iomap: Add iomap_get_folio helper

Add an iomap_get_folio() helper that gets a folio reference based on
an iomap iterator and an offset into the address space.  Use it in
iomap_write_begin().
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/iomap/buffered-io.c

@@ -457,6 +457,33 @@ bool iomap_is_partially_uptodate(struct folio *folio, size_t from, size_t count)
 }
 EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
 
+/**
+ * iomap_get_folio - get a folio reference for writing
+ * @iter: iteration structure
+ * @pos: start offset of write
+ *
+ * Returns a locked reference to the folio at @pos, or an error pointer if the
+ * folio could not be obtained.
+ */
+struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos)
+{
+	unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
+	struct folio *folio;
+
+	if (iter->flags & IOMAP_NOWAIT)
+		fgp |= FGP_NOWAIT;
+
+	folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
+			fgp, mapping_gfp_mask(iter->inode->i_mapping));
+	if (folio)
+		return folio;
+
+	if (iter->flags & IOMAP_NOWAIT)
+		return ERR_PTR(-EAGAIN);
+	return ERR_PTR(-ENOMEM);
+}
+EXPORT_SYMBOL_GPL(iomap_get_folio);
+
 bool iomap_release_folio(struct folio *folio, gfp_t gfp_flags)
 {
 	trace_iomap_release_folio(folio->mapping->host, folio_pos(folio),
@@ -603,12 +630,8 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
 	const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
 	struct folio *folio;
-	unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
 	int status = 0;
 
-	if (iter->flags & IOMAP_NOWAIT)
-		fgp |= FGP_NOWAIT;
-
 	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
 	if (srcmap != &iter->iomap)
 		BUG_ON(pos + len > srcmap->offset + srcmap->length);
@@ -625,12 +648,10 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
 			return status;
 	}
 
-	folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
-			fgp, mapping_gfp_mask(iter->inode->i_mapping));
-	if (!folio) {
-		status = (iter->flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOMEM;
+	folio = iomap_get_folio(iter, pos);
+	if (IS_ERR(folio)) {
 		__iomap_put_folio(iter, pos, 0, NULL);
-		return status;
+		return PTR_ERR(folio);
 	}
 
 	/*

include/linux/iomap.h

@@ -261,6 +261,7 @@ int iomap_file_buffered_write_punch_delalloc(struct inode *inode,
 int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops);
 void iomap_readahead(struct readahead_control *, const struct iomap_ops *ops);
 bool iomap_is_partially_uptodate(struct folio *, size_t from, size_t count);
+struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos);
 bool iomap_release_folio(struct folio *folio, gfp_t gfp_flags);
 void iomap_invalidate_folio(struct folio *folio, size_t offset, size_t len);
 int iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,