Merge tag 'fsverity-for-linus' of git://git.kernel.org/pub/scm/fs/fsverity/linux

Pull fsverity fixes from Eric Biggers:
 "Fix two significant performance issues with fsverity"

* tag 'fsverity-for-linus' of git://git.kernel.org/pub/scm/fs/fsverity/linux:
  fsverity: don't drop pagecache at end of FS_IOC_ENABLE_VERITY
  fsverity: Remove WQ_UNBOUND from fsverity read workqueue

Author: torvalds

fs/verity/enable.c

@@ -8,7 +8,6 @@
 #include "fsverity_private.h"
 
 #include <linux/mount.h>
-#include <linux/pagemap.h>
 #include <linux/sched/signal.h>
 #include <linux/uaccess.h>
 
@@ -367,25 +366,27 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
 		goto out_drop_write;
 
 	err = enable_verity(filp, &arg);
-	if (err)
-		goto out_allow_write_access;
 
 	/*
-	 * Some pages of the file may have been evicted from pagecache after
-	 * being used in the Merkle tree construction, then read into pagecache
-	 * again by another process reading from the file concurrently.  Since
-	 * these pages didn't undergo verification against the file digest which
-	 * fs-verity now claims to be enforcing, we have to wipe the pagecache
-	 * to ensure that all future reads are verified.
+	 * We no longer drop the inode's pagecache after enabling verity.  This
+	 * used to be done to try to avoid a race condition where pages could be
+	 * evicted after being used in the Merkle tree construction, then
+	 * re-instantiated by a concurrent read.  Such pages are unverified, and
+	 * the backing storage could have filled them with different content, so
+	 * they shouldn't be used to fulfill reads once verity is enabled.
+	 *
+	 * But, dropping the pagecache has a big performance impact, and it
+	 * doesn't fully solve the race condition anyway.  So for those reasons,
+	 * and also because this race condition isn't very important relatively
+	 * speaking (especially for small-ish files, where the chance of a page
+	 * being used, evicted, *and* re-instantiated all while enabling verity
+	 * is quite small), we no longer drop the inode's pagecache.
 	 */
-	filemap_write_and_wait(inode->i_mapping);
-	invalidate_inode_pages2(inode->i_mapping);
 
 	/*
 	 * allow_write_access() is needed to pair with deny_write_access().
 	 * Regardless, the filesystem won't allow writing to verity files.
 	 */
-out_allow_write_access:
 	allow_write_access(filp);
 out_drop_write:
 	mnt_drop_write_file(filp);

fs/verity/verify.c

@@ -387,15 +387,15 @@ EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
 int __init fsverity_init_workqueue(void)
 {
 	/*
-	 * Use an unbound workqueue to allow bios to be verified in parallel
-	 * even when they happen to complete on the same CPU.  This sacrifices
-	 * locality, but it's worthwhile since hashing is CPU-intensive.
+	 * Use a high-priority workqueue to prioritize verification work, which
+	 * blocks reads from completing, over regular application tasks.
 	 *
-	 * Also use a high-priority workqueue to prioritize verification work,
-	 * which blocks reads from completing, over regular application tasks.
+	 * For performance reasons, don't use an unbound workqueue.  Using an
+	 * unbound workqueue for crypto operations causes excessive scheduler
+	 * latency on ARM64.
 	 */
 	fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
-						  WQ_UNBOUND | WQ_HIGHPRI,
+						  WQ_HIGHPRI,
 						  num_online_cpus());
 	if (!fsverity_read_workqueue)
 		return -ENOMEM;