Re: [PATCH] ummunotify: Userspace support for MMU notifications

[Peter Zijlstra <peterz@infradead.org>]

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.