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

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Date:   Tue, 23 Apr 2019 17:05:02 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>, akpm@...ux-foundation.org,
        mhocko@...nel.org, peterz@...radead.org, kirill@...temov.name,
        ak@...ux.intel.com, dave@...olabs.net, jack@...e.cz,
        Matthew Wilcox <willy@...radead.org>,
        aneesh.kumar@...ux.ibm.com, benh@...nel.crashing.org,
        mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        sergey.senozhatsky.work@...il.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        vinayak menon <vinayakm.list@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        zhong jiang <zhongjiang@...wei.com>,
        Haiyan Song <haiyanx.song@...el.com>,
        Balbir Singh <bsingharora@...il.com>, sj38.park@...il.com,
        Michel Lespinasse <walken@...gle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, npiggin@...il.com,
        paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v12 00/31] Speculative page faults

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