From 020853b6f5ead2849b055e9873ff7267ce584256 Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" Date: Mon, 10 May 2021 16:33:22 -0400 Subject: [PATCH] mm: Add folio_try_get_rcu() This is the equivalent of page_cache_get_speculative(). Also add folio_ref_try_add_rcu (the equivalent of page_cache_add_speculative) and folio_get_unless_zero() (the equivalent of get_page_unless_zero()). The new kernel-doc attempts to explain from the user's point of view when to use folio_try_get_rcu() and when to use folio_get_unless_zero(), because there seems to be some confusion currently between the users of page_cache_get_speculative() and get_page_unless_zero(). Reimplement page_cache_add_speculative() and page_cache_get_speculative() as wrappers around the folio equivalents, but leave get_page_unless_zero() alone for now. This commit reduces text size by 3 bytes due to slightly different register allocation & instruction selections. Signed-off-by: Matthew Wilcox (Oracle) Acked-by: Vlastimil Babka Reviewed-by: William Kucharski Reviewed-by: Christoph Hellwig Acked-by: Kirill A. Shutemov Acked-by: Mike Rapoport --- include/linux/page_ref.h | 66 +++++++++++++++++++++++++++++++++++++ include/linux/pagemap.h | 84 +++--------------------------------------------- mm/filemap.c | 20 ++++++++++++ 3 files changed, 90 insertions(+), 80 deletions(-) diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h index 717d53c9ddf1..2e677e6ad09f 100644 --- a/include/linux/page_ref.h +++ b/include/linux/page_ref.h @@ -247,6 +247,72 @@ static inline bool folio_ref_add_unless(struct folio *folio, int nr, int u) return page_ref_add_unless(&folio->page, nr, u); } +/** + * folio_try_get - Attempt to increase the refcount on a folio. + * @folio: The folio. + * + * If you do not already have a reference to a folio, you can attempt to + * get one using this function. It may fail if, for example, the folio + * has been freed since you found a pointer to it, or it is frozen for + * the purposes of splitting or migration. + * + * Return: True if the reference count was successfully incremented. + */ +static inline bool folio_try_get(struct folio *folio) +{ + return folio_ref_add_unless(folio, 1, 0); +} + +static inline bool folio_ref_try_add_rcu(struct folio *folio, int count) +{ +#ifdef CONFIG_TINY_RCU + /* + * The caller guarantees the folio will not be freed from interrupt + * context, so (on !SMP) we only need preemption to be disabled + * and TINY_RCU does that for us. + */ +# ifdef CONFIG_PREEMPT_COUNT + VM_BUG_ON(!in_atomic() && !irqs_disabled()); +# endif + VM_BUG_ON_FOLIO(folio_ref_count(folio) == 0, folio); + folio_ref_add(folio, count); +#else + if (unlikely(!folio_ref_add_unless(folio, count, 0))) { + /* Either the folio has been freed, or will be freed. */ + return false; + } +#endif + return true; +} + +/** + * folio_try_get_rcu - Attempt to increase the refcount on a folio. + * @folio: The folio. + * + * This is a version of folio_try_get() optimised for non-SMP kernels. + * If you are still holding the rcu_read_lock() after looking up the + * page and know that the page cannot have its refcount decreased to + * zero in interrupt context, you can use this instead of folio_try_get(). + * + * Example users include get_user_pages_fast() (as pages are not unmapped + * from interrupt context) and the page cache lookups (as pages are not + * truncated from interrupt context). We also know that pages are not + * frozen in interrupt context for the purposes of splitting or migration. + * + * You can also use this function if you're holding a lock that prevents + * pages being frozen & removed; eg the i_pages lock for the page cache + * or the mmap_sem or page table lock for page tables. In this case, + * it will always succeed, and you could have used a plain folio_get(), + * but it's sometimes more convenient to have a common function called + * from both locked and RCU-protected contexts. + * + * Return: True if the reference count was successfully incremented. + */ +static inline bool folio_try_get_rcu(struct folio *folio) +{ + return folio_ref_try_add_rcu(folio, 1); +} + static inline int page_ref_freeze(struct page *page, int count) { int ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count); diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 62db6b0176b9..f3ec26551082 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -172,91 +172,15 @@ static inline struct address_space *page_mapping_file(struct page *page) return page_mapping(page); } -/* - * speculatively take a reference to a page. - * If the page is free (_refcount == 0), then _refcount is untouched, and 0 - * is returned. Otherwise, _refcount