Re: [PATCH 2/5 v0.6] sched/umcg: RFC: add userspace atomic helpers

[Peter Oskolkov <posk@posk.io>]

Thanks for the review, Thomas!

I'll work on a patch(set) to put this stuff into mm/ somewhere. Let's
see how quickly that can happen...

Thanks,
Peter

On Tue, Sep 28, 2021 at 2:58 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Peter,
>
> On Fri, Sep 17 2021 at 11:03, Peter Oskolkov wrote:
>
> > Add helper functions to work atomically with userspace 32/64 bit values -
> > there are some .*futex.* named helpers, but they are not exactly
> > what is needed for UMCG; I haven't found what else I could use, so I
> > rolled these.
> >
> > At the moment only X86_64 is supported.
> >
> > Note: the helpers should probably go into arch/ somewhere; I have
> > them in kernel/sched/umcg_uaccess.h temporarily for convenience. Please
> > let me know where I should put them.
>
> Again: This does not qualify as a changelog, really.
>
> That aside, you already noticed that there are futex helpers. Your
> reasoning that they can't be reused is only partially correct.
>
> What you named __try_cmpxchg_user_32() is pretty much a verbatim copy of
> X86 futex_atomic_cmpxchg_inatomic(). The only difference is that you placed
> the uaccess_begin()/end() into the inline.
>
> Not going anywhere. You have the bad luck to have the second use case
> for such an infrastucture and therefore you have the honours of mopping
> it up by making it a generic facility which replaces the futex specific
> variant.
>
> Also some of the other instances are just a remix of the futex_op()
> mechanics so your argument is even more weak.
>
> > +static inline int fix_pagefault(unsigned long uaddr, bool write_fault, int bytes)
> > +{
> > +     struct mm_struct *mm = current->mm;
> > +     int ret;
> > +
> > +     /* Validate proper alignment. */
> > +     if (uaddr % bytes)
> > +             return -EINVAL;
>
> Why do you want to make this check _after_ the page fault? Checks
> on user supplied pointers have to be done _before_ trying to access
> them.
>
> > +
> > +     if (mmap_read_lock_killable(mm))
> > +             return -EINTR;
> > +     ret = fixup_user_fault(mm, uaddr, write_fault ? FAULT_FLAG_WRITE : 0,
> > +                     NULL);
> > +     mmap_read_unlock(mm);
> > +
> > +     return ret < 0 ? ret : 0;
> > +}
>
> There is no point in making this inline. Fault handling is not a hotpath
> by any means.
>
> Aside of that it's pretty much what futex.c::fault_in_user_writeable()
> does. So it's pretty obvious to factor this out in the first step into
> mm/gup.c or whatever place the mm people fancy and make the futex code
> use it.
>
> > +/**
> > + * cmpxchg_32_user_nosleep - compare_exchange 32-bit values
> > + *
> > + * Return:
> > + * 0 - OK
> > + * -EFAULT: memory access error
> > + * -EAGAIN: @expected did not match; consult @prev
> > + */
> > +static inline int cmpxchg_user_32_nosleep(u32 __user *uaddr, u32 *old, u32 new)
> > +{
> > +     int ret = -EFAULT;
> > +     u32 __old = *old;
> > +
> > +     if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
> > +             return -EFAULT;
> > +
> > +     pagefault_disable();
> > +
> > +     __uaccess_begin_nospec();
>
> Why exactly do you need _nospec() here? Just to make sure that this code
> is x86 only or just because you happened to copy a x86 implementation
> which uses these nospec() variants?
>
> Again, 90% of this is generic and not at all x86 specific and this
> nospec() thing is very well hidden in the architecture code for a good
> reason while
>
>        if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
>                 return -EFAULT;
>
>         pagefault_disable();
>         ret = user_op(.....);
>         pagefault_enable();
>
> is completely generic and does not have any x86 or other architecture
> dependency in it.
>
> > +     ret = __try_cmpxchg_user_32(old, uaddr, __old, new);
> > +     user_access_end();
> > +
> > +     if (!ret)
> > +             ret =  *old == __old ? 0 : -EAGAIN;
> > +
> > +     pagefault_enable();
> > +     return ret;
> > +}
>
> Aside of that this should go into mm/maccess.c or some other reasonable
> place where people can find it along with other properly named
> _nofault() helpers.
>
> Nothing except the ASM wrappers is architecture specific here. So 90% of
> this can be made generic infrastructure as out of line library code.
>
> And yes, I mean out of line library code unless you can come up with a
> compelling reason backed by actual numbers why this has to be inlined.
>
> May I recommend to read this for inspiration:
>
>   https://lore.kernel.org/lkml/alpine.LFD.2.00.1001251002430.3574@localhost.localdomain/
>
> Thanks,
>
>         tglx