Re: [PATCH v2 02/26] mm: userfault: return VM_FAULT_RETRY on signals

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 21, 2019 at 10:29:56AM -0500, Jerome Glisse wrote:
> On Tue, Feb 12, 2019 at 10:56:08AM +0800, Peter Xu wrote:
> > The idea comes from the upstream discussion between Linus and Andrea:
> > 
> >   https://lkml.org/lkml/2017/10/30/560
> > 
> > A summary to the issue: there was a special path in handle_userfault()
> > in the past that we'll return a VM_FAULT_NOPAGE when we detected
> > non-fatal signals when waiting for userfault handling.  We did that by
> > reacquiring the mmap_sem before returning.  However that brings a risk
> > in that the vmas might have changed when we retake the mmap_sem and
> > even we could be holding an invalid vma structure.
> > 
> > This patch removes the special path and we'll return a VM_FAULT_RETRY
> > with the common path even if we have got such signals.  Then for all
> > the architectures that is passing in VM_FAULT_ALLOW_RETRY into
> > handle_mm_fault(), we check not only for SIGKILL but for all the rest
> > of userspace pending signals right after we returned from
> > handle_mm_fault().  This can allow the userspace to handle nonfatal
> > signals faster than before.
> > 
> > This patch is a preparation work for the next patch to finally remove
> > the special code path mentioned above in handle_userfault().
> > 
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> See maybe minor improvement
> 
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> 
> [...]
> 
> > diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> > index 58f69fa07df9..c41c021bbe40 100644
> > --- a/arch/arm/mm/fault.c
> > +++ b/arch/arm/mm/fault.c
> > @@ -314,12 +314,12 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
> >  
> >  	fault = __do_page_fault(mm, addr, fsr, flags, tsk);
> >  
> > -	/* If we need to retry but a fatal signal is pending, handle the
> > +	/* If we need to retry but a 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. */
> > -	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
> > -		if (!user_mode(regs))
> > +	if (unlikely(fault & VM_FAULT_RETRY && signal_pending(current))) {
> 
> I rather see (fault & VM_FAULT_RETRY) ie with the parenthesis as it
> avoids the need to remember operator precedence rules :)

Yes it's good practise.  I've been hit by the lock_page() days ago
already so I think I'll remember (though this patch was earlier :)

I'll fix all the places in the patch.  Actually I noticed that there
are four of them.  And I've taken the r-b after the changes.  Thanks,

> 
> [...]
> 
> > diff --git a/arch/nds32/mm/fault.c b/arch/nds32/mm/fault.c
> > index 68d5f2a27f38..9f6e477b9e30 100644
> > --- a/arch/nds32/mm/fault.c
> > +++ b/arch/nds32/mm/fault.c
> > @@ -206,12 +206,12 @@ void do_page_fault(unsigned long entry, unsigned long addr,
> >  	fault = handle_mm_fault(vma, addr, flags);
> >  
> >  	/*
> > -	 * If we need to retry but a fatal signal is pending, handle the
> > +	 * If we need to retry but a 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.
> >  	 */
> > -	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
> > -		if (!user_mode(regs))
> > +	if (fault & VM_FAULT_RETRY && signal_pending(current)) {
> 
> Same as above parenthesis maybe.
> 
> [...]
> 
> > diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
> > index 0e8b6158f224..09baf37b65b9 100644
> > --- a/arch/um/kernel/trap.c
> > +++ b/arch/um/kernel/trap.c
> > @@ -76,8 +76,11 @@ int handle_page_fault(unsigned long address, unsigned long ip,
> >  
> >  		fault = handle_mm_fault(vma, address, flags);
> >  
> > -		if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> > +		if (fault & VM_FAULT_RETRY && signal_pending(current)) {
> 
> Same as above parenthesis maybe.
> 
> [...]

Regards,

-- 
Peter Xu