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

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