Date:   Mon, 21 Aug 2023 15:22:07 +0100
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     Ingo Molnar <mingo@...hat.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        André Almeida <andrealmeid@...lia.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] futex: Use a folio instead of a page

The futex code already handles compound pages correctly, but using a
folio lets us tell the compiler that we already have the head page and
it doesn't need to call compound_head() again.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 kernel/futex/core.c | 67 ++++++++++++++++++++++-----------------------
 1 file changed, 33 insertions(+), 34 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index f10587d1d481..d1d7b3c175a4 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -222,7 +222,8 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 {
 	unsigned long address = (unsigned long)uaddr;
 	struct mm_struct *mm = current->mm;
-	struct page *page, *tail;
+	struct page *page;
+	struct folio *folio;
 	struct address_space *mapping;
 	int err, ro = 0;
 
@@ -273,54 +274,52 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 		err = 0;
 
 	/*
-	 * The treatment of mapping from this point on is critical. The page
-	 * lock protects many things but in this context the page lock
+	 * The treatment of mapping from this point on is critical. The folio
+	 * lock protects many things but in this context the folio lock
 	 * stabilizes mapping, prevents inode freeing in the shared
 	 * file-backed region case and guards against movement to swap cache.
 	 *
-	 * Strictly speaking the page lock is not needed in all cases being
-	 * considered here and page lock forces unnecessarily serialization
+	 * Strictly speaking the folio lock is not needed in all cases being
+	 * considered here and folio lock forces unnecessarily serialization.
 	 * From this point on, mapping will be re-verified if necessary and
-	 * page lock will be acquired only if it is unavoidable
+	 * folio lock will be acquired only if it is unavoidable
 	 *
-	 * Mapping checks require the head page for any compound page so the
-	 * head page and mapping is looked up now. For anonymous pages, it
-	 * does not matter if the page splits in the future as the key is
-	 * based on the address. For filesystem-backed pages, the tail is
-	 * required as the index of the page determines the key. For
-	 * base pages, there is no tail page and tail == page.
+	 * Mapping checks require the folio so it is looked up now. For
+	 * anonymous pages, it does not matter if the folio is split
+	 * in the future as the key is based on the address. For
+	 * filesystem-backed pages, the precise page is required as the
+	 * index of the page determines the key.
 	 */
-	tail = page;
-	page = compound_head(page);
-	mapping = READ_ONCE(page->mapping);
+	folio = page_folio(page);
+	mapping = READ_ONCE(folio->mapping);
 
 	/*
-	 * If page->mapping is NULL, then it cannot be a PageAnon
+	 * If folio->mapping is NULL, then it cannot be an anonymous
 	 * page; but it might be the ZERO_PAGE or in the gate area or
 	 * in a special mapping (all cases which we are happy to fail);
 	 * or it may have been a good file page when get_user_pages_fast
 	 * found it, but truncated or holepunched or subjected to
-	 * invalidate_complete_page2 before we got the page lock (also
+	 * invalidate_complete_page2 before we got the folio lock (also
 	 * cases which we are happy to fail).  And we hold a reference,
 	 * so refcount care in invalidate_inode_page's remove_mapping
 	 * prevents drop_caches from setting mapping to NULL beneath us.
 	 *
 	 * The case we do have to guard against is when memory pressure made
 	 * shmem_writepage move it from filecache to swapcache beneath us:
-	 * an unlikely race, but we do need to retry for page->mapping.
+	 * an unlikely race, but we do need to retry for folio->mapping.
 	 */
 	if (unlikely(!mapping)) {
 		int shmem_swizzled;
 
 		/*
-		 * Page lock is required to identify which special case above
-		 * applies. If this is really a shmem page then the page lock
+		 * Folio lock is required to identify which special case above
+		 * applies. If this is really a shmem page then the folio lock
 		 * will prevent unexpected transitions.
 		 */
-		lock_page(page);
-		shmem_swizzled = PageSwapCache(page) || page->mapping;
-		unlock_page(page);
-		put_page(page);
+		folio_lock(folio);
+		shmem_swizzled = folio_test_swapcache(folio) || folio->mapping;
+		folio_unlock(folio);
+		folio_put(folio);
 
 		if (shmem_swizzled)
 			goto again;
@@ -331,14 +330,14 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 	/*
 	 * Private mappings are handled in a simple way.
 	 *
-	 * If the futex key is stored on an anonymous page, then the associated
+	 * If the futex key is stored in anonymous memory, then the associated
 	 * object is the mm which is implicitly pinned by the calling process.
 	 *
 	 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
 	 * it's a read-only handle, it's expected that futexes attach to
 	 * the object not the particular process.
 	 */
-	if (PageAnon(page)) {
+	if (folio_test_anon(folio)) {
 		/*
 		 * A RO anonymous page will never change and thus doesn't make
 		 * sense for futex operations.
@@ -357,10 +356,10 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 
 		/*
 		 * The associated futex object in this case is the inode and
-		 * the page->mapping must be traversed. Ordinarily this should
-		 * be stabilised under page lock but it's not strictly
+		 * the folio->mapping must be traversed. Ordinarily this should
+		 * be stabilised under folio lock but it's not strictly
 		 * necessary in this case as we just want to pin the inode, not
-		 * update the radix tree or anything like that.
+		 * update i_pages or anything like that.
 		 *
 		 * The RCU read lock is taken as the inode is finally freed
 		 * under RCU. If the mapping still matches expectations then the
@@ -368,9 +367,9 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 		 */
 		rcu_read_lock();
 
-		if (READ_ONCE(page->mapping) != mapping) {
+		if (READ_ONCE(folio->mapping) != mapping) {
 			rcu_read_unlock();
-			put_page(page);
+			folio_put(folio);
 
 			goto again;
 		}
@@ -378,19 +377,19 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 		inode = READ_ONCE(mapping->host);
 		if (!inode) {
 			rcu_read_unlock();
-			put_page(page);
+			folio_put(folio);
 
 			goto again;
 		}
 
 		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
 		key->shared.i_seq = get_inode_sequence_number(inode);
-		key->shared.pgoff = page_to_pgoff(tail);
+		key->shared.pgoff = folio->index + folio_page_idx(folio, page);
 		rcu_read_unlock();
 	}
 
 out:
-	put_page(page);
+	folio_put(folio);
 	return err;
 }
 
-- 
2.40.1

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