linux-kernel - Re: [PATCH 5/4] update ksm userspace interfaces

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Message-ID: <20090402151251.GA10392@x200.localdomain>
Date:	Thu, 2 Apr 2009 08:12:51 -0700
From:	Chris Wright <chrisw@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Chris Wright <chrisw@...hat.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, avi@...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 5/4] update ksm userspace interfaces

* Andrea Arcangeli (aarcange@...hat.com) wrote:
> On Wed, Apr 01, 2009 at 10:31:14PM -0700, Chris Wright wrote:
> >   - register only ATM, can add MADV_UNSHAREABLE to allow an app to proactively
> >     unregister, but need a cleanup when ->mm goes away via exit/exec
> 
> The unregister cleanup must happen at the vma level (with unregister
> when vma goes away or is overwritten) for this to provide sane madvise
> semantics (not just in exit/exec, but in unmap/mmap too). Otherwise
> this is all but madvise. Basically we need a chunk of code in core VM
> when KSM=y/m, even if we keep returning -EINVAL when KSM=n (for
> backwards compatibility, -ENOSYS not). Example, vma must be split in
> two if you MAP_SHARABLE only part of it etc...

Yes, of course.  I mentioned that (push whole thing into vma).
Current api is really at ->mm level, it's vma agnostic.  Simply put:
watch for pages in this ->mm between start and start+len and (more or
less regardless of the vma).  To do it purely at the vma level would
mean a vma unmap would cause the watch to go away.  So, question is...do
we need something in ->mm as well (like mlockall)?
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