Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2019-05-25 11:09:15 [-0700], Hugh Dickins wrote:
> On Sat, 25 May 2019, Sebastian Andrzej Siewior wrote:
> > On 2019-05-24 15:22:51 [-0700], Hugh Dickins wrote:
> > > I've now run a couple of hours of load successfully with Mike's patch
> > > to GUP, no problem; but whatever the merits of that patch in general,
> > > I agree with Andrew that fault_in_pages_writeable() seems altogether
> > > more appropriate for copy_fpstate_to_sigframe(), and have now run a
> > > couple of hours of load successfully with this instead (rewrite to taste):
> > 
> > so this patch instead of Mike's GUP patch fixes the issue you observed?
> 
> Yes.
> 
> > Is this just a taste question or limitation of the function in general?
> 
> I'd say it's just a taste question. Though the the fact that your
> usage showed up a bug in the get_user_pages_unlocked() implementation,
> demanding a fix, does indicate that it's a more fragile and complex
> route, better avoided if there's a good simple alternative. If it were
> not already on your slowpath, I'd also argue fault_in_pages_writeable()
> is a more efficient way to do it.

Okay. The GUP functions are not properly documented for my taste. There
is no indication whether or not the mm_sem has to be acquired prior
invoking it. Following the call chain of get_user_pages() I ended up in
__get_user_pages_locked() `locked = NULL' indicated that mm_sem is no
acquired and then I saw this:
|                 if (!locked)
|                         /* VM_FAULT_RETRY couldn't trigger, bypass */
|                         return ret;

kind of suggesting that it is okay to invoke it without holding the
mm_sem prefault. It passed a few tests and then
	https://lkml.kernel.org/r/1556657902.6132.13.camel@lca.pw

happened. After that, I switched to the locked variant and the problem
disappeared (also I noticed that MPX code is invoked within ->mmap()).

> > I'm asking because it has been suggested and is used in MPX code (in the
> > signal path but .mmap) and I'm not aware of any limitation. But as I
> > wrote earlier to akpm, if the MM folks suggest to use this instead I am
> > happy to switch.
> 
> I know nothing of MPX, beyond that Dave Hansen has posted patches to
> remove that support entirely, so I'm surprised arch/x86/mm/mpx.c is
> still in the tree.
I need to poke at that. I has been removed but then KVM folks complained
that they kind of depend on that if it has been exposed to the guest. We
need to fade it out slowly…

>                    But peering at it now, it looks as if it's using
> get_user_pages() while holding mmap_sem, whereas you (sensibly enough)
> used get_user_pages_unlocked() to handle the mmap_sem for you -
> the trouble with that is that since it knows it's in control of
> mmap_sem, it feels free to drop it internally, and that takes it
> down the path of the premature return when pages NULL that Mike is
> fixing. MPX's get_user_pages() is not free to go that way.
oki.

> > > --- 5.2-rc1/arch/x86/kernel/fpu/signal.c
> > > +++ linux/arch/x86/kernel/fpu/signal.c
> > > @@ -3,6 +3,7 @@
> > >   * FPU signal frame handling routines.
> > >   */
> > >  
> > > +#include <linux/pagemap.h>
> > >  #include <linux/compat.h>
> > >  #include <linux/cpu.h>
> > >  
> > > @@ -189,15 +190,7 @@ retry:
> > >  	fpregs_unlock();
> > >  
> > >  	if (ret) {
> > > -		int aligned_size;
> > > -		int nr_pages;
> > > -
> > > -		aligned_size = offset_in_page(buf_fx) + fpu_user_xstate_size;
> > > -		nr_pages = DIV_ROUND_UP(aligned_size, PAGE_SIZE);
> > > -
> > > -		ret = get_user_pages_unlocked((unsigned long)buf_fx, nr_pages,
> > > -					      NULL, FOLL_WRITE);
> > > -		if (ret == nr_pages)
> > > +		if (!fault_in_pages_writeable(buf_fx, fpu_user_xstate_size))
> > >  			goto retry;
> > >  		return -EFAULT;
> > >  	}
> > > 
> > > (I did wonder whether there needs to be an access_ok() check on buf_fx;
> > > but if so, then I think it would already have been needed before the
> > > earlier copy_fpregs_to_sigframe(); but I didn't get deep enough into
> > > that to be sure, nor into whether access_ok() check on buf covers buf_fx.)
> > 
> > There is an access_ok() at the begin of copy_fpregs_to_sigframe(). The
> > memory is allocated from user's stack and there is (later) an
> > access_ok() for the whole region (which can be more than the memory used
> > by the FPU code).
> 
> Yes, but remember I know nothing of this FPU signal code, so I cannot
> tell whether an access_ok(buf, size) is good enough to cover the range
> of an access_ok(buf_fx, fpu_user_xstate_size).

yes, because size >= fpu_user_xstate_size

> Your "(later)" worries me a little - I hope you're not writing first
> and checking the limits later; but what you're doing may be perfectly
> correct, I'm just too far from understanding the details to say; but
> raised the matter because (I think) get_user_pages_unlocked() would
> entail an access_ok() check where fault_in_pages_writable() would not.

no, we first check the range and then write. It is later checked again
after the size has been extended.

> Hugh

Sebastian