[PATCH 3/9] mm: Return head pages from grab_cache_page_write_begin

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

This function is only called from filesystems on their own mapping,
so no caller will be surprised by getting back a head page when they
were expecting a tail page.  This lets us remove a call to thp_head()
in wait_for_stable_page().

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/filemap.c        | 12 ++++++++++--
 mm/page-writeback.c |  2 +-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 2214a2c48dd1..62bc6affeb70 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3327,15 +3327,23 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 }
 EXPORT_SYMBOL(generic_file_direct_write);
 
-/*
+/**
+ * grab_cache_page_write_begin - Find or create a page for buffered writes.
+ * @mapping: The address space we're writing to.
+ * @index: The index we're writing to.
+ * @flags: %AOP_FLAG_NOFS to prevent memory reclaim calling the filesystem.
+ *
  * Find or create a page at the given pagecache position. Return the locked
  * page. This function is specifically for buffered writes.
+ *
+ * Return: The head page found in the cache, or NULL if no page could be
+ * created (due to lack of memory).
  */
 struct page *grab_cache_page_write_begin(struct address_space *mapping,
 					pgoff_t index, unsigned flags)
 {
 	struct page *page;
-	int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT;
+	int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT|FGP_HEAD;
 
 	if (flags & AOP_FLAG_NOFS)
 		fgp_flags |= FGP_NOFS;
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7709f0e223f5..3671568d433f 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2849,7 +2849,7 @@ EXPORT_SYMBOL_GPL(wait_on_page_writeback);
  */
 void wait_for_stable_page(struct page *page)
 {
-	page = thp_head(page);
+	VM_BUG_ON_PGFLAGS(PageTail(page), page);
 	if (page->mapping->host->i_sb->s_iflags & SB_I_STABLE_WRITES)
 		wait_on_page_writeback(page);
 }
-- 
2.28.0