Documentation/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.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Robert Ho <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Xiao Guangrong <[email protected]>
Cc: Robert Hu <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Gleb Natapov <[email protected]>
Cc: Marcelo Tosatti <[email protected]>
Cc: Stefan Hajnoczi <[email protected]>
Cc: Ross Zwisler <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 12 insertions, 0 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index fcc1ac094282..219ffd41a911 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).