is incremented by 1 and 1 is returned. - * - * This function must be called inside the same rcu_read_lock() section as has - * been used to lookup the page in the pagecache radix-tree (or page table): - * this allows allocators to use a synchronize_rcu() to stabilize _refcount. - * - * Unless an RCU grace period has passed, the count of all pages coming out - * of the allocator must be considered unstable. page_count may return higher - * than expected, and put_page must be able to do the right thing when the - * page has been finished with, no matter what it is subsequently allocated - * for (because put_page is what is used here to drop an invalid speculative - * reference). - * - * This is the interesting part of the lockless pagecache (and lockless - * get_user_pages) locking protocol, where the lookup-side (eg. find_get_page) - * has the following pattern: - * 1. find page in radix tree - * 2. conditionally increment refcount - * 3. check the page is still in pagecache (if no, goto 1) - * - * Remove-side that cares about stability of _refcount (eg. reclaim) has the - * following (with the i_pages lock held): - * A. atomically check refcount is correct and set it to 0 (atomic_cmpxchg) - * B. remove page from pagecache - * C. free the page - * - * There are 2 critical interleavings that matter: - * - 2 runs before A: in this case, A sees elevated refcount and bails out - * - A runs before 2: in this case, 2 sees zero refcount and retries; - * subsequently, B will complete and 1 will find no page, causing the - * lookup to return NULL. - * - * It is possible that between 1 and 2, the page is removed then the exact same - * page is inserted into the same position in pagecache. That's OK: the - * old find_get_page using a lock could equally have run before or after - * such a re-insertion, depending on order that locks are granted. - * - * Lookups racing against pagecache insertion isn't a big problem: either 1 - * will find the page or it will not. Likewise, the old find_get_page could run - * either before the insertion or afterwards, depending on timing. - */ -static inline int __page_cache_add_speculative(struct page *page, int count) +static inline bool page_cache_add_speculative(struct page *page, int count) { -#ifdef CONFIG_TINY_RCU -# ifdef CONFIG_PREEMPT_COUNT - VM_BUG_ON(!in_atomic() && !irqs_disabled()); -# endif - /* - * Preempt must be disabled here - we rely on rcu_read_lock doing - * this for us. - * - * Pagecache won't be truncated from interrupt context, so if we have - * found a page in the radix tree here, we have pinned its refcount by - * disabling preempt, and hence no need for the "speculative get" that - * SMP requires. - */ - VM_BUG_ON_PAGE(page_count(page) == 0, page); - page_ref_add(page, count); - -#else - if (unlikely(!page_ref_add_unless(page, count, 0))) { - /* - * Either the page has been freed, or will be freed. - * In either case, retry here and the caller should - * do the right thing (see comments above). - */ - return 0; - } -#endif VM_BUG_ON_PAGE(PageTail(page), page); - - return 1; -} - -static inline int page_cache_get_speculative(struct page *page) -{ - return __page_cache_add_speculative(page, 1); + return folio_ref_try_add_rcu((struct folio *)page, count); } -static inline int page_cache_add_speculative(struct page *page, int count) +static inline bool page_cache_get_speculative(struct page *page) { - return __page_cache_add_speculative(page, count); + return page_cache_add_speculative(page, 1); } /** diff --git a/mm/filemap.c b/mm/filemap.c index dae481293b5d..9fcc3d94cfcd 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1802,6 +1802,26 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping, EXPORT_SYMBOL(page_cache_prev_miss); /* + * Lockless page cache protocol: + * On the lookup side: + * 1. Load the folio from i_pages + * 2. Increment the refcount if it's not zero + * 3. If the folio is not found by xas_reload(), put the refcount and retry + * + * On the removal side: + * A. Freeze the page (by zeroing the refcount if nobody else has a reference) + * B. Remove the page from i_pages + * C. Return the page to the page allocator + * + * This means that any page may have its reference count temporarily + * increased by a speculative page cache (or fast GUP) lookup as it can + * be allocated by another user before the RCU grace period expires. + * Because the refcount temporarily acquired here may end up being the + * last refcount on the page, any page allocation must be freeable by + * folio_put(). + */ + +/* * mapping_get_entry - Get a page cache entry. * @mapping: the address_space to search * @index: The page cache index. -- 2.11.0