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authorDavid Herrmann <dh.herrmann@gmail.com>2014-08-08 21:25:36 (GMT)
committerMaciej Wereski <m.wereski@partner.samsung.com>2014-12-29 08:31:35 (GMT)
commitab7b5b245af738ad66dbcfa17b3a905852e354c0 (patch)
tree8e8e5cf686c2c5cceab8341822f9f2462177ca8d
parent2d63a8517a762037ed240509d6a2362744748b05 (diff)
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shm: wait for pins to be released when sealingrefs/changes/09/32909/1
If we set SEAL_WRITE on a file, we must make sure there cannot be any ongoing write-operations on the file. For write() calls, we simply lock the inode mutex, for mmap() we simply verify there're no writable mappings. However, there might be pages pinned by AIO, Direct-IO and similar operations via GUP. We must make sure those do not write to the memfd file after we set SEAL_WRITE. As there is no way to notify GUP users to drop pages or to wait for them to be done, we implement the wait ourself: When setting SEAL_WRITE, we check all pages for their ref-count. If it's bigger than 1, we know there's some user of the page. We then mark the page and wait for up to 150ms for those ref-counts to be dropped. If the ref-counts are not dropped in time, we refuse the seal operation. Signed-off-by: David Herrmann <dh.herrmann@gmail.com> Acked-by: Hugh Dickins <hughd@google.com> Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Ryan Lortie <desrt@desrt.ca> Cc: Lennart Poettering <lennart@poettering.net> Cc: Daniel Mack <zonque@gmail.com> Cc: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Change-Id: I952289df3c4261be68ab4dc590890fe20b0906a4 Origin: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=05f65b5c70909ef686f865f0a85406d74d75f70f Backported-by: Maciej Wereski <m.wereski@partner.samsung.com> Signed-off-by: Maciej Wereski <m.wereski@partner.samsung.com>
-rw-r--r--mm/shmem.c110
1 files changed, 109 insertions, 1 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index e590564..27cb455 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1892,9 +1892,117 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
return offset;
}
+/*
+ * 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.
+ */
+#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;
+
+ lru_add_drain();
+ start = 0;
+ rcu_read_lock();
+
+restart:
+ radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
+ page = radix_tree_deref_slot(slot);
+ if (!page || radix_tree_exception(page)) {
+ if (radix_tree_deref_retry(page))
+ goto restart;
+ } else if (page_count(page) - page_mapcount(page) > 1) {
+ 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()) {
+ cond_resched_rcu();
+ start = iter.index + 1;
+ goto 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)
{
- return 0;
+ 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;
}
#define F_ALL_SEALS (F_SEAL_SEAL | \