mm, dev_pagemap: Do not clear ->mapping on final put

MEMORY_DEVICE_FS_DAX relies on typical page semantics whereby ->mapping
is only ever cleared by truncation, not final put.

Without this fix dax pages may forget their mapping association at the
end of every page pin event.

Move this atypical behavior that HMM wants into the HMM ->page_free()
callback.

Cc: <stable@vger.kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Fixes: d2c997c0f145 ("fs, dax: use page->mapping...")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/mm/hmm.c b/mm/hmm.c
index de7b6bf77201..f9d1d89dec4d 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -963,6 +963,8 @@ static void hmm_devmem_free(struct page *page, void *data)
 {
 	struct hmm_devmem *devmem = data;
 
+	page->mapping = NULL;
+
 	devmem->ops->free(devmem, page);
 }