Re: faults and signals

[Nick Piggin]

Benjamin Herrenschmidt wrote:
> OK, so we made some good progress... now remains my pet issue... faults
> and signals :)
>
> So in SPUfs, I have cases where apps trying to access user-space
> registers of an SPE that is scheduled out might end up blocking a long
> time in the page fault handler. I'm not 100% sure about DRM at this
> point but I suppose they might have good use of a similar ability I'm
> trying to provide which is for a page fault to be interruptible. That
> would allow various cases of processes stuck in kernel for a logn time
> (or forever if something goes wrong).
>
> I think your new fault() thingy is the perfect way to get there. In the
> normal page fault case, a signal is easy to handle: just refault
> (NOPAGE_REFAULT without your patch or return NULL; with your patch,
> though we might want to define a -EINTR result explicitely) and the
> signals will be handled on the return to userland path. However, we
> can't really handle them in get_user_pages() nor on kernel own faults
> (__get_user etc...), at least not until we define versions of these that
> can return -EINTR (we might want to do that for get_user_pages, but
> let's assume not for now).
>
> Thus what is needed is a way to inform the page fault handler wether it
> can be interruptible or not. This could be done using the flags you have
> in there, or some other bits in the argument structure.

Yep, the flags field should be able to do that for you. Since we have
the handle_mm_fault wrapper for machine faults, it isn't too hard to
change the arguments: we should probably turn `write_access` into a
flag so we don't have to push too many arguments onto the stack.

This way we can distinguish get_user_pages faults. And your
architecture will have to switch over to using __handle_mm_fault, and
distinguish kernel faults. Something like that?

> That way, faults could basically check if coming from userland (testing
> the ptregs) and set interruptible in that case (and possibly a flag to
> get_user_pages() telling it can be interruptible for use by drivers who
> can deal with it).
>
> At the vm_ops level, existing things are fine, they are not
> interruptible, and I can modify spufs to check that new flag and return
> -EINTR on signals when waiting.
>
> In fact, it might be that filemap and some filesystems might even want
> to handle interruptible page faults :) but that's a different matter.

--
SUSE Labs, Novell Inc.
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