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

Matthew Wilcox
2021-11-24T20:04:20Z

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.