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

[Andreas Gruenbacher]

On Wed, Nov 24, 2021 at 9:37 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> On Wed, Nov 24, 2021 at 08:03:58PM +0000, Matthew Wilcox wrote:
> > 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 ...
>
> I had a similar attempt initially but I concluded that it doesn't work:
>
> https://lore.kernel.org/r/YS40qqmXL7CMFLGq@xxxxxxx
>
> > @@ -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)
>
> There is no requirement for the arch implementation to be exact and copy
> the maximum number of bytes possible. It can fail early while there are
> still some bytes left that would not fault. The only requirement is that
> if it is restarted from where it faulted, it makes some progress (on
> arm64 there is one extra byte).
>
> >                         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;
>
> So after this point, ubuf+sk_offset+next_offset is writeable by
> fault_in_writable(). If copy_to_user() was attempted on
> ubuf+sk_offset+next_offset, all would be fine, but copy_to_sk() restarts
> the copy from ubuf+sk_offset, so it returns exacting the same ret as in
> the previous iteration.

So this means that after a short copy_to_user_nofault(), copy_to_sk()
needs to figure out the actual point of failure. We'll have the same
problem elsewhere, so this should probably be a generic helper. The
alignment hacks are arch specific, so maybe we can have a generic
version that assumes no alignment restrictions, with arch-specific
overrides.

Once we know the exact point of failure, a
fault_in_writeable(point_of_failure, 1) in search_ioctl() will tell if
the failure is pertinent. Once we know that the failure isn't
pertinent, we're safe to retry the original fault_in_writeable().

Thanks,
Andreas