On 4/10/26 8:00 PM, Dev Jain wrote:
The original version of mremap_test (7df666253f26: "kselftests: vm: add
mremap tests") validated remapped contents byte-by-byte and printed a
mismatch index in case the bytes streams are not equal. That made
validation expensive in both cases: for "no mismatch" (the common case when
mremap is not buggy), it still walked all bytes in C; for "mismatch", it
broke out of the loop after printing the mismatch index.
Later, my commit 7033c6cc9620 ("selftests/mm: mremap_test: optimize
execution time from minutes to seconds using chunkwise memcmp") tried to
optimize both cases by using chunk-wise memcmp() and only scanning bytes
within a range which has been determined by memcmp as mismatching.
But get_sqrt() in that commit is buggy: `high = mid - 1` is applied
unconditionally. This makes the speed of checking the mismatch index
suboptimal.
The mismatch index does not provide useful debugging value here: if
validation fails, we know mremap behavior is wrong, and the specific byte
offset does not make root-causing easier.
So instead of fixing get_sqrt(), bite the bullet, drop mismatch index
scanning and just compare the two byte streams with memcmp().
Reported-by: Sarthak Sharma <[email protected]>
Signed-off-by: Dev Jain <[email protected]>
I have tested this patch with
./mremap_test -t 0
and the selftest results are unchanged.
I did not observe any significant change in execution time across
multiple runs.
Tested-by: Sarthak Sharma <[email protected]>
---
Sorry for sending two patchsets the same day - the problem was made known
to me today, and I couldn't help myself but fix it immediately, imagine
my embarrassment when I found out that I made a typo in the binary search
code which I had been writing consistently throughout college :)
Applies on mm-unstable.
tools/testing/selftests/mm/mremap_test.c | 109 +++--------------------
1 file changed, 10 insertions(+), 99 deletions(-)
diff --git a/tools/testing/selftests/mm/mremap_test.c
b/tools/testing/selftests/mm/mremap_test.c
index 308576437228c..131d9d6db8679 100644
--- a/tools/testing/selftests/mm/mremap_test.c
+++ b/tools/testing/selftests/mm/mremap_test.c
@@ -76,27 +76,6 @@ enum {
.expect_failure = should_fail \
}
-/* compute square root using binary search */
-static unsigned long get_sqrt(unsigned long val)
-{
- unsigned long low = 1;
-
- /* assuming rand_size is less than 1TB */
- unsigned long high = (1UL << 20);
-
- while (low <= high) {
- unsigned long mid = low + (high - low) / 2;
- unsigned long temp = mid * mid;
-
- if (temp == val)
- return mid;
- if (temp < val)
- low = mid + 1;
- high = mid - 1;
- }
- return low;
-}
-
/*
* Returns false if the requested remap region overlaps with an
* existing mapping (e.g text, stack) else returns true.
@@ -995,11 +974,9 @@ static long long remap_region(struct config c, unsigned
int threshold_mb,
char *rand_addr)
{
void *addr, *tmp_addr, *src_addr, *dest_addr, *dest_preamble_addr =
NULL;
- unsigned long long t, d;
struct timespec t_start = {0, 0}, t_end = {0, 0};
long long start_ns, end_ns, align_mask, ret, offset;
unsigned long long threshold;
- unsigned long num_chunks;
if (threshold_mb == VALIDATION_NO_THRESHOLD)
threshold = c.region_size;
@@ -1068,87 +1045,21 @@ static long long remap_region(struct config c, unsigned
int threshold_mb,
goto clean_up_dest_preamble;
}
- /*
- * Verify byte pattern after remapping. Employ an algorithm with a
- * square root time complexity in threshold: divide the range into
- * chunks, if memcmp() returns non-zero, only then perform an
- * iteration in that chunk to find the mismatch index.
- */
- num_chunks = get_sqrt(threshold);
- for (unsigned long i = 0; i < num_chunks; ++i) {
- size_t chunk_size = threshold / num_chunks;
- unsigned long shift = i * chunk_size;
-
- if (!memcmp(dest_addr + shift, rand_addr + shift, chunk_size))
- continue;
-
- /* brute force iteration only over mismatch segment */
- for (t = shift; t < shift + chunk_size; ++t) {
- if (((char *) dest_addr)[t] != rand_addr[t]) {
- ksft_print_msg("Data after remap doesn't match at
offset %llu\n",
- t);
- ksft_print_msg("Expected: %#x\t Got: %#x\n",
rand_addr[t] & 0xff,
- ((char *) dest_addr)[t] & 0xff);
- ret = -1;
- goto clean_up_dest;
- }
- }
- }
-
- /*
- * if threshold is not divisible by num_chunks, then check the
- * last chunk
- */
- for (t = num_chunks * (threshold / num_chunks); t < threshold; ++t) {
- if (((char *) dest_addr)[t] != rand_addr[t]) {
- ksft_print_msg("Data after remap doesn't match at offset
%llu\n",
- t);
- ksft_print_msg("Expected: %#x\t Got: %#x\n", rand_addr[t]
& 0xff,
- ((char *) dest_addr)[t] & 0xff);
- ret = -1;
- goto clean_up_dest;
- }
+ /* Verify byte pattern after remapping */
+ if (memcmp(dest_addr, rand_addr, threshold)) {
+ ksft_print_msg("Data after remap doesn't match\n");
+ ret = -1;
+ goto clean_up_dest;
}
/* Verify the dest preamble byte pattern after remapping */
- if (!c.dest_preamble_size)
- goto no_preamble;
-
- num_chunks = get_sqrt(c.dest_preamble_size);
-
- for (unsigned long i = 0; i < num_chunks; ++i) {
- size_t chunk_size = c.dest_preamble_size / num_chunks;
- unsigned long shift = i * chunk_size;
-
- if (!memcmp(dest_preamble_addr + shift, rand_addr + shift,
- chunk_size))
- continue;
-
- /* brute force iteration only over mismatched segment */
- for (d = shift; d < shift + chunk_size; ++d) {
- if (((char *) dest_preamble_addr)[d] != rand_addr[d]) {
- ksft_print_msg("Preamble data after remap doesn't
match at offset %llu\n",
- d);
- ksft_print_msg("Expected: %#x\t Got: %#x\n",
rand_addr[d] & 0xff,
- ((char *) dest_preamble_addr)[d]
& 0xff);
- ret = -1;
- goto clean_up_dest;
- }
- }
- }
-
- for (d = num_chunks * (c.dest_preamble_size / num_chunks); d <
c.dest_preamble_size; ++d) {
- if (((char *) dest_preamble_addr)[d] != rand_addr[d]) {
- ksft_print_msg("Preamble data after remap doesn't match at
offset %llu\n",
- d);
- ksft_print_msg("Expected: %#x\t Got: %#x\n", rand_addr[d]
& 0xff,
- ((char *) dest_preamble_addr)[d] &
0xff);
- ret = -1;
- goto clean_up_dest;
- }
+ if (c.dest_preamble_size &&
+ memcmp(dest_preamble_addr, rand_addr, c.dest_preamble_size)) {
+ ksft_print_msg("Preamble data after remap doesn't match\n");
+ ret = -1;
+ goto clean_up_dest;
}
-no_preamble:
start_ns = t_start.tv_sec * NS_PER_SEC + t_start.tv_nsec;
end_ns = t_end.tv_sec * NS_PER_SEC + t_end.tv_nsec;
ret = end_ns - start_ns;
