Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults

[Matthew Wilcox <willy@infradead.org>]

On Wed, Nov 24, 2021 at 07:20:24PM +0000, Catalin Marinas wrote:
> +++ b/fs/btrfs/ioctl.c
> @@ -2223,7 +2223,8 @@ static noinline int search_ioctl(struct inode *inode,
>  
>  	while (1) {
>  		ret = -EFAULT;
> -		if (fault_in_writeable(ubuf + sk_offset, *buf_size - sk_offset))
> +		if (fault_in_exact_writeable(ubuf + sk_offset,
> +					     *buf_size - sk_offset))
>  			break;
>  
>  		ret = btrfs_search_forward(root, &key, path, sk->min_transid);

Couldn't we avoid all of this nastiness by doing ...

@@ -2121,10 +2121,9 @@ static noinline int copy_to_sk(struct btrfs_path *path,
                 * problem. Otherwise we'll fault and then copy the buffer in
                 * properly this next time through
                 */
-               if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh))) {
-                       ret = 0;
+               ret = __copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh));
+               if (ret)
                        goto out;
-               }
 
                *sk_offset += sizeof(sh);
@@ -2196,6 +2195,7 @@ static noinline int search_ioctl(struct inode *inode,
        int ret;
        int num_found = 0;
        unsigned long sk_offset = 0;
+       unsigned long next_offset = 0;
 
        if (*buf_size < sizeof(struct btrfs_ioctl_search_header)) {
                *buf_size = sizeof(struct btrfs_ioctl_search_header);
@@ -2223,7 +2223,8 @@ static noinline int search_ioctl(struct inode *inode,
 
        while (1) {
                ret = -EFAULT;
-               if (fault_in_writeable(ubuf + sk_offset, *buf_size - sk_offset))
+               if (fault_in_writeable(ubuf + sk_offset + next_offset,
+                                       *buf_size - sk_offset - next_offset))
                        break;
 
                ret = btrfs_search_forward(root, &key, path, sk->min_transid);
@@ -2235,11 +2236,12 @@ static noinline int search_ioctl(struct inode *inode,
                ret = copy_to_sk(path, &key, sk, buf_size, ubuf,
                                 &sk_offset, &num_found);
                btrfs_release_path(path);
-               if (ret)
+               if (ret > 0)
+                       next_offset = ret;
+               else if (ret < 0)
                        break;
-
        }
-       if (ret > 0)
+       if (ret == -ENOSPC || ret > 0)
                ret = 0;
 err:
        sk->nr_items = num_found;

(not shown: the tedious bits where the existing 'ret = 1' are converted
to 'ret = -ENOSPC' in copy_to_sk())
 
(where __copy_to_user_nofault() is a new function that does exactly what
copy_to_user_nofault() does, but returns the number of bytes copied)

That way, the existing fault_in_writable() will get the fault, and we
don't need to probe every 16 bytes.