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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
Date: Tue, 3 Feb 2015 12:53:01 +0200	[thread overview]
Message-ID: <20150203105301.GC14259@node.dhcp.inet.fi> (raw)
In-Reply-To: <54D08483.40209@suse.cz>

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

  reply	other threads:[~2015-02-03 10:53 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-02 16:55 Mel Gorman
2015-02-02 22:05 ` Andrew Morton
2015-02-02 22:18   ` Mel Gorman
2015-02-02 22:35     ` Andrew Morton
2015-02-03  0:26       ` Davidlohr Bueso
2015-02-03 10:50       ` Mel Gorman
2015-02-05 21:44     ` Rik van Riel
2015-02-02 22:22 ` Dave Hansen
2015-02-03  8:19   ` MADV_DONTNEED semantics? Was: " Vlastimil Babka
2015-02-03 10:53     ` Kirill A. Shutemov [this message]
2015-02-03 11:42       ` Vlastimil Babka
2015-02-03 16:20         ` Michael Kerrisk (man-pages)
2015-02-04 13:46           ` Vlastimil Babka
2015-02-04 14:00             ` Michael Kerrisk (man-pages)
2015-02-04 17:02               ` Vlastimil Babka
2015-02-04 19:24                 ` Michael Kerrisk (man-pages)
2015-02-05  1:07                   ` Minchan Kim
2015-02-06 15:41                     ` Michael Kerrisk (man-pages)
2015-02-09  6:46                       ` Minchan Kim
2015-02-09  9:13                         ` Michael Kerrisk (man-pages)
2015-02-05 15:41                   ` Michal Hocko
2015-02-06 15:57                     ` Michael Kerrisk (man-pages)
2015-02-06 20:45                       ` Michal Hocko
2015-02-09  6:50                       ` Minchan Kim
2015-02-04  0:09         ` Minchan Kim
2015-02-03 11:16     ` Mel Gorman
2015-02-03 15:21       ` Michal Hocko
2015-02-03 16:25         ` Michael Kerrisk (man-pages)
2015-02-03  9:47   ` Mel Gorman
2015-02-03 10:47     ` Kirill A. Shutemov
2015-02-03 11:21       ` Mel Gorman

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