| Message ID | 20241018035304.1050135-1-zhengyejian@huaweicloud.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/damon/vaddr: Fix issue in damon_va_evenly_split_region() | expand |
Hi Zheng, Thank you for sharing this nice finding and fix! I have a few comments below. On Fri, 18 Oct 2024 11:53:04 +0800 Zheng Yejian <zhengyejian@huaweicloud.com> wrote: > According to the logic of damon_va_evenly_split_region(), currently at > least following split cases would not meet the expectation: > > Suppose DAMON_MIN_REGION=0x1000, > Case1: Split [0x0, 0x1100) into 1 pieces, then the result would be > acutually [0x0, 0x1000), but NOT the expected [0x0, 0x1100) !!! Nice finding! However, as long as DAMON_MIN_REGION is respected, [0x0, 0x1100] region could not be created. So, the problematic case cannot happen in real? Please let me know if I'm missing something. And, why would someone call the function with nr_pieces 1? > Case2: 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) !!! Nice finding! > > 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: > 1. For the above Case1, the 'end' value of the split 1 piece is > aligned but not updated!!! > 2. For the above Case2, more pieces are split out than expected!!! > > To fix it, in this patch: > - As for the expect to split 1 piece, just return 0; As mentioned above, I think this is not needed, since the problematic case is unreal. > - Count for each piece split and make sure no more than 'nr_pieces'; > - Add above two cases into damon_test_split_evenly(). Thank you for adding tests! > > BTW, currently when running kunit test, DAMON_MIN_REGION is redefined > as 1, then above ALIGN_DOWN cases may not be test, since every int > value is ALIGN-ed to 1. > > 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> > --- > mm/damon/tests/vaddr-kunit.h | 2 ++ > mm/damon/vaddr.c | 13 +++++++++---- > 2 files changed, 11 insertions(+), 4 deletions(-) > > diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h > index a339d117150f..b9a03e4e29e5 100644 > --- a/mm/damon/tests/vaddr-kunit.h > +++ b/mm/damon/tests/vaddr-kunit.h > @@ -300,6 +300,8 @@ 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, 4, 6, 1); If my above assumption (the first problem is unreal) is not wrong, maybe this test is not needed? > + damon_test_split_evenly_succ(test, 0, 3, 2); Nice. > damon_test_split_evenly_fail(test, 5, 6, 2); > } > > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c > index 08cfd22b5249..1f3cebd20829 100644 > --- a/mm/damon/vaddr.c > +++ b/mm/damon/vaddr.c > @@ -67,10 +67,14 @@ 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; > + int i; Purpose of this variable is counting the number of splitted regions, and comparing it against 'nr_pieces', right? Because nr_pieces is 'unsigned int', let's make this 'unsigned int' type, too. > > if (!r || !nr_pieces) > return -EINVAL; > > + if (nr_pieces == 1) > + return 0; > + As mentioned above, I don't think this is not needed. > orig_end = r->ar.end; > sz_orig = damon_sz_region(r); > sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION); > @@ -79,9 +83,11 @@ static int damon_va_evenly_split_region(struct damon_target *t, > return -EINVAL; > > r->ar.end = r->ar.start + sz_piece; > + /* origin region will be updated as the first one after splitting */ I don't think this comment is easy to understand. Let's just remove it. > + i = 1; > + n = r; Why we need this? for 'nr_pieces == 1' case? If so, I don't think we need to take care about the case for the above mentioned reason. Please let me know if I'm missing something. > next = damon_next_region(r); > - for (start = r->ar.end; start + sz_piece <= orig_end; > - start += sz_piece) { > + for (start = r->ar.end; i < nr_pieces; start += sz_piece, i++) { > n = damon_new_region(start, start + sz_piece); > if (!n) > return -ENOMEM; > @@ -89,8 +95,7 @@ static int damon_va_evenly_split_region(struct damon_target *t, > r = n; > } > /* complement last region for possible rounding error */ > - if (n) > - n->ar.end = orig_end; > + n->ar.end = orig_end; Maybe this change is related with the above 'n = r' line? But, I don't think we need that, as commented there. > > return 0; > } > -- > 2.25.1 Thanks, SJ
