Re: [PATCH rfc v2 05/10] powerpc: mm: use try_vma_locked_page_fault()

[Kefeng Wang]

On 2023/8/22 17:38, Christophe Leroy wrote:
> Le 21/08/2023 à 14:30, Kefeng Wang a écrit :
> > Use new try_vma_locked_page_fault() helper to simplify code.
> > No functional change intended.
>
> Does it really simplifies code ? It's 32 insertions versus 34 deletions
> so only removing 2 lines.

Yes，it is unfriendly for powerpc as the arch's vma access check is much
complex than other arch,
> I don't like the struct vm_fault you are adding because when it was four
> independant variables it was handled through local registers. Now that
> it is a struct it has to go via the stack, leading to unnecessary memory
> read and writes. And going back and forth between architecture code and
> generic code may also be counter-performant.

Because different arch has different var to check vma access, so the
easy way to add them into vmf, I don' find a better way.
> Did you make any performance analysis ? Page faults are really a hot
> path when dealling with minor faults.

no, this is only built and rfc to see the feedback about the conversion.

Thanks.

> Thanks
> Christophe
>
> > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > ---
> >    arch/powerpc/mm/fault.c | 66 ++++++++++++++++++++---------------------
> >    1 file changed, 32 insertions(+), 34 deletions(-)
> >
> > diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> > index b1723094d464..52f9546e020e 100644
> > --- a/arch/powerpc/mm/fault.c
> > +++ b/arch/powerpc/mm/fault.c
> > @@ -391,6 +391,22 @@ static int page_fault_is_bad(unsigned long err)
> >    #define page_fault_is_bad(__err)	((__err) & DSISR_BAD_FAULT_32S)
> >    #endif
> >    
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +bool arch_vma_access_error(struct vm_area_struct *vma, struct vm_fault *vmf)
> > +{
> > +	int is_exec = TRAP(vmf->regs) == INTERRUPT_INST_STORAGE;
> > +	int is_write = page_fault_is_write(vmf->fault_code);
> > +
> > +	if (unlikely(access_pkey_error(is_write, is_exec,
> > +				(vmf->fault_code & DSISR_KEYFAULT), vma)))
> > +		return true;
> > +
> > +	if (unlikely(access_error(is_write, is_exec, vma)))
> > +		return true;
> > +	return false;
> > +}
> > +#endif
> > +
> >    /*
> >     * For 600- and 800-family processors, the error_code parameter is DSISR
> >     * for a data fault, SRR1 for an instruction fault.
> > @@ -407,12 +423,18 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
> >    {
> >    	struct vm_area_struct * vma;
> >    	struct mm_struct *mm = current->mm;
> > -	unsigned int flags = FAULT_FLAG_DEFAULT;
> >    	int is_exec = TRAP(regs) == INTERRUPT_INST_STORAGE;
> >    	int is_user = user_mode(regs);
> >    	int is_write = page_fault_is_write(error_code);
> >    	vm_fault_t fault, major = 0;
> >    	bool kprobe_fault = kprobe_page_fault(regs, 11);
> > +	struct vm_fault vmf = {
> > +		.real_address = address,
> > +		.fault_code = error_code,
> > +		.regs = regs,
> > +		.flags = FAULT_FLAG_DEFAULT,
> > +	};
> > +
> >    
> >    	if (unlikely(debugger_fault_handler(regs) || kprobe_fault))
> >    		return 0;
> > @@ -463,45 +485,21 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
> >    	 * mmap_lock held
> >    	 */
> >    	if (is_user)
> > -		flags |= FAULT_FLAG_USER;
> > +		vmf.flags |= FAULT_FLAG_USER;
> >    	if (is_write)
> > -		flags |= FAULT_FLAG_WRITE;
> > +		vmf.flags |= FAULT_FLAG_WRITE;
> >    	if (is_exec)
> > -		flags |= FAULT_FLAG_INSTRUCTION;
> > +		vmf.flags |= FAULT_FLAG_INSTRUCTION;
> >    
> > -	if (!(flags & FAULT_FLAG_USER))
> > -		goto lock_mmap;
> > -
> > -	vma = lock_vma_under_rcu(mm, address);
> > -	if (!vma)
> > -		goto lock_mmap;
> > -
> > -	if (unlikely(access_pkey_error(is_write, is_exec,
> > -				       (error_code & DSISR_KEYFAULT), vma))) {
> > -		vma_end_read(vma);
> > -		goto lock_mmap;
> > -	}
> > -
> > -	if (unlikely(access_error(is_write, is_exec, vma))) {
> > -		vma_end_read(vma);
> > -		goto lock_mmap;
> > -	}
> > -
> > -	fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> > -	if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> > -		vma_end_read(vma);
> > -
> > -	if (!(fault & VM_FAULT_RETRY)) {
> > -		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> > +	fault = try_vma_locked_page_fault(&vmf);
> > +	if (fault == VM_FAULT_NONE)
> > +		goto retry;
> > +	if (!(fault & VM_FAULT_RETRY))
> >    		goto done;
> > -	}
> > -	count_vm_vma_lock_event(VMA_LOCK_RETRY);
> >    
> >    	if (fault_signal_pending(fault, regs))
> >    		return user_mode(regs) ? 0 : SIGBUS;
> >    
> > -lock_mmap:
> > -
> >    	/* When running in the kernel we expect faults to occur only to
> >    	 * addresses in user space.  All other faults represent errors in the
> >    	 * kernel and should generate an OOPS.  Unfortunately, in the case of an
> > @@ -528,7 +526,7 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
> >    	 * make sure we exit gracefully rather than endlessly redo
> >    	 * the fault.
> >    	 */
> > -	fault = handle_mm_fault(vma, address, flags, regs);
> > +	fault = handle_mm_fault(vma, address, vmf.flags, regs);
> >    
> >    	major |= fault & VM_FAULT_MAJOR;
> >    
> > @@ -544,7 +542,7 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
> >    	 * case.
> >    	 */
> >    	if (unlikely(fault & VM_FAULT_RETRY)) {
> > -		flags |= FAULT_FLAG_TRIED;
> > +		vmf.flags |= FAULT_FLAG_TRIED;
> >    		goto retry;
> >    	}
> >