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>

diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
index 3dad8dfd9005fcb4828dd4712978e685cee8815f..fcdccb614fd8554a973a12fb7440a640f6d38976 100644 (file)
--- 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 821990d0141a01cd560e646ed27048364553f6e0..86f612fbf8866f330a6257de97d6e1604db3905e 100644 (file)
--- 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;