We currently fail setting SEAL_WRITE in case there're pending page
references. This patch extends the pin-tests to wait up to 150ms for all
references to be dropped. This is still not perfect in that it doesn't
account for harmless read-only pins, but it's much better than a hard
failure.
Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxx>
---
mm/shmem.c | 97 ++++++++++++++++++++++++++++++++++++++++++++++++++++----------
1 file changed, 82 insertions(+), 15 deletions(-)
diff --git a/mm/shmem.c b/mm/shmem.c
index e7c5fe1..ddc3998 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1735,25 +1735,19 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
}
/*
- * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
- * via get_user_pages(), drivers might have some pending I/O without any active
- * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
- * and see whether it has an elevated ref-count. If so, we abort.
- * The caller must guarantee that no new user will acquire writable references
- * to those pages to avoid races.
+ * We need a tag: a new tag would expand every radix_tree_node by 8 bytes,
+ * so reuse a tag which we firmly believe is never set or cleared on shmem.
*/
-static int shmem_test_for_pins(struct address_space *mapping)
+#define SHMEM_TAG_PINNED PAGECACHE_TAG_TOWRITE
+#define LAST_SCAN 4 /* about 150ms max */
+
+static void shmem_tag_pins(struct address_space *mapping)
{
struct radix_tree_iter iter;
void **slot;
pgoff_t start;
struct page *page;
- int error;
-
- /* flush additional refs in lru_add early */
- lru_add_drain_all();
- error = 0;
start = 0;
rcu_read_lock();
@@ -1764,8 +1758,10 @@ restart:
if (radix_tree_deref_retry(page))
goto restart;
} else if (page_count(page) - page_mapcount(page) > 1) {
- error = -EBUSY;
- break;
+ spin_lock_irq(&mapping->tree_lock);
+ radix_tree_tag_set(&mapping->page_tree, iter.index,
+ SHMEM_TAG_PINNED);
+ spin_unlock_irq(&mapping->tree_lock);
}
if (need_resched()) {
@@ -1775,6 +1771,77 @@ restart:
}
}
rcu_read_unlock();
+}
+
+/*
+ * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
+ * via get_user_pages(), drivers might have some pending I/O without any active
+ * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
+ * and see whether it has an elevated ref-count. If so, we tag them and wait for
+ * them to be dropped.
+ * The caller must guarantee that no new user will acquire writable references
+ * to those pages to avoid races.
+ */
+static int shmem_wait_for_pins(struct address_space *mapping)
+{
+ struct radix_tree_iter iter;
+ void **slot;
+ pgoff_t start;
+ struct page *page;
+ int error, scan;
+
+ shmem_tag_pins(mapping);
+
+ error = 0;
+ for (scan = 0; scan <= LAST_SCAN; scan++) {
+ if (!radix_tree_tagged(&mapping->page_tree, SHMEM_TAG_PINNED))
+ break;
+
+ if (!scan)
+ lru_add_drain_all();
+ else if (schedule_timeout_killable((HZ << scan) / 200))
+ scan = LAST_SCAN;
+
+ start = 0;
+ rcu_read_lock();
+restart:
+ radix_tree_for_each_tagged(slot, &mapping->page_tree, &iter,
+ start, SHMEM_TAG_PINNED) {
+
+ page = radix_tree_deref_slot(slot);
+ if (radix_tree_exception(page)) {
+ if (radix_tree_deref_retry(page))
+ goto restart;
+
+ page = NULL;
+ }
+
+ if (page &&
+ page_count(page) - page_mapcount(page) != 1) {
+ if (scan < LAST_SCAN)
+ goto continue_resched;
+
+ /*
+ * On the last scan, we clean up all those tags
+ * we inserted; but make a note that we still
+ * found pages pinned.
+ */
+ error = -EBUSY;
+ }
+
+ spin_lock_irq(&mapping->tree_lock);
+ radix_tree_tag_clear(&mapping->page_tree,
+ iter.index, SHMEM_TAG_PINNED);
+ spin_unlock_irq(&mapping->tree_lock);
+continue_resched:
+ if (need_resched()) {
+ cond_resched_rcu();
+ start = iter.index + 1;
+ goto restart;
+ }
+ }
+ rcu_read_unlock();
+ }
return error;
}
@@ -1840,7 +1907,7 @@ int shmem_add_seals(struct file *file, unsigned int seals)
if (error)
goto unlock;
- error = shmem_test_for_pins(file->f_mapping);
+ error = shmem_wait_for_pins(file->f_mapping);
if (error) {
mapping_allow_writable(file->f_mapping);
goto unlock;
--
2.0.0
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