linux-kernel - Re: [PATCH] [31/31] HWPOISON: Add a madvise() injector for soft page offlining

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Date:	Sun, 27 Jun 2010 07:30:29 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Michael Kerrisk <mtk.manpages@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] [31/31] HWPOISON: Add a madvise() injector for soft
 page 	offlining

On Sat, Jun 26, 2010 at 09:18:52PM +0800, Michael Kerrisk wrote:
> Hi Fengguang,
> 
> On Sun, Jun 20, 2010 at 9:14 AM, Wu Fengguang <fengguang.wu@...el.com> wrote:
> > On Sun, Jun 20, 2010 at 02:19:35PM +0800, Michael Kerrisk wrote:
> >> Hi Andi,
> >> On Sat, Jun 19, 2010 at 9:52 PM, Andi Kleen <andi@...stfloor.org> wrote:
> >> >> .TP
> >> >> .BR MADV_SOFT_OFFLINE " (Since Linux 2.6.33)
> >> >> Soft offline the pages in the range specified by
> >> >> .I addr
> >> >> and
> >> >> .IR length .
> >> >> This memory of each page in the specified range is copied to a new page,
> >> >
> >> > Actually there are some cases where it's also dropped if it's cached page.
> >> >
> >> > Perhaps better would be something more fuzzy like
> >> >
> >> > "the contents are preserved"
> >>
> >> The problem to me is that this gets so fuzzy that it's hard to
> >> understand the meaning (I imagine many readers will ask: "What does it
> >> mean that the contents are preserved"?). Would you be able to come up
> >> with a wording that is a little miore detailed?
> >
> > That is, MADV_SOFT_OFFLINE won't lose data.
> >
> > If a process writes "1" to some virtual address and then called
> > madvice(MADV_SOFT_OFFLINE) on that virtual address, it can continue
> > to read "1" from that virtual address.
> >
> > MADV_SOFT_OFFLINE "transparently" replaces the underlying physical page
> > frame with a new one that contains the same data "1". The original page
> > frame is offlined, and the new page frame may be installed lazily.
> 
> Thanks. That helps me come up with a description that is I think a bit clearer:
> 
>        MADV_SOFT_OFFLINE (Since Linux 2.6.33)
>               Soft offline the pages in the range specified by
>               addr and length.  The memory of each page in the
>               specified  range  is  preserved (i.e., when next
>               accessed, the same content will be visible,  but
>               in  a new physical page frame), and the original
>               page is offlined  (i.e.,  no  longer  used,  and
>               taken  out  of  normal  memory management).  The
>               effect of  the  MADV_SOFT_OFFLINE  operation  is
>               invisible  to  (i.e., does not change the seman-
>               tics of) the calling process. ...
> 
> The actual patch for man-pages-3.26 is below.

Thanks. The change looks good to me.

Note that the other perceivable change may be a little access delay.
The kernel could choose to simply drop the in-memory data when there
is another copy in disk. When accessed again, the content for the new
physical page will be populated from disk IO.

Thanks,
Fengguang

> 
> --- a/man2/madvise.2
> +++ b/man2/madvise.2
> @@ -163,12 +163,14 @@ Soft offline the pages in the range specified by
>  .I addr
>  and
>  .IR length .
> -The memory of each page in the specified range is copied to a new page,
> +The memory of each page in the specified range is preserved
> +(i.e., when next accessed, the same content will be visible,
> +but in a new physical page frame),
>  and the original page is offlined
>  (i.e., no longer used, and taken out of normal memory management).
>  The effect of the
>  .B MADV_SOFT_OFFLINE
> -operation is normally invisible to (i.e., does not change the semantics of)
> +operation is invisible to (i.e., does not change the semantics of)
>  the calling process.
>  This feature is intended for testing of memory error-handling code;
>  it is only available if the kernel was configured with
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