Re: [PATCH 02/11] iomap: copy the xfs writeback code to iomap.c

[Christoph Hellwig]

On Tue, Oct 08, 2019 at 08:43:53AM +1100, Dave Chinner wrote:
> > +static int
> > +iomap_ioend_compare(void *priv, struct list_head *a, struct list_head *b)
> > +{
> > +	struct iomap_ioend *ia, *ib;
> > +
> > +	ia = container_of(a, struct iomap_ioend, io_list);
> > +	ib = container_of(b, struct iomap_ioend, io_list);
> > +	if (ia->io_offset < ib->io_offset)
> > +		return -1;
> > +	else if (ia->io_offset > ib->io_offset)
> > +		return 1;
> > +	return 0;
> 
> No need for the else here.

That is usually my comment :)  But in this case it is just copied over
code, so I didn't want to do cosmetic changes.

> > +	/*
> > +	 * Given that we do not allow direct reclaim to call us, we should
> > +	 * never be called while in a filesystem transaction.
> > +	 */
> > +	if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
> > +		goto redirty;
> 
> Is this true for all expected callers of these functions rather than
> just XFS? i.e. PF_MEMALLOC_NOFS is used by transactions in XFS to
> prevent transaction context recursion, but other filesystems do not
> do this..
> 
> FWIW, I can also see that this is going to cause us problems if high
> level code starts using memalloc_nofs_save() and then calling
> filemap_datawrite() and friends...

We have the check for direct reclaim just above, so any file system
using this iomap code will not allow direct reclaim.  Which I think is
a very good idea given that direct reclaim through the file system is
a very bad idea.

That leaves with only the filemap_datawrite case, which so far is
theoretical.  If that ever becomes a think it is very obvious and we
can just remove the debug check.

> > +iomap_writepage(struct page *page, struct writeback_control *wbc,
> > +		struct iomap_writepage_ctx *wpc,
> > +		const struct iomap_writeback_ops *ops)
> > +{
> > +	int ret;
> > +
> > +	wpc->ops = ops;
> > +	ret = iomap_do_writepage(page, wbc, wpc);
> > +	if (!wpc->ioend)
> > +		return ret;
> > +	return iomap_submit_ioend(wpc, wpc->ioend, ret);
> > +}
> > +EXPORT_SYMBOL_GPL(iomap_writepage);
> 
> Can we kill ->writepage for iomap users, please? After all, we don't
> mostly don't allow memory reclaim to do writeback of dirty pages,
> and that's the only caller of ->writepage.

I'd rather not do this as part of this move.  But if you could expedite
your patch to kill ->writepage from the large block size support patch
and submit it ASAP on top of this series I would be very much in favor.