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@huaweicloud.com>
---

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@huaweicloud.com/

 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

On Tue, 22 Oct 2024 16:39:26 +0800 Zheng Yejian <zhengyejian@huaweicloud.com> wrote:

[...]
> Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
> Signed-off-by: Zheng Yejian <zhengyejian@huaweicloud.com>
> ---
> 
> 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@huaweicloud.com/

Nit.  If the patch series has a cover letter, we usually put this kind of
revision history on the cover letter.


Thanks,
SJ

[...]

Hi Zheng,


We Cc kunit folks for any DAMON kunit test changes, so I Cc-ed them.

On Tue, 22 Oct 2024 16:39:26 +0800 Zheng Yejian <zhengyejian@huaweicloud.com> wrote:

> 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:

Just for a clarification.  damon-operations test doesn't fail without this
patch.  This patch introduces two changes.  A new kunit test, and a bug fix.
Without the bug fix, the new kunit test fails.

I usually prefer separating test changes from fixes (introduc a fix first, and
then the test for it, to avoid unnecessary test failures).  But, given the
small size and the simplicity of the kunit change for this patch, I think
introducing it together with the fix is ok.

> 
>  # ./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@huaweicloud.com>

Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

[...]