According to the logic of damon_va_evenly_split_region(), currently
following split case would not meet the expectation:
Suppose DAMON_MIN_REGION=0x1000,
Case: Split [0x0, 0x3000) into 2 pieces, then the result would be
acutually 3 regions:
[0x0, 0x1000), [0x1000, 0x2000), [0x2000, 0x3000)
but NOT the expected 2 regions:
[0x0, 0x1000), [0x1000, 0x3000) !!!
The root cause is that when calculating size of each split piece in
damon_va_evenly_split_region():
`sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION);`
both the dividing and the ALIGN_DOWN may cause loss of precision,
then each time split one piece of size 'sz_piece' from origin 'start' to
'end' would cause more pieces are split out than expected!!!
To fix it, count for each piece split and make sure no more than
'nr_pieces'. In addition, add above case into damon_test_split_evenly().
After this patch, damon-operations test passed:
# ./tools/testing/kunit/kunit.py run damon-operations
[...]
============== damon-operations (6 subtests) ===============
[PASSED] damon_test_three_regions_in_vmas
[PASSED] damon_test_apply_three_regions1
[PASSED] damon_test_apply_three_regions2
[PASSED] damon_test_apply_three_regions3
[PASSED] damon_test_apply_three_regions4
[PASSED] damon_test_split_evenly
================ [PASSED] damon-operations =================
Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
Signed-off-by: Zheng Yejian <zhengyejian@xxxxxxxxxxxxxxx>
---
v1 -> v2:
- Move 'nr_pieces == 1' check into separate patch and update commit messages;
- Change type of variable 'i' to be 'unsigned int';
- Remove a puzzling comment;
v1: https://lore.kernel.org/all/20241018035304.1050135-1-zhengyejian@xxxxxxxxxxxxxxx/
mm/damon/tests/vaddr-kunit.h | 1 +
mm/damon/vaddr.c | 4 ++--
2 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
index a339d117150f..a149e354bb26 100644
--- a/mm/damon/tests/vaddr-kunit.h
+++ b/mm/damon/tests/vaddr-kunit.h
@@ -300,6 +300,7 @@ static void damon_test_split_evenly(struct kunit *test)
damon_test_split_evenly_fail(test, 0, 100, 0);
damon_test_split_evenly_succ(test, 0, 100, 10);
damon_test_split_evenly_succ(test, 5, 59, 5);
+ damon_test_split_evenly_succ(test, 0, 3, 2);
damon_test_split_evenly_fail(test, 5, 6, 2);
}
diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 08cfd22b5249..dba3b2f4d758 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -67,6 +67,7 @@ static int damon_va_evenly_split_region(struct damon_target *t,
unsigned long sz_orig, sz_piece, orig_end;
struct damon_region *n = NULL, *next;
unsigned long start;
+ unsigned int i;
if (!r || !nr_pieces)
return -EINVAL;
@@ -80,8 +81,7 @@ static int damon_va_evenly_split_region(struct damon_target *t,
r->ar.end = r->ar.start + sz_piece;
next = damon_next_region(r);
- for (start = r->ar.end; start + sz_piece <= orig_end;
- start += sz_piece) {
+ for (start = r->ar.end, i = 1; i < nr_pieces; start += sz_piece, i++) {
n = damon_new_region(start, start + sz_piece);
if (!n)
return -ENOMEM;
--
2.25.1
