memfd: fix F_SEAL_WRITE after shmem huge page allocated

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>

diff --git a/mm/memfd.c b/mm/memfd.c
index 9f80f162791a5a51cab843759c5353bb27741da4..08f5f8304746fb675f744ed93eaa7eaf7e53a6d6 100644 (file)
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -31,20 +31,28 @@
 static void memfd_tag_pins(struct xa_state *xas)
 {
        struct page *page;
-       unsigned int tagged = 0;
+       int latency = 0;
+       int cache_count;
 
        lru_add_drain();
 
        xas_lock_irq(xas);
        xas_for_each(xas, page, ULONG_MAX) {
-               if (xa_is_value(page))
-                       continue;
-               page = find_subpage(page, xas->xa_index);
-               if (page_count(page) - page_mapcount(page) > 1)
+               cache_count = 1;
+               if (!xa_is_value(page) &&
+                   PageTransHuge(page) && !PageHuge(page))
+                       cache_count = HPAGE_PMD_NR;
+
+               if (!xa_is_value(page) &&
+                   page_count(page) - total_mapcount(page) != cache_count)
                        xas_set_mark(xas, MEMFD_TAG_PINNED);
+               if (cache_count != 1)
+                       xas_set(xas, page->index + cache_count);
 
-               if (++tagged % XA_CHECK_SCHED)
+               latency += cache_count;
+               if (latency < XA_CHECK_SCHED)
                        continue;
+               latency = 0;
 
                xas_pause(xas);
                xas_unlock_irq(xas);
@@ -73,7 +81,8 @@ static int memfd_wait_for_pins(struct address_space *mapping)
 
        error = 0;
        for (scan = 0; scan <= LAST_SCAN; scan++) {
-               unsigned int tagged = 0;
+               int latency = 0;
+               int cache_count;
 
                if (!xas_marked(&xas, MEMFD_TAG_PINNED))
                        break;
@@ -87,10 +96,14 @@ static int memfd_wait_for_pins(struct address_space *mapping)
                xas_lock_irq(&xas);
                xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
                        bool clear = true;
-                       if (xa_is_value(page))
-                               continue;
-                       page = find_subpage(page, xas.xa_index);
-                       if (page_count(page) - page_mapcount(page) != 1) {
+
+                       cache_count = 1;
+                       if (!xa_is_value(page) &&
+                           PageTransHuge(page) && !PageHuge(page))
+                               cache_count = HPAGE_PMD_NR;
+
+                       if (!xa_is_value(page) && cache_count !=
+                           page_count(page) - total_mapcount(page)) {
                                /*
                                 * On the last scan, we clean up all those tags
                                 * we inserted; but make a note that we still
@@ -103,8 +116,11 @@ static int memfd_wait_for_pins(struct address_space *mapping)
                        }
                        if (clear)
                                xas_clear_mark(&xas, MEMFD_TAG_PINNED);
-                       if (++tagged % XA_CHECK_SCHED)
+
+                       latency += cache_count;
+                       if (latency < XA_CHECK_SCHED)
                                continue;
+                       latency = 0;
 
                        xas_pause(&xas);
                        xas_unlock_irq(&xas);