Date: Tue, 3 Feb 2015 12:53:01 +0200
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
        linux-mm@kvack.org, Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        mtk.manpages@gmail.com, linux-man@vger.kernel.org
Subject: Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore
 repeated MADV_DONTNEED hints
Message-ID: <20150203105301.GC14259@node.dhcp.inet.fi>
References: <20150202165525.GM2395@suse.de>
 <54CFF8AC.6010102@intel.com>
 <54D08483.40209@suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <54D08483.40209@suse.cz>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1985
Lines: 42

On Tue, Feb 03, 2015 at 09:19:15AM +0100, Vlastimil Babka wrote:
> [CC linux-api, man pages]
> 
> On 02/02/2015 11:22 PM, Dave Hansen wrote:
> > On 02/02/2015 08:55 AM, Mel Gorman wrote:
> >> This patch identifies when a thread is frequently calling MADV_DONTNEED
> >> on the same region of memory and starts ignoring the hint. On an 8-core
> >> single-socket machine this was the impact on ebizzy using glibc 2.19.
> > 
> > The manpage, at least, claims that we zero-fill after MADV_DONTNEED is
> > called:
> > 
> >>      MADV_DONTNEED
> >>               Do  not  expect  access in the near future.  (For the time being, the application is finished with the given range, so the kernel can free resources
> >>               associated with it.)  Subsequent accesses of pages in this range will succeed, but will result either in reloading of the memory contents  from  the
> >>               underlying mapped file (see mmap(2)) or zero-fill-on-demand pages for mappings without an underlying file.
> > 
> > So if we have anything depending on the behavior that it's _always_
> > zero-filled after an MADV_DONTNEED, this will break it.
> 
> OK, so that's a third person (including me) who understood it as a zero-fill
> guarantee. I think the man page should be clarified (if it's indeed not
> guaranteed), or we have a bug.
> 
> The implementation actually skips MADV_DONTNEED for
> VM_LOCKED|VM_HUGETLB|VM_PFNMAP vma's.

It doesn't skip. It fails with -EINVAL. Or I miss something.

> - The word "will result" did sound as a guarantee at least to me. So here it
> could be changed to "may result (unless the advice is ignored)"?

It's too late to fix documentation. Applications already depends on the
beheviour.

-- 
 Kirill A. Shutemov
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/