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

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  [PATCH v4 2/2] Dcumentation/filesystems/proc.txt: Add more description for maps/smaps Robert Ho <robert.hu@intel.com> - 2016-10-01 06:50 +0200
    Re: [PATCH v4 2/2] Dcumentation/filesystems/proc.txt: Add more  description for maps/smaps Michal Hocko <mhocko@kernel.org> - 2016-10-03 14:00 +0200

#1494354 — [PATCH v4 2/2] Dcumentation/filesystems/proc.txt: Add more description for maps/smaps

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@intel.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

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#1494845 — 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@intel.com>

Acked-by: Michal Hocko <mhocko@suse.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

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