linux-kernel - Re: [PATCH 4/5] mm: soft-dirty bits for user memory changes tracking

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Date:	Thu, 11 Jul 2013 11:46:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Pavel Emelyanov <xemul@...allels.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Glauber Costa <glommer@...allels.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Matt Mackall <mpm@...enic.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 4/5] mm: soft-dirty bits for user memory changes tracking

On 04/30/2013 09:12 AM, Pavel Emelyanov wrote:
> The soft-dirty is a bit on a PTE which helps to track which pages a task
> writes to. In order to do this tracking one should
> 
>   1. Clear soft-dirty bits from PTEs ("echo 4 > /proc/PID/clear_refs)
>   2. Wait some time.
>   3. Read soft-dirty bits (55'th in /proc/PID/pagemap entries)
> 
> To do this tracking, the writable bit is cleared from PTEs when the
> soft-dirty bit is. Thus, after this, when the task tries to modify a page
> at some virtual address the #PF occurs and the kernel sets the soft-dirty
> bit on the respective PTE.
> 
> Note, that although all the task's address space is marked as r/o after the
> soft-dirty bits clear, the #PF-s that occur after that are processed fast.
> This is so, since the pages are still mapped to physical memory, and thus
> all the kernel does is finds this fact out and puts back writable, dirty
> and soft-dirty bits on the PTE.
> 
> Another thing to note, is that when mremap moves PTEs they are marked with
> soft-dirty as well, since from the user perspective mremap modifies the
> virtual memory at mremap's new address.
> 
> 

Sorry I'm late to the party -- I didn't notice this until the lwn
article this week.

How does this get munmap + mmap right?  mremap marks things soft-dirty,
but unmapping and remapping seems like it will result in the soft-dirty
bit being cleared.  For that matter, won't this sequence also end up wrong:

 - clear_refs
 - Write to mapping
 - Page and pte evicted due to memory pressure
 - Read from mapping -- clean page faulted back in
 - pte soft-dirty is now clear ?!?

--Andy
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