linux-kernel - Re: [PATCH v2] kvm: Faults which trigger IO release the mmap

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Date:	Thu, 18 Sep 2014 08:29:17 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Andres Lagar-Cavilla <andreslc@...gle.com>
Cc:	Gleb Natapov <gleb@...nel.org>, Radim Krcmar <rkrcmar@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Paul Cassella <cassella@...y.com>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] kvm: Faults which trigger IO release the mmap_sem

Hi Andres,
On Wed, Sep 17, 2014 at 10:51:48AM -0700, Andres Lagar-Cavilla wrote:
[...]
> static inline int check_user_page_hwpoison(unsigned long addr)
> {
> 	int rc, flags = FOLL_TOUCH | FOLL_HWPOISON | FOLL_WRITE;
>@@ -1177,9 +1214,15 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
> 		npages = get_user_page_nowait(current, current->mm,
> 					      addr, write_fault, page);
> 		up_read(&current->mm->mmap_sem);
>-	} else
>-		npages = get_user_pages_fast(addr, 1, write_fault,
>-					     page);
>+	} else {
>+		/*
>+		 * By now we have tried gup_fast, and possibly async_pf, and we
>+		 * are certainly not atomic. Time to retry the gup, allowing
>+		 * mmap semaphore to be relinquished in the case of IO.
>+		 */
>+		npages = kvm_get_user_page_io(current, current->mm, addr,
>+					      write_fault, page);
>+	}

try_async_pf 
 gfn_to_pfn_async 
  __gfn_to_pfn  			async = false 
   __gfn_to_pfn_memslot
    hva_to_pfn 
	 hva_to_pfn_fast 
	 hva_to_pfn_slow 
	  kvm_get_user_page_io

page will always be ready after kvm_get_user_page_io which leads to APF
don't need to work any more.

Regards,
Wanpeng Li

> 	if (npages != 1)
> 		return npages;
> 
>-- 
>2.1.0.rc2.206.gedb03e5
>
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