On 2024/10/19 02:33, SeongJae Park wrote: > Hi Zheng, > > > Thank you for sharing this nice finding and fix! I have a few comments below. > Thanks for your review! > On Fri, 18 Oct 2024 11:53:04 +0800 Zheng Yejian <zhengyejian@huaweicloud.com> wrote: > >> According to the logic of damon_va_evenly_split_region(), currently at >> least following split cases would not meet the expectation: >> >> Suppose DAMON_MIN_REGION=0x1000, >> Case1: Split [0x0, 0x1100) into 1 pieces, then the result would be >> acutually [0x0, 0x1000), but NOT the expected [0x0, 0x1100) !!! > > Nice finding! However, as long as DAMON_MIN_REGION is respected, [0x0, 0x1100] > region could not be created. So, the problematic case cannot happen in real? > Please let me know if I'm missing something. Currently when DAMON_MIN_REGION is defined as PAGE_SIZE, and both vm start and end are commonly page-aligned, then the [0x, 0x1100) could not be created, but I'm not sure either. > > And, why would someone call the function with nr_pieces 1? > damon_va_evenly_split_region() is called in __damon_va_init_regions(), and nr_pieces is calculated by: `nr_pieces = (regions[i].end - regions[i].start) / sz;` Above regions[i].start/regions[i].end/sz is determine at runtime, and sz can beaffected by minimum number of regions, user can change that, am I right? Then nr_pieces can be 1 ! On the other hand, I think damon_va_evenly_split_region() itself should handle the 'nr_pieces == 1' case, or if we make sure that case is unreal, would it be better to add some assertion? >> Case2: 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) !!! > > Nice finding! > >> >> 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: >> 1. For the above Case1, the 'end' value of the split 1 piece is >> aligned but not updated!!! >> 2. For the above Case2, more pieces are split out than expected!!! >> >> To fix it, in this patch: >> - As for the expect to split 1 piece, just return 0; > > As mentioned above, I think this is not needed, since the problematic case is > unreal. I think this case exists, as above reply. > >> - Count for each piece split and make sure no more than 'nr_pieces'; >> - Add above two cases into damon_test_split_evenly(). > > Thank you for adding tests! > >> >> BTW, currently when running kunit test, DAMON_MIN_REGION is redefined >> as 1, then above ALIGN_DOWN cases may not be test, since every int >> value is ALIGN-ed to 1. >> >> 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> >> --- >> mm/damon/tests/vaddr-kunit.h | 2 ++ >> mm/damon/vaddr.c | 13 +++++++++---- >> 2 files changed, 11 insertions(+), 4 deletions(-) >> >> diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h >> index a339d117150f..b9a03e4e29e5 100644 >> --- a/mm/damon/tests/vaddr-kunit.h >> +++ b/mm/damon/tests/vaddr-kunit.h >> @@ -300,6 +300,8 @@ 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, 4, 6, 1); > > If my above assumption (the first problem is unreal) is not wrong, maybe this > test is not needed? > As an unit test, damon_va_evenly_split_region() itself should be able to handle the 'nr_pieces == 1' case, right? I think this testcase can be added in case something goes wrong one day. >> + damon_test_split_evenly_succ(test, 0, 3, 2); > > Nice. > >> damon_test_split_evenly_fail(test, 5, 6, 2); >> } >> >> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c >> index 08cfd22b5249..1f3cebd20829 100644 >> --- a/mm/damon/vaddr.c >> +++ b/mm/damon/vaddr.c >> @@ -67,10 +67,14 @@ 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; >> + int i; > > Purpose of this variable is counting the number of splitted regions, and > comparing it against 'nr_pieces', right? Because nr_pieces is 'unsigned int', > let's make