Re: [PATCH] [31/31] HWPOISON: Add a madvise() injector for softpage offlining
From: Wu Fengguang
Date: Sat Jun 26 2010 - 19:30:45 EST
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@xxxxxxxxx> 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@xxxxxxxxxxxxxx> 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.
> --- a/man2/madvise.2
> +++ b/man2/madvise.2
> @@ -163,12 +163,14 @@ Soft offline the pages in the range specified by
> .I addr
> .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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