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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
Reviewed-by: SeongJae Park <sj@xxxxxxxxxx>
Thanks,
SJ
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