this 'unsigned int' type, too. > Well, yes, I'll do it in v2 after all the discussions for this version are complete! >> >> if (!r || !nr_pieces) >> return -EINVAL; >> >> + if (nr_pieces == 1) >> + return 0; >> + > > As mentioned above, I don't think this is not needed. > >> orig_end = r->ar.end; >> sz_orig = damon_sz_region(r); >> sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION); >> @@ -79,9 +83,11 @@ static int damon_va_evenly_split_region(struct damon_target *t, >> return -EINVAL; >> >> r->ar.end = r->ar.start + sz_piece; >> + /* origin region will be updated as the first one after splitting */ > > I don't think this comment is easy to understand. Let's just remove it. > Thanks, I'll remove it in next version! >> + i = 1; >> + n = r; > > Why we need this? for 'nr_pieces == 1' case? If so, I don't think we need to > take care about the case for the above mentioned reason. Please let me know if > I'm missing something. Yes, this is for 'nr_pieces == 1' case, and if we have above `if (nr_pieces == 1) return 0;` line, then this is not needed since nr_pieces > 1, and following loop will at least two times > >> next = damon_next_region(r); >> - for (start = r->ar.end; start + sz_piece <= orig_end; >> - start += sz_piece) { >> + for (start = r->ar.end; i < nr_pieces; start += sz_piece, i++) { >> n = damon_new_region(start, start + sz_piece); >> if (!n) >> return -ENOMEM; >> @@ -89,8 +95,7 @@ static int damon_va_evenly_split_region(struct damon_target *t, >> r = n; >> } >> /* complement last region for possible rounding error */ >> - if (n) >> - n->ar.end = orig_end; >> + n->ar.end = orig_end; > > Maybe this change is related with the above 'n = r' line? But, I don't think > we need that, as commented there. Yes, they related. > >> >> return 0; >> } >> -- >> 2.25.1 > > > Thanks, > SJ
On Mon, 21 Oct 2024 11:56:04 +0800 Zheng Yejian <zhengyejian@huaweicloud.com> wrote: > On 2024/10/19 02:33, SeongJae Park wrote: > > Hi Zheng, > > > > > > Thank you for sharing this nice finding and fix! I have a few comments below. > > > > Thanks for your review! > > > On Fri, 18 Oct 2024 11:53:04 +0800 Zheng Yejian <zhengyejian@huaweicloud.com> wrote: > > > >> According to the logic of damon_va_evenly_split_region(), currently at > >> least following split cases would not meet the expectation: > >> > >> Suppose DAMON_MIN_REGION=0x1000, > >> Case1: Split [0x0, 0x1100) into 1 pieces, then the result would be > >> acutually [0x0, 0x1000), but NOT the expected [0x0, 0x1100) !!! > > > > Nice finding! However, as long as DAMON_MIN_REGION is respected, [0x0, 0x1100] > > region could not be created. So, the problematic case cannot happen in real? > > Please let me know if I'm missing something. > > Currently when DAMON_MIN_REGION is defined as PAGE_SIZE, and both vm start > and end are commonly page-aligned, then the [0x, 0x1100) could not be created, > but I'm not sure either. Thank you for confirming. If there is a way that DAMON could generate [0x, 0x1100], that's a bug that deserves its own fix. So let's assume it cannot happen for now. > > > > > And, why would someone call the function with nr_pieces 1? > > > > damon_va_evenly_split_region() is called in __damon_va_init_regions(), and nr_pieces > is calculated by: > > `nr_pieces = (regions[i].end - regions[i].start) / sz;` > > Above regions[i].start/regions[i].end/sz is determine at runtime, and sz can > beaffected by minimum number of regions, user can change that, am I right? > Then nr_pieces can be 1 ! You're right, thank you. Now, the next question would be, could that ('damon_va_evenly_split_region()' being called with 1 'nr_pieces') trigger some issues? Based on the code, I don't think so. Please let me know if I'm missing some corner cases. > On the other hand, I think damon_va_evenly_split_region() itself should > handle the 'nr_pieces == 1' case, or if we make sure that case is unreal, > would it be better to add some assertion? Nice