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

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Date:   Fri, 10 May 2019 09:36:12 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     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 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.