Re: [Xen-devel] [PATCH] turn off writable page tables

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 26 Jul 2006, at 09:18, Gerd Hoffmann wrote:

> > I'd like to make sure there's no 'dumb stuff' happening, and the
> > writeable pagetables isn't being used erroneously where we don't 
> > expect
> > it (hence crippling the scores), and that its actually functioning as
> > intended i.e. that we get one fault to unhook, and then a fault 
> > causing
> > a rehook once we move to the next page in the fork.
> >
> > If you write a little test program that dirties a large chunk of 
> > memory
> > just before the fork, we should see writeable pagetables winning 
> > easily.
> Just an idea:  Any chance mm_pin() and mm_unpin() cause this?  The bulk
> page table updates for the new process created by fork() are not seen 
> by
> xen anyway I think.  The first schedule of the new process triggers
> pinning, i.e. r/o mapping and verification ...
The batching should still benefit the write-protecting of the parent 
pagetables, which are visible to Xen during fork() (since the fork() 
runs on them!).
Hence the suggestion of dirtying pages before the fork -- that will 
ensure that lots of PTEs are definitely writable, and so they will have 
to be updated to make them read-only.
 -- Keir


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