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

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Message-Id: <20130411142417.bb58d519b860d06ab84333c2@linux-foundation.org>
Date:	Thu, 11 Apr 2013 14:24:17 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Linux MM <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] mm: Soft-dirty bits for user memory changes
 tracking

On Thu, 11 Apr 2013 15:30:00 +0400 Pavel Emelyanov <xemul@...allels.com> 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/pagemap2 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.
> 
> ...
>
> +config MEM_SOFT_DIRTY
> +	bool "Track memory changes"
> +	depends on CHECKPOINT_RESTORE && X86

I guess we can add the CHECKPOINT_RESTORE dependency for now, but it is
a general facility and I expect others will want to get their hands on
it for unrelated things.

>From that perspective, the dependency on X86 is awful.  What's the
problem here and what do other architectures need to do to be able to
support the feature?


You have a test application, I assume.  It would be helpful if we could
get that into tools/testing/selftests.
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