Re: Why doesn't zap_pte_range() call page_mkwrite()

[Nick Piggin <npiggin@suse.de>]

On Tue, Sep 08, 2009 at 12:31:49PM -0400, Chris Mason wrote:
> On Tue, Sep 08, 2009 at 05:41:32PM +0200, Nick Piggin wrote:
> > It hasn't fallen completely off my radar. fsblock has the same issue
> > (although I've just been ignoring gup writes into fsblock fs for the
> > time being).
> 
> Ok, I'll change my detection code a bit then.

OK.


> > I have a basic idea of what to do... It would be nice to change calling
> > convention of get_user_pages and take the page lock. Database people might
> > scream, in which case we could only take the page lock for filesystems that
> > define ->page_mkwrite (so shared mem segments avoid the overhead). Lock
> > ordering might get a bit interesting, but if we can have callers ensure they
> > always submit and release partially fulfilled requirests, then we can always
> > trylock them.
> 
> I think everyone will have page_mkwrite eventually, at least everyone
> who the databases will care about ;)

Ah, the problem is not where the DIO write goes, it's where the read
goes :) (ie. the read writes into get_user_pages pages).

So for databases this should typically be shared memory segments I'd
say (tmpfs), or maybe anonymous memory.