+ lib-introduce-hierarchical-per-cpu-counters-fix.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: lib: fix compare_delta parameter order in percpu_counter_tree
has been added to the -mm mm-unstable branch.  Its filename is
     lib-introduce-hierarchical-per-cpu-counters-fix.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-introduce-hierarchical-per-cpu-counters-fix.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: David Carlier <devnexen@gmail.com>
Subject: lib: fix compare_delta parameter order in percpu_counter_tree
Date: Fri, 13 Mar 2026 17:54:24 +0000

The compare_delta() helper takes (delta, accuracy_neg, accuracy_pos), but
every call site passes (delta, accuracy_pos, accuracy_neg) — the last
two arguments are consistently swapped.

The documented invariant (include/linux/percpu_counter_tree.h) is:

  (precise_sum - under) <= approx_sum <= (precise_sum + over)

Which means precise_sum is in [approx_sum - over, approx_sum + under].

For a positive delta (v - approx_sum >= 0), accuracy_pos must be
"under" (the maximum amount precise_sum can exceed approx_sum).
For a negative delta, accuracy_neg must be "over". Since under > over
always (batch_size * M vs (batch_size - 1) * M), swapping them causes
false definitive results: the functions return 1 ("v > counter") when
the correct answer is 0 (indeterminate).

This affects all comparison functions:
- percpu_counter_tree_approximate_compare_value()
- percpu_counter_tree_approximate_compare()
- percpu_counter_tree_precise_compare_value()
- percpu_counter_tree_precise_compare()

The precise variants are also affected because their approximate
fast-path can short-circuit with a wrong result, skipping the precise
sum computation.

Fix by swapping the parameter order in compare_delta() itself, since all
call sites are consistently swapped.

Link: https://lkml.kernel.org/r/20260313175424.132735-1-devnexen@gmail.com
Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Josh Law <objecting@objecting.org>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/percpu_counter_tree.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/lib/percpu_counter_tree.c~lib-introduce-hierarchical-per-cpu-counters-fix
+++ a/lib/percpu_counter_tree.c
@@ -458,7 +458,7 @@ long percpu_counter_tree_precise_sum(str
 EXPORT_SYMBOL_GPL(percpu_counter_tree_precise_sum);
 
 static
-int compare_delta(long delta, unsigned long accuracy_neg, unsigned long accuracy_pos)
+int compare_delta(long delta, unsigned long accuracy_pos, unsigned long accuracy_neg)
 {
 	if (delta >= 0) {
 		if (delta <= accuracy_pos)
_

Patches currently in -mm which might be from devnexen@gmail.com are

lib-introduce-hierarchical-per-cpu-counters-fix.patch