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madvise.2: Added documentation of MADV_MERGEABLE and MADV_UNMERGEABLE 

These flags (used for Kernel Samepage Mergeing, KSM)
are new in 2.6.32.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2010-06-19 07:36:46 +02:00
parent cf037473e7
commit 5baa8f0928
1 changed files with 49 additions and 2 deletions
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51
man2/madvise.2
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@ -25,8 +25,9 @@
.\" Based on comments from mm/filemap.c. Last modified on 10-06-2001
.\" Modified, 25 Feb 2002, Michael Kerrisk, <mtk.manpages@gmail.com>
.\" Added notes on MADV_DONTNEED
.\" 2010-06-19, mtk, Added documentation of MADV_MERGEABLE and
.\" MADV_UNMERGEABLE
.\" FIXME
.\" 2.6.32 added MADV_MERGEABLE, and MADV_UNMERGEABLE (only if CONFIG_KSM)
.\" 2.6.33 added MADV_SOFT_OFFLINE (only if CONFIG_MEMORY_FAILURE)
.\"
.TH MADVISE 2 2008-04-22 "Linux" "Linux Programmer's Manual"
@ -156,6 +157,42 @@ and the page being unmapped.
This feature is intended for memory testing.
This feature is only available if the kernel was configured with
.BR CONFIG_MEMORY_FAILURE .
.TP
.BR MADV_MERGEABLE " (since Linux 2.6.32)"
Enable Kernel Samepage Merging (KSM) for the pages in the range specified by
.I addr
and
.IR len .
The KSM daemon
.RI ( ksmd )
periodically scans those areas of user memory that have
been marked as mergeable,
looking for pages with identical content.
These are replaced by a single write-protected page (which is automatically
copied if a process later wants to update the content of the page).
KSM only merges private anonymous pages (see
.BR mmap (2)).
The KSM feature is intended for applications that generate many
instances of the same data (e.g., virtualization systems such as KVM).
It can consume a lot of processing power; use with care.
See the kernel source file
.I Documentation/vm/ksm.txt
for more details.
The
.BR MADV_MERGEABLE
and
.BR MADV_UNMERGEABLE
operations are only available if the kernel was configured with
.BR CONFIG_KSM.
.TP
.BR MADV_UNMERGEABLE " (since Linux 2.6.32)"
Undo the effect of an earlier
.BR MADV_MERGEABLE
operation on the specified address range;
KSM unmerges whatever pages it had merged in the address rnage specified by
.IR addr
and
.IR length .
.SH "RETURN VALUE"
On success
.BR madvise ()
@ -189,6 +226,14 @@ is not a valid value
.IP *
The application is attempting to release locked or shared pages (with
.BR MADV_DONTNEED ).
.IP *
.BR MADV_MERGEABLE
or
.BR MADV_UNMERGEABLE
was specified in
.IR advice ,
but the kernel was not configured with
.BR CONFIG_KSM .
.RE
.TP
.B EIO
@ -221,8 +266,10 @@ for file access.
.BR MADV_REMOVE ,
.BR MADV_DONTFORK ,
.BR MADV_DOFORK ,
.BR MAD_HWPOISON ,
.BR MADVISE_MERGEABLE ,
and
.BR MAD_HWPOISON
.BR MADVISE_UNMERGEABLE
are Linux-specific.
.SH NOTES
.SS "Linux Notes"