[PATCH v13 009/137] mm: Add folio_try_get_rcu()

Matthew Wilcox (Oracle)
2021-07-12T03:14:14Z

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) <willy@infradead.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
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 ed02aa522263..db1726b1bc1c 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 d1458ecf2f51..634adeacc4c1 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1746,6 +1746,26 @@ 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
+ * put_folio().
+ */
+
/*
* mapping_get_entry - Get a page cache entry.
* @mapping: the address_space to search
--
2.30.2