[RFC PATCH 09/10] iomap: Add writethrough for O_SYNC

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

For O_SYNC writes, if the filesystem has already allocated blocks for
the range, we can avoid marking the page as dirty and skip straight to
marking the page as writeback.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 fs/iomap/buffered-io.c | 74 ++++++++++++++++++++++++++++++++++++------
 1 file changed, 64 insertions(+), 10 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 6c540390eec3..5050adbd4bc8 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -531,6 +531,12 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
 EXPORT_SYMBOL_GPL(iomap_migrate_page);
 #endif /* CONFIG_MIGRATION */
 
+struct iomap_write_ctx {
+	struct iomap_ioend *ioend;
+	struct list_head iolist;
+	bool write_through;
+};
+
 static void
 iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
 {
@@ -875,8 +881,38 @@ static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
 	return status;
 }
 
-static size_t __iomap_write_end(struct inode *inode, loff_t pos, size_t len,
-		size_t copied, struct folio *folio)
+/* Returns true if we can skip dirtying the page */
+static bool iomap_write_through(struct iomap_write_ctx *iwc,
+		struct iomap *iomap, struct inode *inode, struct folio *folio,
+		loff_t pos, size_t len)
+{
+	unsigned int blksize = i_blocksize(inode);
+
+	if (!iwc || !iwc->write_through)
+		return false;
+	if (folio_test_dirty(folio))
+		return true;
+	if (folio_test_writeback(folio))
+		return false;
+
+	/* Can't allocate blocks here because we don't have ->prepare_ioend */
+	if (iomap->type != IOMAP_MAPPED || iomap->type != IOMAP_UNWRITTEN ||
+	    iomap->flags & IOMAP_F_SHARED)
+		return false;
+
+	len = round_up(pos + len - 1, blksize);
+	pos = round_down(pos, blksize);
+	len -= pos;
+	iwc->ioend = iomap_add_to_ioend(inode, pos, len, folio,
+			iomap_page_create(inode, folio), iomap, iwc->ioend,
+			NULL, &iwc->iolist);
+	folio_start_writeback(folio);
+	return true;
+}
+
+static size_t __iomap_write_end(struct iomap_write_ctx *iwc,
+		struct iomap *iomap, struct inode *inode, loff_t pos,
+		size_t len, size_t copied, struct folio *folio)
 {
 	struct iomap_page *iop = to_iomap_page(folio);
 	flush_dcache_folio(folio);
@@ -895,7 +931,8 @@ static size_t __iomap_write_end(struct inode *inode, loff_t pos, size_t len,
 	if (unlikely(copied < len && !folio_test_uptodate(folio)))
 		return 0;
 	iomap_set_range_uptodate(folio, iop, offset_in_folio(folio, pos), len);
-	filemap_dirty_folio(inode->i_mapping, folio);
+	if (!iomap_write_through(iwc, iomap, inode, folio, pos, len))
+		filemap_dirty_folio(inode->i_mapping, folio);
 	return copied;
 }
 
@@ -918,7 +955,8 @@ static size_t iomap_write_end_inline(const struct iomap_iter *iter,
 }
 
 /* Returns the number of bytes copied.  May be 0.  Cannot be an errno. */
-static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
+static size_t iomap_write_end(struct iomap_write_ctx *iwc,
+		struct iomap_iter *iter, loff_t pos, size_t len,
 		size_t copied, struct folio *folio)
 {
 	const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
@@ -932,7 +970,8 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
 		ret = block_write_end(NULL, iter->inode->i_mapping, pos, len,
 				copied, &folio->page, NULL);
 	} else {
-		ret = __iomap_write_end(iter->inode, pos, len, copied, folio);
+		ret = __iomap_write_end(iwc, &iter->iomap, iter->inode, pos,
+				len, copied, folio);
 	}
 
 	/*
@@ -957,7 +996,8 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
 	return ret;
 }
 
-static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
+static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i,
+		struct iomap_write_ctx *iwc)
 {
 	loff_t length = iomap_length(iter);
 	loff_t pos = iter->pos;
@@ -999,7 +1039,7 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 
 		copied = copy_page_from_iter_atomic(page, offset, bytes, i);
 
-		status = iomap_write_end(iter, pos, bytes, copied, folio);
+		status = iomap_write_end(iwc, iter, pos, bytes, copied, folio);
 
 		if (unlikely(copied != status))
 			iov_iter_revert(i, copied - status);
@@ -1036,10 +1076,24 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
 		.len		= iov_iter_count(i),
 		.flags		= IOMAP_WRITE,
 	};
+	struct iomap_write_ctx iwc = {
+		.iolist = LIST_HEAD_INIT(iwc.iolist),
+		.write_through = iocb->ki_flags & IOCB_SYNC,
+	};
+	struct iomap_ioend *ioend, *next;
 	int ret;
 
 	while ((ret = iomap_iter(&iter, ops)) > 0)
-		iter.processed = iomap_write_iter(&iter, i);
+		iter.processed = iomap_write_iter(&iter, i, &iwc);
+
+	list_for_each_entry_safe(ioend, next, &iwc.iolist, io_list) {
+		list_del_init(&ioend->io_list);
+		ret = iomap_submit_ioend(NULL, ioend, ret);
+	}
+
+	if (iwc.ioend)
+		ret = iomap_submit_ioend(NULL, iwc.ioend, ret);
+
 	if (iter.pos == iocb->ki_pos)
 		return ret;
 	return iter.pos - iocb->ki_pos;
@@ -1071,7 +1125,7 @@ static loff_t iomap_unshare_iter(struct iomap_iter *iter)
 		if (unlikely(status))
 			return status;
 
-		status = iomap_write_end(iter, pos, bytes, bytes, folio);
+		status = iomap_write_end(NULL, iter, pos, bytes, bytes, folio);
 		if (WARN_ON_ONCE(status == 0))
 			return -EIO;
 
@@ -1133,7 +1187,7 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
 		folio_zero_range(folio, offset, bytes);
 		folio_mark_accessed(folio);
 
-		bytes = iomap_write_end(iter, pos, bytes, bytes, folio);
+		bytes = iomap_write_end(NULL, iter, pos, bytes, bytes, folio);
 		if (WARN_ON_ONCE(bytes == 0))
 			return -EIO;
 
-- 
2.34.1