fuse: use mtime for readdir cache verification

Store the modification time of the directory in the cache, obtained before
starting to fill the cache.

When reading the cache, verify that the directory hasn't changed, by
checking if current modification time is the same as the one stored in the
cache.

This only needs to be done when the current file position is at the
beginning of the directory, as mandated by POSIX.

Signed-off-by: Miklos Szeredi <[e-mail]>

fs/fuse/fuse_i.h

···
 		/* version of the cache */
 		u64 version;
 
+		/* modification time of directory when cache was started */
+		struct timespec64 mtime;
+
 		/* protects above fields */
 		spinlock_t lock;
 	} rdc;

fs/fuse/readdir.c

···
 	return res;
 }
 
-static void fuse_rdc_reset(struct fuse_inode *fi)
+static void fuse_rdc_reset(struct inode *inode)
 {
+	struct fuse_inode *fi = get_fuse_inode(inode);
+
 	fi->rdc.cached = false;
 	fi->rdc.version++;
 	fi->rdc.size = 0;
···
 {
 	struct fuse_file *ff = file->private_data;
 	struct inode *inode = file_inode(file);
+	struct fuse_conn *fc = get_fuse_conn(inode);
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	enum fuse_parse_result res;
 	pgoff_t index;
···
 		ff->readdir.cache_off = 0;
 	}
 
+	/*
+	 * We're just about to start reading into the cache or reading the
+	 * cache; both cases require an up-to-date mtime value.
+	 */
+	if (!ctx->pos && fc->auto_inval_data) {
+		int err = fuse_update_attributes(inode, file);
+
+		if (err)
+			return err;
+	}
+
 retry:
 	spin_lock(&fi->rdc.lock);
+retry_locked:
 	if (!fi->rdc.cached) {
+		/* Starting cache? Set cache mtime. */
+		if (!ctx->pos && !fi->rdc.size) {
+			fi->rdc.mtime = inode->i_mtime;
+		}
 		spin_unlock(&fi->rdc.lock);
 		return UNCACHED;
 	}
+	/*
+	 * When at the beginning of the directory (i.e. just after opendir(3) or
+	 * rewinddir(3)), then need to check whether directory contents have
+	 * changed, and reset the cache if so.
+	 */
+	if (!ctx->pos) {
+		if (!timespec64_equal(&fi->rdc.mtime, &inode->i_mtime)) {
+			fuse_rdc_reset(inode);
+			goto retry_locked;
+		}
+	}
+
 	/*
 	 * If cache version changed since the last getdents() call, then reset
 	 * the cache stream.
···
 		 * Uh-oh: page gone missing, cache is useless
 		 */
 		if (fi->rdc.version == ff->readdir.version)
-			fuse_rdc_reset(fi);
-		spin_unlock(&fi->rdc.lock);
-		return UNCACHED;
+			fuse_rdc_reset(inode);
+		goto retry_locked;
 	}
 
 	/* Make sure it's still the same version after getting the page. */