[PATCH v4 4/5] mm/filemap: add write_begin_get_folio() helper function

["陈涛涛 Taotao Chen" <chentaotao@didiglobal.com>]

From: Taotao Chen <chentaotao@didiglobal.com>

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>
---
 include/linux/pagemap.h |  3 +++
 mm/filemap.c            | 30 ++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e63fbfbd5b0f..cbf8539ba11b 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -749,6 +749,9 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 		fgf_t fgp_flags, gfp_t gfp);
 struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 		fgf_t fgp_flags, gfp_t gfp);
+struct folio *write_begin_get_folio(const struct kiocb *iocb,
+				    struct address_space *mapping,
+				    pgoff_t index, size_t len);
 
 /**
  * filemap_get_folio - Find and get a folio.
diff --git a/mm/filemap.c b/mm/filemap.c
index ba089d75fc86..9520f65c287a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2026,6 +2026,36 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
 }
 EXPORT_SYMBOL(__filemap_get_folio);
 
+
+/**
+ * write_begin_get_folio - Get folio for write_begin with flags
+ * @iocb: kiocb passed from write_begin (may be NULL)
+ * @mapping: the address space to search in
+ * @index: 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.
+ *
+ * Returns a folio or an ERR_PTR.
+ */
+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));
+}
+EXPORT_SYMBOL(write_begin_get_folio);
+
 static inline struct folio *find_get_entry(struct xa_state *xas, pgoff_t max,
 		xa_mark_t mark)
 {
-- 
2.34.1