Re: [PATCH v9 29/51] mm/mprotect, powerpc/mm/pkeys, x86/mm/pkeys: Add sysfs interface

From: Ram Pai
Date: Tue Dec 19 2017 - 11:32:47 EST

On Tue, Dec 19, 2017 at 09:50:24PM +1100, Michael Ellerman wrote:
> Dave Hansen <dave.hansen@xxxxxxxxx> writes:
> > On 11/06/2017 12:57 AM, Ram Pai wrote:
> >> Expose useful information for programs using memory protection keys.
> >> Provide implementation for powerpc and x86.
> >>
> >> On a powerpc system with pkeys support, here is what is shown:
> >>
> >> $ head /sys/kernel/mm/protection_keys/*
> >> ==> /sys/kernel/mm/protection_keys/disable_access_supported <==
> >> true
> >
> > This is cute, but I don't think it should be part of the ABI. Put it in
> > debugfs if you want it for cute tests. The stuff that this tells you
> > can and should come from pkey_alloc() for the ABI.
> Yeah I agree this is not sysfs material.
> In particular the total/usable numbers are completely useless vs other
> threads allocating pkeys out from under you.

The usable number is the minimum number of keys available for use by the
application, not the number of keys **currently** available. Its a
static number.

I am dropping this patch. We can revisit this when a clear request for
such a feature emerges.

> >
> >> Any application wanting to use protection keys needs to be able to
> >> function without them. They might be unavailable because the
> >> hardware that the application runs on does not support them, the
> >> kernel code does not contain support, the kernel support has been
> >> disabled, or because the keys have all been allocated, perhaps by a
> >> library the application is using. It is recommended that
> >> applications wanting to use protection keys should simply call
> >> pkey_alloc(2) and test whether the call succeeds, instead of
> >> attempting to detect support for the feature in any other way.
> >
> > Do you really not have standard way on ppc to say whether hardware
> > features are supported by the kernel? For instance, how do you know if
> > a given set of registers are known to and are being context-switched by
> > the kernel?
> Yes we do, we emit feature bits in the AT_HWCAP entry of the aux vector,
> same as some other architectures.

Ah. I was not aware of this.