Re: gfs2 iomap dealock, IOMAP_F_UNBALANCED

[Andreas Gruenbacher <agruenba@redhat.com>]

On Thu, 21 Mar 2019 at 22:43, Dave Chinner <david@fromorbit.com> wrote:
> The problem is calling balance_dirty_pages() inside the
> ->iomap_begin/->iomap_end calls and not that it is called by the
> iomap infrastructure itself, right?
>
> Is so, I'd prefer to see this in iomap_apply() after the call to
> ops->iomap_end because iomap_file_buffered_write() can iterate and
> call iomap_apply() multiple times. This would keep the balancing to
> a per-iomap granularity, rather than a per-syscall granularity.
>
> i.e. if we do write(2GB), we want more than one balancing call
> during that syscall, so it would be up to the filesystem to a) limit
> the size of write mappings to something smaller (e.g. 1024 pages)
> so that there are still frequent balancing calls for large writes.

Hmm. The looping across multiple mappings isn't done in iomap_apply
but in iomap_file_buffered_write, so the balancing could go into
iomap_apply or iomap_file_buffered_write, but can't go further up the
stack. Given that, iomap_file_buffered_write seems the better place,
but this is still quite horrible.

Thanks,
Andreas