linux-kernel - Re: [PATCH] ummunotify: Userspace support for MMU notifications

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Date:	Thu, 15 Apr 2010 15:48:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Roland Dreier <rdreier@...co.com>,
	Håkon Bugge <Haakon.Bugge@....COM>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric B Munson <ebmunson@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
	rolandd@...co.com, pavel@....cz, mingo@...e.hu, jsquyres@...co.com
Subject: Re: [PATCH] ummunotify: Userspace support for MMU notifications

On Wed, 2010-04-14 at 11:52 +0300, Gleb Natapov wrote:
> On Tue, Apr 13, 2010 at 08:02:54PM +0200, Peter Zijlstra wrote:
> > On Tue, 2010-04-13 at 10:57 -0700, Roland Dreier wrote:
> > > Are those system calls the only possible way that virtual to physical
> > > mappings can change?  Can't page migration or something like that
> > > potentially affect things?  And even if you did have hooks into every
> > > system call that mattered (keep in mind that relying on glibc is not
> > > enough, since an MPI application may not use glibc) would decoding them
> > > and figuring out what happened really be preferable to a single event
> > > type that tells you exactly what address range was affected? 
> > 
> > Yeah, virtual<->physical maps can change through swapping, page
> > migration, memory compaction, huge-page aggregation (the latter two not
> > yet being upstream).
> > 
> > Even mlock() doesn't pin virtual<->physical maps.
> Pages registered for RDMA are GUPed so no method above should touch
> them. Fork+cow or unmap/map on the other hand can change
> virtual<->physical maps. GUPed pages are still GUPed, but they are no
> longer mapped into process' virtual address space. MPI copes with
> Fork+cow by marking registered memory as MADV_DONTFORK.

Sure, holding a page-ref will pin the physical page, but that is not
something userspace usually has means to.

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