ARC: Improve handling of fatal signals in do_page_fault()

This was triggered by investigation of a deadlock after OOM killer invocation,
see [1] for more details.

Looks like our handling of fatal signal in do_page_fault() has some issues:

1. We only want to do special (read "early") handling of fatal signal
   if handle_mm_fault() returned VM_FAULT_RETRY so that we don't loop
   in retry loop endlessly, otherwise we'll handle that signal normally
   on exit from exception handler.

2. up_read() is not needed as indeed it will be done in __lock_page_or_retry()
   in mm/filemap.c.

With above comments in mind simplified version should be like that:
------------------------------->8---------------------------
	if (fatal_signal_pending(current)
		if (fault & VM_FAULT_RETRY)
			if (user_mode(regs))
				return;
------------------------------->8---------------------------

But looks like there's a room for improvement, see [2].
Instead of proceeding forward and then inevitably hitting retry path we
short-cut right to kernel fix-up code in no_context.

[1] http://lists.infradead.org/pipermail/linux-snps-arc/2018-February/003403.html
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=746a272e44141af24a02f6c9b0f65f4c4598ed42

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
---
 arch/arc/mm/fault.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Hi Vineet,

On Fri, 2018-06-29 at 11:20 -0700, Alexey Brodkin wrote:
> This was triggered by investigation of a deadlock after OOM killer invocation,
> see [1] for more details.
> 
> Looks like our handling of fatal signal in do_page_fault() has some issues:
> 
> 1. We only want to do special (read "early") handling of fatal signal
>    if handle_mm_fault() returned VM_FAULT_RETRY so that we don't loop
>    in retry loop endlessly, otherwise we'll handle that signal normally
>    on exit from exception handler.
> 
> 2. up_read() is not needed as indeed it will be done in __lock_page_or_retry()
>    in mm/filemap.c.
> 
> With above comments in mind simplified version should be like that:
> ------------------------------->8---------------------------
> 	if (fatal_signal_pending(current)
> 		if (fault & VM_FAULT_RETRY)
> 			if (user_mode(regs))
> 				return;
> ------------------------------->8---------------------------
> 
> But looks like there's a room for improvement, see [2].
> Instead of proceeding forward and then inevitably hitting retry path we
> short-cut right to kernel fix-up code in no_context.
> 
> [1] http://lists.infradead.org/pipermail/linux-snps-arc/2018-February/003403.html
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=746a272e44141af24a02f6c9b0f65f4c4598ed42
> 
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

I should have put that in commit message right away:)

This patch fixes OOM lock-up we were seeing previously,
see STAR 9001304674 "OOM killer hangs system".

-Alexey

Hi Alexey,

I was finally forced to revisit this for my glibc tst-tls3-malloc deadlock. And
indeed with this change we don'tsee the deadlock. But see below..


> @@ -139,12 +139,16 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
>  	 */
>  	fault = handle_mm_fault(vma, address, flags);
>  
> -	/* If Pagefault was interrupted by SIGKILL, exit page fault "early" */
> +	/* If we need to retry but a fatal signal is pending, handle the
> +	 * signal first. We do not need to release the mmap_sem because
> +	 * it would already be released in __lock_page_or_retry in
> +	 * mm/filemap.c. */

Right and we were already doing that: up_read() was called for !VM_FAULT_RETRY
meaning we relied on the core mm to do that already for VM_FAULT_RETRY case.

The issue here was additional check for VM_FAULT_ERROR. Typically this is not set
by handle_mm_fault() meaning for common user faults with signal pending, we were
not calling up_read, hence the ensuing deadlock.


>  	if (unlikely(fatal_signal_pending(current))) {
> -		if ((fault & VM_FAULT_ERROR) && !(fault & VM_FAULT_RETRY))
> -			up_read(&mm->mmap_sem);
> -		if (user_mode(regs))
> +		if (fault & VM_FAULT_RETRY) {
> +			if (!user_mode(regs))
> +				goto no_context;

Given this code is really tricky, lets only solve one problem with 1 one patch.

>  			return;
> +		}
>  	}

The fault handling is spaghetti mess of checks and more checks and has not really
been touched since upstreaming. I need to clean it up and essentially rewrite it
for v4.19

Hi Vineet,

On Wed, 2018-08-01 at 12:49 -0700, Vineet Gupta wrote:
> Hi Alexey,
> 
> I was finally forced to revisit this for my glibc tst-tls3-malloc deadlock. And
> indeed with this change we don'tsee the deadlock. But see below..
> 
> 
> > @@ -139,12 +139,16 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
> >  	 */
> >  	fault = handle_mm_fault(vma, address, flags);
> >  
> > -	/* If Pagefault was interrupted by SIGKILL, exit page fault "early" */
> > +	/* If we need to retry but a fatal signal is pending, handle the
> > +	 * signal first. We do not need to release the mmap_sem because
> > +	 * it would already be released in __lock_page_or_retry in
> > +	 * mm/filemap.c. */
> 
> Right and we were already doing that: up_read() was called for !VM_FAULT_RETRY
> meaning we relied on the core mm to do that already for VM_FAULT_RETRY case.
> 
> The issue here was additional check for VM_FAULT_ERROR. Typically this is not set
> by handle_mm_fault() meaning for common user faults with signal pending, we were
> not calling up_read, hence the ensuing deadlock.

Right.

> >  	if (unlikely(fatal_signal_pending(current))) {
> > -		if ((fault & VM_FAULT_ERROR) && !(fault & VM_FAULT_RETRY))
> > -			up_read(&mm->mmap_sem);
> > -		if (user_mode(regs))
> > +		if (fault & VM_FAULT_RETRY) {
> > +			if (!user_mode(regs))
> > +				goto no_context;
> 
> Given this code is really tricky, lets only solve one problem with 1 one patch.

Agree.

> >  			return;
> > +		}
> >  	}
> 
> The fault handling is spaghetti mess of checks and more checks and has not really
> been touched since upstreaming. I need to clean it up and essentially rewrite it
> for v4.19

So would you like me to send a re-spin with less changes as discussed above so
we have something better for now and for back-porting to stable branches.

Or you're going to rewrite all that sometime soon yourself?

-Alexey

On 08/02/2018 12:08 AM, Alexey Brodkin wrote:
>> The fault handling is spaghetti mess of checks and more checks and has not really
>> been touched since upstreaming. I need to clean it up and essentially rewrite it
>> for v4.19
> So would you like me to send a re-spin with less changes as discussed above so
> we have something better for now and for back-porting to stable branches.

No that's fine, I'll fix it up here - minimally for the stable backports and do
the rewrite later.

BTW I tried to reproduce the Old OOM hangs from LTP (STARS 9001304674, 9001281305)
to prove that this fix solves those too, but I can't seem to reproduce the same
deadlock without this fix on 4.17 kernel.

-Vineet