Re: [patch 3/3] mm: fault handler to replace nopage and populate

[Thomas Hellström <thomas@tungstengraphics.com>]

Benjamin Herrenschmidt wrote:

> > > Could it be an option to make it safe for the fault handler to 
> > > temporarily drop the mmap_sem read lock given that some conditions TBD 
> > > are met?
> > > In that case it can retake the mmap_sem write lock, do the VMA flags 
> > > modifications, downgrade and do the pte modifications using a helper, or 
> > > even use remap_pfn_range() during the time the write lock is held?
> > >      
> > When you drop and retake the mmap_sem, you need to start again from
> > find_vma. At which point you technically probably want to start again
> > from the architecture specfic fault code. It sounds difficult but I
> > won't say it can't be done.
> >    
>
> I can be done with returning NOPAGE_REFAULT but as you said, I don't
> think it's necessary.
>  
Still, even with NOPAGE_REFAULT or the equivalent with the new fault() code,
in the case we need to take this route, (and it looks like we won't have 
to),
I guess we still need to restart from find_vma() in the fault()/nopage() 
handler to make sure the VMA is still present. The object mutex need to 
be dropped as well to avoid deadlocks. Sounds complicated.

> Cheers,
> Ben.
>
>
>  
/Thomas

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