linux-kernel - Re: [PATCH] mm: vmscan: correct nr

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Date:   Fri, 10 May 2019 16:06:20 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>, hannes@...xchg.org,
        mhocko@...e.com, mgorman@...hsingularity.net,
        kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: vmscan: correct nr_reclaimed for THP

On Fri, May 10, 2019 at 03:54:56PM -0700, Ira Weiny wrote:
> On Fri, May 10, 2019 at 09:36:12AM -0700, Matthew Wilcox wrote:
> > On Fri, May 10, 2019 at 10:12:40AM +0800, Huang, Ying wrote:
> > > > +		nr_reclaimed += (1 << compound_order(page));
> > > 
> > > How about to change this to
> > > 
> > > 
> > >         nr_reclaimed += hpage_nr_pages(page);
> > 
> > Please don't.  That embeds the knowledge that we can only swap out either 
> > normal pages or THP sized pages.  I'm trying to make the VM capable of 
> > supporting arbitrary-order pages, and this would be just one more place
> > to fix.
> > 
> > I'm sympathetic to the "self documenting" argument.  My current tree has
> > a patch in it:
> > 
> >     mm: Introduce compound_nr
> >     
> >     Replace 1 << compound_order(page) with compound_nr(page).  Minor
> >     improvements in readability.
> > 
> > It goes along with this patch:
> > 
> >     mm: Introduce page_size()
> > 
> >     It's unnecessarily hard to find out the size of a potentially huge page.
> >     Replace 'PAGE_SIZE << compound_order(page)' with page_size(page).
> > 
> > Better suggestions on naming gratefully received.  I'm more happy with 
> > page_size() than I am with compound_nr().  page_nr() gives the wrong
> > impression; page_count() isn't great either.
> 
> Stupid question : what does 'nr' stand for?

NumbeR.  It's relatively common argot in the Linux kernel (as you can
see from the earlier example ...

> > >         nr_reclaimed += hpage_nr_pages(page);

willy@...o:~/kernel/xarray-2$ git grep -w nr mm |wc -l
388
willy@...o:~/kernel/xarray-2$ git grep -w nr fs |wc -l
1067