Re: [PATCH v2 11/28] mm: Make compound_pincount always available

[John Hubbard]

On 1/10/22 20:38, Matthew Wilcox wrote:
> On Mon, Jan 10, 2022 at 08:06:54PM -0800, John Hubbard wrote:
> > > +#ifdef CONFIG_64BIT
> > >    	return page[1].compound_nr;
> > > +#else
> > > +	return 1UL << compound_order(page);
> > > +#endif
> >
> > Now that you are highlighting this, I have this persistent feeling (not
> > yet confirmed by any testing) that compound_nr is a micro-optimization
> > that is actually invisible at runtime--but is now slicing up our code
> > with ifdefs, and using space in a fairly valuable location.
> >
> > Not for this patch or series, but maybe a separate patch or series
> > should just remove the compound_nr field entirely, yes? It is
> > surprising to carry around both compound_order and (1 <<
> > compound_order), right next to each other. It would be different if this
> > were an expensive calculation, but it's just a shift.
> >
> > Maybe testing would prove that that's a bad idea, and maybe someone has
> > already looked into it, but I wanted to point it out.
>
> It' probably worth looking at the patch which added it ... 1378a5ee451a
> in August 2020.  I didn't provide any performance numbers, but code size
> definitely went down.

I looked at that, and the lore link for the conversation, but failed to learn
anything additional. Of course if you recall that there was in fact a measurable
performance improvement, then as of now, it's recorded somewhere. :)

It's far from clear whether we'll need or want this space in page[1] in the
future anyway, just wanted to poke at it though.

> > > @@ -52,7 +51,7 @@ static int page_pincount_sub(struct page *page, int refs)
> > >    {
> > >    	VM_BUG_ON_PAGE(page != compound_head(page), page);
> > > -	if (hpage_pincount_available(page))
> > > +	if (PageHead(page))
> >
> > OK, so we just verified (via VM_BUG_ON_PAGE(), which is not always active)
> > that this is not a tail page. And so PageHead() effectively means PageCompound().
> >
> > I wonder if it would be better to just use PageCompound() here and in similar
> > cases. Because that's what is logically being checked, after all. It seems
> > slightly more accurate.
>
> Well PageCompound() is defined as PageHead() || PageTail().  I don't
> think the intent was for people to always ask "Is this a compound page",
> more "This is a good shorthand to replace PageHead() || PageTail()".
> It's kind of moot anyway because this gets replaced with
> folio_test_large() further down the patch series.

OK.

thanks,
--
John Hubbard
NVIDIA