[PATCH 4.9 31/32] memfd: fix F_SEAL_WRITE after shmem huge page allocated

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Hugh Dickins <hughd@google.com>

commit f2b277c4d1c63a85127e8aa2588e9cc3bd21cb99 upstream.

Wangyong reports: after enabling tmpfs filesystem to support transparent
hugepage with the following command:

  echo always > /sys/kernel/mm/transparent_hugepage/shmem_enabled

the docker program tries to add F_SEAL_WRITE through the following
command, but it fails unexpectedly with errno EBUSY:

  fcntl(5, F_ADD_SEALS, F_SEAL_WRITE) = -1.

That is because memfd_tag_pins() and memfd_wait_for_pins() were never
updated for shmem huge pages: checking page_mapcount() against
page_count() is hopeless on THP subpages - they need to check
total_mapcount() against page_count() on THP heads only.

Make memfd_tag_pins() (compared > 1) as strict as memfd_wait_for_pins()
(compared != 1): either can be justified, but given the non-atomic
total_mapcount() calculation, it is better now to be strict.  Bear in
mind that total_mapcount() itself scans all of the THP subpages, when
choosing to take an XA_CHECK_SCHED latency break.

Also fix the unlikely xa_is_value() case in memfd_wait_for_pins(): if a
page has been swapped out since memfd_tag_pins(), then its refcount must
have fallen, and so it can safely be untagged.

Link: https://lkml.kernel.org/r/a4f79248-df75-2c8c-3df-ba3317ccb5da@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Reported-by: wangyong <wang.yong12@zte.com.cn>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: CGEL ZTE <cgel.zte@gmail.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 mm/shmem.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2478,7 +2478,8 @@ static void shmem_tag_pins(struct addres
 				slot = radix_tree_iter_retry(&iter);
 				continue;
 			}
-		} else if (page_count(page) - page_mapcount(page) > 1) {
+		} else if (!PageTail(page) && page_count(page) !=
+			   hpage_nr_pages(page) + total_mapcount(page)) {
 			radix_tree_tag_set(&mapping->page_tree, iter.index,
 					   SHMEM_TAG_PINNED);
 		}
@@ -2538,8 +2539,8 @@ static int shmem_wait_for_pins(struct ad
 				page = NULL;
 			}
 
-			if (page &&
-			    page_count(page) - page_mapcount(page) != 1) {
+			if (page && page_count(page) !=
+			    hpage_nr_pages(page) + total_mapcount(page)) {
 				if (scan < LAST_SCAN)
 					goto continue_resched;