linux-kernel - Re: [PATCH 4/4] add ksm kernel shared memory driver.

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Date:	Tue, 31 Mar 2009 18:25:25 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, avi@...hat.com,
	chrisw@...hat.com, riel@...hat.com, jeremy@...p.org,
	mtosatti@...hat.com, hugh@...itas.com, corbet@....net,
	yaniv@...hat.com, dmonakhov@...nvz.org
Subject: Re: [PATCH 4/4] add ksm kernel shared memory driver.

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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