mm/pagemap: add write_begin_get_folio() helper function

Add write_begin_get_folio() to simplify the common folio lookup logic
used by filesystem ->write_begin() implementations.

This helper wraps __filemap_get_folio() with common flags such as
FGP_WRITEBEGIN, conditional FGP_DONTCACHE, and set folio order based
on the write length.

Part of a series refactoring address_space_operations write_begin and
write_end callbacks to use struct kiocb for passing write context and
flags.

Signed-off-by: Taotao Chen <chentaotao@didiglobal.com>
Link: https://lore.kernel.org/20250716093559.217344-5-chentaotao@didiglobal.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Taotao Chen

include/linux/pagemap.h

@@ -750,6 +750,33 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 		fgf_t fgp_flags, gfp_t gfp);
 
+/**
+ * write_begin_get_folio - Get folio for write_begin with flags.
+ * @iocb: The kiocb passed from write_begin (may be NULL).
+ * @mapping: The address space to search.
+ * @index: The page cache index.
+ * @len: Length of data being written.
+ *
+ * This is a helper for filesystem write_begin() implementations.
+ * It wraps __filemap_get_folio(), setting appropriate flags in
+ * the write begin context.
+ *
+ * Return: A folio or an ERR_PTR.
+ */
+static inline struct folio *write_begin_get_folio(const struct kiocb *iocb,
+		  struct address_space *mapping, pgoff_t index, size_t len)
+{
+        fgf_t fgp_flags = FGP_WRITEBEGIN;
+
+        fgp_flags |= fgf_set_order(len);
+
+        if (iocb && iocb->ki_flags & IOCB_DONTCACHE)
+                fgp_flags |= FGP_DONTCACHE;
+
+        return __filemap_get_folio(mapping, index, fgp_flags,
+                                   mapping_gfp_mask(mapping));
+}
+
 /**
  * filemap_get_folio - Find and get a folio.
  * @mapping: The address_space to search.