Re: [PATCH v12 00/31] Speculative page faults

[Anshuman Khandual]

On 04/16/2019 07:14 PM, Laurent Dufour wrote:
> In pseudo code, this could be seen as:
>     speculative_page_fault()
>     {
> 	    vma = find_vma_rcu()
> 	    check vma sequence count
> 	    check vma's support
> 	    disable interrupt
> 		  check pgd,p4d,...,pte
> 		  save pmd and pte in vmf
> 		  save vma sequence counter in vmf
> 	    enable interrupt
> 	    check vma sequence count
> 	    handle_pte_fault(vma)
> 		    ..
> 		    page = alloc_page()
> 		    pte_map_lock()
> 			    disable interrupt
> 				    abort if sequence counter has changed
> 				    abort if pmd or pte has changed
> 				    pte map and lock
> 			    enable interrupt
> 		    if abort
> 		       free page
> 		       abort

Would not it be better if the 'page' allocated here can be passed on to handle_pte_fault()
below so that in the fallback path it does not have to enter the buddy again ? Of course
it will require changes to handle_pte_fault() to accommodate a pre-allocated non-NULL
struct page to operate on or free back into the buddy if fallback path fails for some
other reason. This will probably make SPF path less overhead for cases where it has to
fallback on handle_pte_fault() after pte_map_lock() in speculative_page_fault().

> 		    ...
> 	    put_vma(vma)
>     }
>     
>     arch_fault_handler()
>     {
> 	    if (speculative_page_fault(&vma))
> 	       goto done
>     again:
> 	    lock(mmap_sem)
> 	    vma = find_vma();
> 	    handle_pte_fault(vma);
> 	    if retry
> 	       unlock(mmap_sem)
> 	       goto again;
>     done:
> 	    handle fault error
>     }

- Anshuman