filemap: add mapping_read_folio_gfp()

This is like read_cache_page_gfp() except it returns the folio instead
of the precise page.

Link: https://lkml.kernel.org/r/20230206162520.4029022-1-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mark Hemment <markhemm@googlemail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pagemap.h

@@ -756,6 +756,8 @@ static inline struct page *grab_cache_page(struct address_space *mapping,
 
 struct folio *read_cache_folio(struct address_space *, pgoff_t index,
 		filler_t *filler, struct file *file);
+struct folio *mapping_read_folio_gfp(struct address_space *, pgoff_t index,
+		gfp_t flags);
 struct page *read_cache_page(struct address_space *, pgoff_t index,
 		filler_t *filler, struct file *file);
 extern struct page * read_cache_page_gfp(struct address_space *mapping,

mm/filemap.c

@@ -3585,6 +3585,30 @@ struct folio *read_cache_folio(struct address_space *mapping, pgoff_t index,
 }
 EXPORT_SYMBOL(read_cache_folio);
 
+/**
+ * mapping_read_folio_gfp - Read into page cache, using specified allocation flags.
+ * @mapping:	The address_space for the folio.
+ * @index:	The index that the allocated folio will contain.
+ * @gfp:	The page allocator flags to use if allocating.
+ *
+ * This is the same as "read_cache_folio(mapping, index, NULL, NULL)", but with
+ * any new memory allocations done using the specified allocation flags.
+ *
+ * The most likely error from this function is EIO, but ENOMEM is
+ * possible and so is EINTR.  If ->read_folio returns another error,
+ * that will be returned to the caller.
+ *
+ * The function expects mapping->invalidate_lock to be already held.
+ *
+ * Return: Uptodate folio on success, ERR_PTR() on failure.
+ */
+struct folio *mapping_read_folio_gfp(struct address_space *mapping,
+		pgoff_t index, gfp_t gfp)
+{
+	return do_read_cache_folio(mapping, index, NULL, NULL, gfp);
+}
+EXPORT_SYMBOL(mapping_read_folio_gfp);
+
 static struct page *do_read_cache_page(struct address_space *mapping,
 		pgoff_t index, filler_t *filler, struct file *file, gfp_t gfp)
 {