
From: Roman Gushchin <guro@...com>
To: Zi Yan <ziy@...dia.com>
CC: <linux-mm@...ck.org>, Matthew Wilcox <willy@...radead.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Andrew Morton <akpm@...ux-foundation.org>,
<linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
Yang Shi <shy828301@...il.com>,
Michal Hocko <mhocko@...nel.org>,
John Hubbard <jhubbard@...dia.com>,
Ralph Campbell <rcampbell@...dia.com>,
David Nellans <dnellans@...dia.com>
Subject: Re: [RFC PATCH 4/6] mm: thp: add support for split huge page to any
lower order pages.
On Wed, Nov 11, 2020 at 03:40:06PM -0500, 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 | 78 +++++++++++++++++++++++++++++------------
> mm/swap.c | 1 -
> 3 files changed, 63 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 60a907a19f7d..9819cd9b4619 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -189,6 +189,8 @@ bool is_transparent_hugepage(struct page *page);
>
> bool can_split_huge_page(struct page *page, 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);
> @@ -396,6 +398,12 @@ split_huge_page_to_list(struct page *page, struct list_head *list)
> {
> return 0;
> }
> +static inline int
> +split_huge_page_to_order_to_list(struct page *page, struct list_head *list,
> + unsigned int new_order)
It was
int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
unsigned int new_order);
above.
> +{
> + 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 8b7d771ee962..88f50da40c9b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2327,11 +2327,14 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
> static void unmap_page(struct page *page)
> {
> enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
> - TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> + TTU_RMAP_LOCKED;
> bool unmap_success;
>
> VM_BUG_ON_PAGE(!PageHead(page), page);
>
> + if (thp_order(page) >= HPAGE_PMD_ORDER)
> + ttu_flags |= TTU_SPLIT_HUGE_PMD;
> +
> if (PageAnon(page))
> ttu_flags |= TTU_SPLIT_FREEZE;
>
> @@ -2339,21 +2342,22 @@ static void unmap_page(struct page *page)
> VM_BUG_ON_PAGE(!unmap_success, page);
> }
>
> -static void remap_page(struct page *page, unsigned int nr)
> +static void remap_page(struct page *page, unsigned int nr, unsigned int new_nr)
> {
> int i;
> - if (PageTransHuge(page)) {
> + if (thp_nr_pages(page) == nr) {
> remove_migration_ptes(page, page, true);
> } else {
> - for (i = 0; i < nr; i++)
> + for (i = 0; i < nr; i += new_nr)
> remove_migration_ptes(page + i, page + i, true);
> }
> }
>
> 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);
>
> @@ -2377,6 +2381,7 @@ 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 */
> @@ -2395,10 +2400,15 @@ 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);
> + thp_prep(page_tail);
> + }
>
> /* 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);
> @@ -2416,7 +2426,7 @@ 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, unsigned long flags)
> + pgoff_t end, unsigned long flags, unsigned int new_order)
> {
> struct page *head = compound_head(page);
> pg_data_t *pgdat = page_pgdat(head);
> @@ -2424,12 +2434,13 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> struct address_space *swap_cache = NULL;
> unsigned long offset = 0;
> unsigned int nr = thp_nr_pages(head);
> + unsigned int new_nr = 1 << new_order;
> int i;
>
> lruvec = mem_cgroup_page_lruvec(head, pgdat);
>
> /* complete memcg works before add pages to LRU */
> - mem_cgroup_split_huge_fixup(head, 1);
> + mem_cgroup_split_huge_fixup(head, new_nr);
>
> if (PageAnon(head) && PageSwapCache(head)) {
> swp_entry_t entry = { .val = page_private(head) };
> @@ -2439,14 +2450,14 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> xa_lock(&swap_cache->i_pages);
> }
>
> - 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 i_size: drop them from page cache */
> if (head[i].index >= end) {
> ClearPageDirty(head + i);
> __delete_from_page_cache(head + i, NULL);
> if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
> - 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,
> @@ -2457,28 +2468,31 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> }
> }
>
> - ClearPageCompound(head);
> + if (!new_order)
> + ClearPageCompound(head);
> + else
> + set_compound_order(head, new_order);
>
> - split_page_owner(head, nr, 1);
> + split_page_owner(head, nr, new_nr);
>
> /* 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);
> }
>
> spin_unlock_irqrestore(&pgdat->lru_lock, flags);
>
> - remap_page(head, nr);
> + remap_page(head, nr, new_nr);
>
> if (PageSwapCache(head)) {
> swp_entry_t entry = { .val = page_private(head) };
> @@ -2486,7 +2500,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> split_swap_cluster(entry);
> }
>
> - for (i = 0; i < nr; i++) {
> + for (i = 0; i < nr; i += new_nr) {
> struct page *subpage = head + i;
> if (subpage == page)
> continue;
> @@ -2620,21 +2634,39 @@ bool can_split_huge_page(struct page *page, int *pextra_pins)
> * 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);
> +}
> +
> +int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> + unsigned int new_order)
> {
> struct page *head = compound_head(page);
> struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
> 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 count, mapcount, extra_pins, ret;
> unsigned long flags;
> 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);
>
> + if (new_order == 1) {
> + WARN_ONCE(1, "Cannot split THP to order-1 (no order-1 THPs)");
> + return -EINVAL;
> + }
> +
> + if (PageAnon(head) && new_order) {
> + WARN_ONCE(1, "Split anonymous THP to non-zero order not support");
> + return -EINVAL;
> + }
I'd convert those into VM_BUG_ON()'s. Unlikely they will be hit at arbitrary moments
by random users.
Thanks!
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