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Documentation: kmemleak: fix stale statements about scanning

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Three statements in the "false positives/negatives" and "Limitations"
sections have never matched the code:

 - task stack scanning is on by default (kmemleak_stack_scan = 1), as
   the parameter list earlier in the same document already states;

 - MSECS_MIN_AGE has been 5000, not 1000, since the initial commit;

 - scanning is done by a periodic kthread. Reading the debugfs file
   only lists what the last scan found; kmemleak_open() calls
   seq_open() and never scans.

Link: https://lore.kernel.org/20260917-b4-kmemleak-doc-v1-2-84fde6d1f749@debian.org
Fixes: 04f70336c8 ("kmemleak: Add documentation on the memory leak detector")
Fixes: e0a2a1601b ("kmemleak: Enable task stacks scanning by default")
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
This commit is contained in:
Breno Leitao 2026-09-17 06:47:36 -07:00 committed by Andrew Morton
parent 947ab1dd1c
commit b523fc8e6b
1 changed files with 7 additions and 4 deletions
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11
Documentation/dev-tools/kmemleak.rst
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@ -190,7 +190,8 @@ reported by kmemleak because values found during the memory scanning
point to such objects. To reduce the number of false negatives, kmemleak
provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
kmemleak_erase functions (see above). The task stacks also increase the
amount of false negatives and their scanning is not enabled by default.
amount of false negatives and their scanning is enabled by default; it
can be turned off with ``stack=off``.
The false positives are objects wrongly reported as being memory leaks
(orphan). For objects known not to be leaks, kmemleak provides the
@ -200,7 +201,7 @@ longer be scanned.
Some of the reported leaks are only transient, especially on SMP
systems, because of pointers temporarily stored in CPU registers or
stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 5000) representing
the minimum age of an object to be reported as a memory leak.
The ``min_unref_scans`` module parameter requires an object to be seen
@ -217,8 +218,10 @@ Limitations and Drawbacks
-------------------------
The main drawback is the reduced performance of memory allocation and
freeing. To avoid other penalties, the memory scanning is only performed
when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
freeing. To avoid other penalties, the memory scanning is performed by a
periodic thread rather than on every allocation. Reading the
/sys/kernel/debug/kmemleak file only lists the objects found by the last
scan; writing ``scan`` to it triggers a new one. Anyway, this tool is
intended for debugging purposes where the performance might not be the
most important requirement.