Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints

From: Michal Hocko <mhocko@suse.cz>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	linux-man <linux-man@vger.kernel.org>,
	Hugh Dickins <hughd@google.com>
Subject: Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints
Date: Fri, 6 Feb 2015 21:45:36 +0100	[thread overview]
Message-ID: <20150206204536.GA24245@dhcp22.suse.cz> (raw)
In-Reply-To: <54D4E47E.4020509@gmail.com>

On Fri 06-02-15 16:57:50, Michael Kerrisk wrote:
[...]
> > Yes, this wording is better because many users are not aware of
> > MAP_ANON|MAP_SHARED being file backed in fact and mmap man page doesn't
> > mention that.
> 
> (Michal, would you have a text to propose to add to the mmap(2) page?
> Maybe it would be useful to add something there.)

I am half way on vacation, but I can cook a patch after I am back after
week.

> > I am just wondering whether it makes sense to mention that MADV_DONTNEED
> > for shared mappings might be surprising and not freeing the backing
> > pages thus not really freeing memory until there is a memory
> > pressure. But maybe this is too implementation specific for a man
> > page. What about the following wording on top of yours?
> > "
> > Please note that the MADV_DONTNEED hint on shared mappings might not
> > lead to immediate freeing of pages in the range. The kernel is free to
> > delay this until an appropriate moment. RSS of the calling process will
> > be reduced however.
> > "
> 
> Thanks! I added this, but dropped in the word "immediately" in the last 
> sentence, since I assume that was implied. So now we have:
> 
>               After  a  successful MADV_DONTNEED operation, the seman‐
>               tics of  memory  access  in  the  specified  region  are
>               changed:  subsequent accesses of pages in the range will
>               succeed, but will result in either repopulating the mem‐
>               ory  contents from the up-to-date contents of the under‐
>               lying mapped file  (for  shared  file  mappings,  shared
>               anonymous  mappings,  and shmem-based techniques such as
>               System V shared memory segments) or  zero-fill-on-demand
>               pages for anonymous private mappings.
> 
>               Note  that,  when applied to shared mappings, MADV_DONT‐
>               NEED might not lead to immediate freeing of the pages in
>               the  range.   The  kernel  is  free to delay freeing the
>               pages until an appropriate  moment.   The  resident  set
>               size  (RSS)  of  the calling process will be immediately
>               reduced however.

This sounds good to me and it is definitely much better than the current
state. Thanks!

> The current draft of the page can be found in a branch,
> http://git.kernel.org/cgit/docs/man-pages/man-pages.git/log/?h=draft_madvise
> 
> Thanks,
> 
> Michael
> 
> 
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/

-- 
Michal Hocko
SUSE Labs