linux-kernel - [PATCH v14 017/138] mm/filemap: Add folio

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Message-Id: <20210715033704.692967-18-willy@infradead.org>
Date:   Thu, 15 Jul 2021 04:35:03 +0100
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Jeff Layton <jlayton@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        William Kucharski <william.kucharski@...cle.com>,
        David Howells <dhowells@...hat.com>
Subject: [PATCH v14 017/138] mm/filemap: Add folio_unlock()

Convert unlock_page() to call folio_unlock().  By using a folio we
avoid a call to compound_head().  This shortens the function from 39
bytes to 25 and removes 4 instructions on x86-64.  Because we still
have unlock_page(), it's a net increase of 16 bytes of text for the
kernel as a whole, but any path that uses folio_unlock() will execute
4 fewer instructions.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
Reviewed-by: Christoph Hellwig <hch@....de>
Acked-by: Jeff Layton <jlayton@...nel.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Reviewed-by: William Kucharski <william.kucharski@...cle.com>
Reviewed-by: David Howells <dhowells@...hat.com>
---
 include/linux/pagemap.h |  3 ++-
 mm/filemap.c            | 29 ++++++++++++-----------------
 mm/folio-compat.c       |  6 ++++++
 3 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index a0925a89ba11..a13edc7a2916 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -658,7 +658,8 @@ extern int __lock_page_killable(struct page *page);
 extern int __lock_page_async(struct page *page, struct wait_page_queue *wait);
 extern int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 				unsigned int flags);
-extern void unlock_page(struct page *page);
+void unlock_page(struct page *page);
+void folio_unlock(struct folio *folio);
 
 /*
  * Return true if the page was successfully locked
diff --git a/mm/filemap.c b/mm/filemap.c
index 634adeacc4c1..1af67ef94e4c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1435,29 +1435,24 @@ static inline bool clear_bit_unlock_is_negative_byte(long nr, volatile void *mem
 #endif
 
 /**
- * unlock_page - unlock a locked page
- * @page: the page
+ * folio_unlock - Unlock a locked folio.
+ * @folio: The folio.
  *
- * Unlocks the page and wakes up sleepers in wait_on_page_locked().
- * Also wakes sleepers in wait_on_page_writeback() because the wakeup
- * mechanism between PageLocked pages and PageWriteback pages is shared.
- * But that's OK - sleepers in wait_on_page_writeback() just go back to sleep.
+ * Unlocks the folio and wakes up any thread sleeping on the page lock.
  *
- * Note that this depends on PG_waiters being the sign bit in the byte
- * that contains PG_locked - thus the BUILD_BUG_ON(). That allows us to
- * clear the PG_locked bit and test PG_waiters at the same time fairly
- * portably (architectures that do LL/SC can test any bit, while x86 can
- * test the sign bit).
+ * Context: May be called from interrupt or process context.  May not be
+ * called from NMI context.
  */
-void unlock_page(struct page *page)
+void folio_unlock(struct folio *folio)
 {
+	/* Bit 7 allows x86 to check the byte's sign bit */
 	BUILD_BUG_ON(PG_waiters != 7);
-	page = compound_head(page);
-	VM_BUG_ON_PAGE(!PageLocked(page), page);
-	if (clear_bit_unlock_is_negative_byte(PG_locked, &page->flags))
-		wake_up_page_bit(page, PG_locked);
+	BUILD_BUG_ON(PG_locked > 7);
+	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
+	if (clear_bit_unlock_is_negative_byte(PG_locked, folio_flags(folio, 0)))
+		wake_up_page_bit(&folio->page, PG_locked);
 }
-EXPORT_SYMBOL(unlock_page);
+EXPORT_SYMBOL(folio_unlock);
 
 /**
  * end_page_private_2 - Clear PG_private_2 and release any waiters
diff --git a/mm/folio-compat.c b/mm/folio-compat.c
index 5e107aa30a62..91b3d00a92f7 100644
--- a/mm/folio-compat.c
+++ b/mm/folio-compat.c
@@ -11,3 +11,9 @@ struct address_space *page_mapping(struct page *page)
 	return folio_mapping(page_folio(page));
 }
 EXPORT_SYMBOL(page_mapping);
+
+void unlock_page(struct page *page)
+{
+	return folio_unlock(page_folio(page));
+}
+EXPORT_SYMBOL(unlock_page);
-- 
2.30.2