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

[Matthew Wilcox]

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.

> > @@ -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.