Re: [PATCH v4 2/2] Dcumentation/filesystems/proc.txt: Add more description for maps/smaps
From: Michal Hocko
Date: Mon Oct 03 2016 - 07:54:09 EST
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.
> Adopt Dave Hansen's revision from v1 as the description.
> Signed-off-by: Robert Ho <robert.hu@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 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
> +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
> +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).