Re: [PATCH 2/2] iomap: zero cached pages over unwritten extents on zero range

[Brian Foster <bfoster@redhat.com>]

On Tue, Oct 27, 2020 at 06:15:52PM +0000, Christoph Hellwig wrote:
> On Tue, Oct 20, 2020 at 12:21:50PM -0400, Brian Foster wrote:
> > Ugh, so the above doesn't quite describe historical behavior.
> > block_truncate_page() converts an unwritten block if a page exists
> > (dirty or not), but bails out if a page doesn't exist. We could still do
> > the above, but if we wanted something more intelligent I think we need
> > to check for a page before we get the mapping to know whether we can
> > safely skip an unwritten block or need to write over it. Otherwise if we
> > check for a page within the actor, we have no way of knowing whether
> > there was a (possibly dirty) page that had been written back and/or
> > reclaimed since ->iomap_begin(). If we check for the page first, I think
> > that the iolock/mmaplock in the truncate path ensures that a page can't
> > be added before we complete. We might be able to take that further and
> > check for a dirty || writeback page, but that might be safer as a
> > separate patch. See the (compile tested only) diff below for an idea of
> > what I was thinking.
> 
> The idea looks reasonable, but a few comment below:
> 

JFYI, I had posted an implementation of this idea here[1] and followed
up with some details on a similar COW related issue that was exposed
once the unwritten variant was addressed. I was reasoning about a
slightly different approach that might more clearly facilitate handling
both scenarios, but I think I mentioned to Darrick offline that this all
has me back to preferring the original patch to flush the new EOF block
first, at least as a first step.

I have a couple other fixes (one being the discard_page() patch you've
already commented on) related to iomap and I'm going to be offline for a
few weeks after this week so I'll try to collect them in a series and
get them posted together soon..

Brian

[1] https://lore.kernel.org/linux-fsdevel/20201021133329.1337689-1-bfoster@redhat.com/

> > +struct iomap_trunc_priv {
> > +	bool *did_zero;
> 
> I don't think there is any point on using a pointer here, when we
> can trivially copy out the scalar value.
> 
> > +	bool has_page;
> 
> The naming of this flag really confuses me.  Maybe has_data or
> in_pagecache might be better options?
> 
> > +static loff_t
> > +iomap_truncate_page_actor(struct inode *inode, loff_t pos, loff_t count,
> > +		void *data, struct iomap *iomap, struct iomap *srcmap)
> > +{
> > +	struct iomap_trunc_priv	*priv = data;
> > +	unsigned offset;
> > +	int status;
> > +
> > +	if (srcmap->type == IOMAP_HOLE)
> > +		return count;
> > +	if (srcmap->type == IOMAP_UNWRITTEN && !priv->has_page)
> > +		return count;
> 
> Maybe add a comment here to explain why priv->has_page matters?
> 
> > +
> > +	offset = offset_in_page(pos);
> 
> I'd move this on the initialization line.
> 
> > +	ret = iomap_apply(inode, pos, blocksize - off, IOMAP_ZERO, ops, &priv,
> > +			  iomap_truncate_page_actor);
> > +	if (ret <= 0)
> > +		return ret;
> 
> The check could just be < 0 and would be a little more obvious.
>