suggestion, thanks. I agree that making it be handled is better in terms of maintenance. It would make the code much easier to read. It wouldn't be for a fix of a bug, but for making the code easier to read. So I think posting it as a separate patch is better. If you don't mind, please post a patch. > > >> Case2: 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) !!! > > > > Nice finding! > > > >> > >> 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: > >> 1. For the above Case1, the 'end' value of the split 1 piece is > >> aligned but not updated!!! > >> 2. For the above Case2, more pieces are split out than expected!!! > >> > >> To fix it, in this patch: > >> - As for the expect to split 1 piece, just return 0; > > > > As mentioned above, I think this is not needed, since the problematic case is > > unreal. > > I think this case exists, as above reply. A case that damon_va_evenly_split_region() is called with nr_pieces of value 1 exists. A case that the function is called with DAMON_MIN_REGION un-aligned region doesn't exist (unless there is a bug). I was saying about the second case. I still agree doing the nr_pieces check is good for readability, so please post a patch if you don't mind. > > > > >> - Count for each piece split and make sure no more than 'nr_pieces'; > >> - Add above two cases into damon_test_split_evenly(). > > > > Thank you for adding tests! > > > >> > >> BTW, currently when running kunit test, DAMON_MIN_REGION is redefined > >> as 1, then above ALIGN_DOWN cases may not be test, since every int > >> value is ALIGN-ed to 1. > >> > >> 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> > >> --- > >> mm/damon/tests/vaddr-kunit.h | 2 ++ > >> mm/damon/vaddr.c | 13 +++++++++---- > >> 2 files changed, 11 insertions(+), 4 deletions(-) > >> > >> diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h > >> index a339d117150f..b9a03e4e29e5 100644 > >> --- a/mm/damon/tests/vaddr-kunit.h > >> +++ b/mm/damon/tests/vaddr-kunit.h > >> @@ -300,6 +300,8 @@ 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, 4, 6, 1); > > > > If my above assumption (the first problem is unreal) is not wrong, maybe this > > test is not needed? > > > > As an unit test, damon_va_evenly_split_region() itself should be able > to handle the 'nr_pieces == 1' case, right? I think this testcase can > be added in case something goes wrong one day. I agree. Nonetheless, let's make it be separated with the real bug fix. > > >> + damon_test_split_evenly_succ(test, 0, 3, 2); > > > > Nice. > > > >> damon_test_split_evenly_fail(test, 5, 6, 2); > >> } > >> > >> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c > >> index 08cfd22b5249..1f3cebd20829 100644 > >> --- a/mm/damon/vaddr.c > >> +++ b/mm/damon/vaddr.c > >> @@ -67,10 +67,14 @@ 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; > >> + int i; > > > > Purpose of this variable is counting the number of splitted regions, and > > comparing it against 'nr_pieces', right? Because nr_pieces is 'unsigned int', > > let's make this 'unsigned int' type, too. > > > > Well, yes, I'll do it in v2 after all the discussions for this version are complete! Thanks :) > > >> > >> if (!r || !nr_pieces) > >> return -EINVAL; > >> > >> + if (nr_pieces == 1) > >> + return 0; > >> + > > > > As mentioned above, I don't think this is not needed. As mentioned above, now I think having this is good for readability, but let's make it an individual change that separated from the real bug fix. > > > > > > >> orig_end = r->ar.end; > >> sz_orig = damon_sz_region(r); > >> sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION); > >> @@ -79,9 +83,11 @@ static int damon_va_evenly_split_region(struct damon_target *t, > >> return -EINVAL; > >> > >> r->ar.end = r->ar.start + sz_piece; > >> + /* origin region will be updated as the first one after splitting */ > > > > I don't think this comment is easy to understand. Let's just remove it. > > > > Thanks, I'll remove it in next