shmem: document how to "persist" data when using shmem_*file_setup

Add a blurb that simply dirtying the folio will persist data for in-kernel
shmem files.  This is what most of the callers already do.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 95e70e9ea060f36e82e1849a8da06c7875f42c28..fb76da93d369cf6cea017fd82428203277307a24 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2147,6 +2147,10 @@ unlock:
  * Looks up the page cache entry at @inode & @index.  If a folio is
  * present, it is returned locked with an increased refcount.
  *
+ * If the caller modifies data in the folio, it must call folio_mark_dirty()
+ * before unlocking the folio to ensure that the folio is not reclaimed.
+ * There is no need to reserve space before calling folio_mark_dirty().
+ *
  * When no folio is found, the behavior depends on @sgp:
  *  - for SGP_READ, *foliop is %NULL and 0 is returned
  *  - for SGP_NOALLOC, *foliop is %NULL and -ENOENT is returned