linux-kernel - Re: [PATCH v4 2/2] Dcumentation/filesystems/proc.txt: Add more description for maps/smaps

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Message-ID: <20161003115355.GB26768@dhcp22.suse.cz>
Date:   Mon, 3 Oct 2016 13:53:55 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Robert Ho <robert.hu@...el.com>
Cc:     pbonzini@...hat.com, akpm@...ux-foundation.org, oleg@...hat.com,
        dave.hansen@...el.com, dan.j.williams@...el.com,
        guangrong.xiao@...ux.intel.com, gleb@...nel.org,
        mtosatti@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, stefanha@...hat.com,
        yuhuang@...hat.com, linux-mm@...ck.org,
        ross.zwisler@...ux.intel.com
Subject: Re: [PATCH v4 2/2] Dcumentation/filesystems/proc.txt: Add more
 description for maps/smaps

On Sat 01-10-16 12:42:38, Robert Ho wrote:
> Add some more description on the limitations for smaps/maps readings, as well
> as some guaruntees we can make.
> 
> Changelog:
> v2:
> 	Adopt Dave Hansen's revision from v1 as the description.
> 
> Signed-off-by: Robert Ho <robert.hu@...el.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  Documentation/filesystems/proc.txt | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 68080ad..daa096f 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -515,6 +515,18 @@ be vanished or the reverse -- new added.
>  This file is only present if the CONFIG_MMU kernel configuration option is
>  enabled.
>  
> +Note: reading /proc/PID/maps or /proc/PID/smaps is inherently racy (consistent
> +output can be achieved only in the single read call).
> +This typically manifests when doing partial reads of these files while the
> +memory map is being modified.  Despite the races, we do provide the following
> +guarantees:
> +
> +1) The mapped addresses never go backwards, which implies no two
> +   regions will ever overlap.
> +2) If there is something at a given vaddr during the entirety of the
> +   life of the smaps/maps walk, there will be some output for it.
> +
> +
>  The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG
>  bits on both physical and virtual pages associated with a process, and the
>  soft-dirty bit on pte (see Documentation/vm/soft-dirty.txt for details).
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs