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

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Date:   Sat,  1 Oct 2016 12:42:38 +0800
From:   Robert Ho <robert.hu@...el.com>
To:     pbonzini@...hat.com, akpm@...ux-foundation.org, mhocko@...e.com,
        oleg@...hat.com, dave.hansen@...el.com, dan.j.williams@...el.com
Cc:     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, Robert Ho <robert.hu@...el.com>
Subject: [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@...el.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