From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Thu, 16 Jun 2016 10:38:08 +0200
From: Willy Tarreau <w@1wt.eu>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Andy Lutomirski <luto@kernel.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Andrew Morton <akpm@linux-foundation.org>,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] fs: Improve and simplify copy_mount_options
Message-ID: <20160616083807.GB7409@1wt.eu>
References: <20160615235047.GA14480@ZenIV.linux.org.uk>
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 <20160616054525.GA6803@1wt.eu>
 <CA+55aFwMpvtLbkMQqjA2FJa32FMuVX2Jtmevz9p=i8YOyKGZbA@mail.gmail.com>
 <20160616065738.GA7154@1wt.eu>
 <20160616070821.GB14480@ZenIV.linux.org.uk>
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 <20160616080229.GC14480@ZenIV.linux.org.uk>
 <20160616082038.GA7409@1wt.eu>
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On Thu, Jun 16, 2016 at 10:20:38AM +0200, Willy Tarreau wrote:
> However if the initial point was to avoid reading extra
> pages most of the time, we could possibly consider that the string
> copy is an optimization for the case where we have the information
> available. Thus if !fstype || !(type->fs_flags & FS_BINARY_MOUNTDATA)
> then we use copy_mount_options() otherwise we use copy_mount_string().
> It will only leave the full page copy for mount --bind or -o remount
> then.

For example like this (not compile-tested either) :

diff --git a/fs/compat.c b/fs/compat.c
index be6e48b..6407d40 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -795,18 +795,26 @@ COMPAT_SYSCALL_DEFINE5(mount, const char __user *, dev_name,
 	void *options;
 	char *kernel_dev;
 	int retval;
+	bool use_str = false;
+	struct file_system_type *fstype;
 
 	kernel_type = copy_mount_string(type);
 	retval = PTR_ERR(kernel_type);
 	if (IS_ERR(kernel_type))
 		goto out;
 
+	if (kernel_type && (fstype = get_fs_type(kernel_type)) {
+		if (!(fstype->fs_flags & FS_BINARY_MOUNTDATA))
+			use_str = true;
+		put_filesystem(fstype);
+	}
+
 	kernel_dev = copy_mount_string(dev_name);
 	retval = PTR_ERR(kernel_dev);
 	if (IS_ERR(kernel_dev))
 		goto out1;
 
-	options = copy_mount_options(data);
+	options = use_str ? copy_mount_string(data) : copy_mount_options(data);
 	retval = PTR_ERR(options);
 	if (IS_ERR(options))
 		goto out2;
diff --git a/fs/namespace.c b/fs/namespace.c
index 4fb1691..f14089d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2906,18 +2906,26 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 	char *kernel_type;
 	char *kernel_dev;
 	void *options;
+	bool use_str = false;
+	struct file_system_type *fstype;
 
 	kernel_type = copy_mount_string(type);
 	ret = PTR_ERR(kernel_type);
 	if (IS_ERR(kernel_type))
 		goto out_type;
 
+	if (kernel_type && (fstype = get_fs_type(kernel_type)) {
+		if (!(fstype->fs_flags & FS_BINARY_MOUNTDATA))
+			use_str = true;
+		put_filesystem(fstype);
+	}
+
 	kernel_dev = copy_mount_string(dev_name);
 	ret = PTR_ERR(kernel_dev);
 	if (IS_ERR(kernel_dev))
 		goto out_dev;
 
-	options = copy_mount_options(data);
+	options = use_str ? copy_mount_string(data) : copy_mount_options(data);
 	ret = PTR_ERR(options);
 	if (IS_ERR(options))
 		goto out_data;