linux-kernel - Re: [PATCH] mm: Add more comments for MADV

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Date:   Tue, 10 Mar 2020 22:08:54 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH] mm: Add more comments for MADV_FREE

On Wed, 11 Mar 2020, Huang, Ying wrote:

> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 6f2fef7b0784..01144dd02a5f 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -9,10 +9,11 @@
>   * page_is_file_cache - should the page be on a file LRU or anon LRU?
>   * @page: the page to test
>   *
> - * Returns 1 if @page is page cache page backed by a regular filesystem,
> - * or 0 if @page is anonymous, tmpfs or otherwise ram or swap backed.
> - * Used by functions that manipulate the LRU lists, to sort a page
> - * onto the right LRU list.
> + * Returns 1 if @page is page cache page backed by a regular filesystem or
> + * anonymous page lazily freed (e.g. via MADV_FREE).  Returns 0 if @page is
> + * normal anonymous page, tmpfs or otherwise ram or swap backed.  Used by
> + * functions that manipulate the LRU lists, to sort a page onto the right LRU
> + * list.

The function name is misleading: anonymous pages that can be lazily freed 
are not file cache.  This returns 1 because of the question it is asking: 
anonymous lazily freeable pages should be on the file lru, not the anon 
lru.  So before adjusting the comment I'd suggest renaming the function to 
something like page_is_file_lru().

The rest looks fine.