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

[Jerome Glisse <jglisse@redhat.com>]

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 :)

[...]

> 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.

[...]