version! > > >> + i = 1; > >> + n = r; > > > > Why we need this? for 'nr_pieces == 1' case? If so, I don't think we need to > > take care about the case for the above mentioned reason. Please let me know if > > I'm missing something. > > Yes, this is for 'nr_pieces == 1' case, and if we have above `if (nr_pieces == 1) return 0;` line, > then this is not needed since nr_pieces > 1, and following loop will at least two times > > > > >> next = damon_next_region(r); > >> - for (start = r->ar.end; start + sz_piece <= orig_end; > >> - start += sz_piece) { > >> + for (start = r->ar.end; i < nr_pieces; start += sz_piece, i++) { > >> n = damon_new_region(start, start + sz_piece); > >> if (!n) > >> return -ENOMEM; > >> @@ -89,8 +95,7 @@ static int damon_va_evenly_split_region(struct damon_target *t, > >> r = n; > >> } > >> /* complement last region for possible rounding error */ > >> - if (n) > >> - n->ar.end = orig_end; > >> + n->ar.end = orig_end; > > > > Maybe this change is related with the above 'n = r' line? But, I don't think > > we need that, as commented there. > > Yes, they related. Thank you for confirming. > > > > >> > >> return 0; > >> } > >> -- > >> 2.25.1 > > > > > > Thanks, > > SJ > > -- > Thanks, > Zheng Yejian So, let's add the 'nr_pieces == 1' check, but as a change that separated from the real bug fix. I'm looking forward to your next posts, Zheng :) Nonetheless, please note that the real bug is not somewhat critical for users. It only has a potential to slightly degrade the best-effort accuracy of DAMON in corner cases. Thanks, SJ
diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h index a339d117150f..b9a03e4e29e5 100644 --- a/mm/damon/tests/vaddr-kunit.h +++ b/mm/damon/tests/vaddr-kunit.h @@ -300,6 +300,8 @@ 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, 4, 6, 1); + 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..1f3cebd20829 100644 --- a/mm/damon/vaddr.c +++ b/mm/damon/vaddr.c @@ -67,10 +67,14 @@ 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; + int i; if (!r || !nr_pieces) return -EINVAL; + if (nr_pieces == 1) + return 0; + orig_end = r->ar.end; sz_orig = damon_sz_region(r); sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION); @@ -79,9 +83,11 @@ static int damon_va_evenly_split_region(struct damon_target *t, return -EINVAL; r->ar.end = r->ar.start + sz_piece; + /* origin region will be updated as the first one after splitting */ + i = 1; + n = r; next = damon_next_region(r); - for (start = r->ar.end; start + sz_piece <= orig_end; - start += sz_piece) { + for (start = r->ar.end; i < nr_pieces; start += sz_piece, i++) { n = damon_new_region(start, start + sz_piece); if (!n) return -ENOMEM; @@ -89,8 +95,7 @@ static int damon_va_evenly_split_region(struct damon_target *t, r = n; } /* complement last region for possible rounding error */ - if (n) - n->ar.end = orig_end; + n->ar.end = orig_end; return 0; }
According to the logic of damon_va_evenly_split_region(), currently at least following split cases would not meet the expectation: Suppose DAMON_MIN_REGION=0x1000, Case1: Split [0x0, 0x1100) into 1 pieces, then the result would be acutually [0x0, 0x1000), but NOT the expected [0x0, 0x1100) !!! Case2: 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: 1. For the above Case1, the 'end' value of the split 1 piece is aligned but not updated!!! 2. For the above Case2, more pieces are split out than expected!!! To fix it, in this patch: - As for the expect to split 1 piece, just return 0; - Count for each piece split and make sure no more than 'nr_pieces'; - Add above two cases into damon_test_split_evenly(). BTW, currently when running kunit test, DAMON_MIN_REGION is redefined as 1, then above ALIGN_DOWN cases may not be test, since every int value is ALIGN-ed to 1. 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> --- mm/damon/tests/vaddr-kunit.h | 2 ++ mm/damon/vaddr.c | 13 +++++++++---- 2 files changed, 11 insertions(+), 4 deletions(-)