Docs/admin-guide/mm/damon/reclaim: warn commit_inputs vs param updates race

commit 0beba407d4585a15b0dc09f2064b5b3ddcb0e857 upstream.

Patch series "Docs/admin-guide/mm/damon: warn commit_inputs vs other
params race".

Writing 'Y' to the commit_inputs parameter of DAMON_RECLAIM and
DAMON_LRU_SORT, and writing other parameters before the commit_inputs
request is completely processed can cause race conditions.  While the
consequence can be bad, the documentation is not clearly describing that.
Add clear warnings.

The issue was discovered [1,2] by sashiko.

This patch (of 2):

DAMON_RECLAIM handles commit_inputs request inside kdamond thread,
reading the module parameters.  If the user updates the module
parameters while the kdamond thread is reading those, races can happen.
To avoid this, the commit_inputs parameter shows whether it is still in
the progress, assuming users wouldn't update parameters in the middle of
the work.  Some users might ignore that.  Add a warning about the
behavior.

The issue was discovered in [1] by sashiko.

Link: https://lore.kernel.org/20260329153052.46657-2-sj@kernel.org
Link: https://lore.kernel.org/20260319161620.189392-3-objecting@objecting.org
Link: https://lore.kernel.org/20260319161620.189392-2-objecting@objecting.org
Fixes: 81a84182c343 ("Docs/admin-guide/mm/damon/reclaim: document 'commit_inputs' parameter")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 5.19.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/mm/damon/reclaim.rst b/Documentation/admin-guide/mm/damon/reclaim.rst
index 8eba3da8dcee0af38396e5d8bd7a0a78cfefd1a3..1d68db2aa27ea1613f1ca5d44e4d8506fa7833f4 100644 (file)
--- a/Documentation/admin-guide/mm/damon/reclaim.rst
+++ b/Documentation/admin-guide/mm/damon/reclaim.rst
@@ -71,6 +71,10 @@ of parametrs except ``enabled`` again.  Once the re-reading is done, this
 parameter is set as ``N``.  If invalid parameters are found while the
 re-reading, DAMON_RECLAIM will be disabled.
 
+Once ``Y`` is written to this parameter, the user must not write to any
+parameters until reading ``commit_inputs`` again returns ``N``.  If users
+violate this rule, the kernel may exhibit undefined behavior.
+
 min_age
 -------