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Date: Tue, 31 Mar 2009 18:25:25 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Izik Eidus <ieidus@redhat.com>, linux-kernel@vger.kernel.org,
       kvm@vger.kernel.org, linux-mm@kvack.org, avi@redhat.com,
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       mtosatti@redhat.com, hugh@veritas.com, corbet@lwn.net, yaniv@redhat.com,
       dmonakhov@openvz.org
Subject: Re: [PATCH 4/4] add ksm kernel shared memory driver.
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On Tue, Mar 31, 2009 at 10:54:57AM -0500, Anthony Liguori wrote:
> You can still disable ksm and simply return ENOSYS for the MADV_ flag.  You 

-EINVAL if something, -ENOSYS would tell userland that it shall stop
trying to use madvise, including the other MADV_ too.

> could even keep it as a module if you liked by separating the madvise bits 
> from the ksm bits.  The madvise() bits could just provide the tracking 
> infrastructure for determine which vmas were currently marked as sharable.
> You could then have ksm as loadable module that consumed that interface to 
> then perform scanning.

What's the point of making ksm a module if one has part of ksm code
loaded in the kernel and not being possible to avoid compiling in?
People that says KSM=N in their .config (like embedded running with 1M
of ram), don't want that tracking overhead compiled into the kernel.

Returning -EINVAL would be an option but again I think madvise is core
syscall for SuS and I don't like that those core VM parts returns
-EINVAL at will depend on certain kernel modules being loaded.

> A number of MADV_ flags are Linux specific (like 
> MADV_DOFORK/MADV_DONTFORK).

But those aren't kernel module related, so they're in line with the
standard ones and could be adapted by other OS.

KSM is not a core VM functionality, madvise is a core VM
functionality, so I don't see fit. KSM as ioctl or KSM creating
/proc/<pid>/ksm when loaded, sounds fine to me instead. If open of
either one fails, application won't register in. It's up to you to
choose KSM=M/N, if you want it as core functionality just build as
KSM=Y but leave the option to others to save memory.
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