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

[John Hubbard <jhubbard@nvidia.com>]

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