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mm/damon/vaddr: fix issue in damon_va_evenly_split_region()

Patch series "mm/damon/vaddr: Fix issue in
damon_va_evenly_split_region()". v2.

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().

And add 'nr_piece == 1' check in damon_va_evenly_split_region() for better
code readability and add a corresponding kunit testcase.


This patch (of 2):

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 =================

Link: https://lkml.kernel.org/r/20241022083927.3592237-1-zhengyejian@huaweicloud.com
Link: https://lkml.kernel.org/r/20241022083927.3592237-2-zhengyejian@huaweicloud.com
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>
Cc: Fernand Sieber <sieberf@amazon.com>
Cc: Leonard Foerster <foersleo@amazon.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Ye Weihua <yeweihua4@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Zheng Yejian and committed by
Andrew Morton f3c7a1ed ab505e8b

+3 -2
+1
mm/damon/tests/vaddr-kunit.h
··· 300 300 damon_test_split_evenly_fail(test, 0, 100, 0); 301 301 damon_test_split_evenly_succ(test, 0, 100, 10); 302 302 damon_test_split_evenly_succ(test, 5, 59, 5); 303 + damon_test_split_evenly_succ(test, 0, 3, 2); 303 304 damon_test_split_evenly_fail(test, 5, 6, 2); 304 305 } 305 306
+2 -2
mm/damon/vaddr.c
··· 67 67 unsigned long sz_orig, sz_piece, orig_end; 68 68 struct damon_region *n = NULL, *next; 69 69 unsigned long start; 70 + unsigned int i; 70 71 71 72 if (!r || !nr_pieces) 72 73 return -EINVAL; ··· 81 80 82 81 r->ar.end = r->ar.start + sz_piece; 83 82 next = damon_next_region(r); 84 - for (start = r->ar.end; start + sz_piece <= orig_end; 85 - start += sz_piece) { 83 + for (start = r->ar.end, i = 1; i < nr_pieces; start += sz_piece, i++) { 86 84 n = damon_new_region(start, start + sz_piece); 87 85 if (!n) 88 86 return -ENOMEM;