Re: [PATCH v5 27/32] PCI/P2PDMA: Convert to printbuf

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Aug 08, 2022 at 02:42:03PM -0400, Kent Overstreet wrote:
> On 8/8/22 13:51, Bjorn Helgaas wrote:
> > I don't object to the patch, but it would be nice if the commit log
> > hinted at what the advantage is.  I assume it's faster/safer/better in
> > some way, but I have no idea what.
> 
> Printbufs have some additional features over seq_buf but they're not used
> here. The main one you might be interested in is heap allocation: that means
> no need to statically allocate buffers on the stack and no need to calculate
> the buffer size, printbufs will reallocate as necessary.

This doesn't tell me what the advantage of this patch is, since I
don't think *this* patch uses heap allocation.

As far as I can tell, this particular patch doesn't improve safety or
readability, so "convert X to Y even though we don't use any fancy Y
features" is a pointless message.

But if printbufs are better than seq_buf overall, and converting this
gets us closer to the goal of removing seq_buf completely, that's a
perfectly acceptable reason.  Just say that.

> I generally haven't been converting code to use that unless it's obvious
> that we're in a context where it's safe to allocate memory and can deal with
> allocation failures.
> 
> I notice that in calc_map_type_and_dist() you're using xa_store() which can
> fail, but you're not checking for that or returning errors properly :)
> perhaps a fix for that could also switch to using printbuf in
> heap-allocation mode.
> 
> > Also, cpu_show_meltdown() doesn't appear in this patch, so maybe
> > that's relevant to some other patch but not this one?
> 
> Whoops, was copying the commit message from another patch, yeah.n