On 4/10/26 16:30, 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.
So is that the only problem with 7033c6cc9620: the speed?
>
> 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.
Fully agreed.
>
> So instead of fixing get_sqrt(), bite the bullet, drop mismatch index
> scanning and just compare the two byte streams with memcmp().
How does this affect the execution time of the test?
>
> Reported-by: Sarthak Sharma <[email protected]>
> Signed-off-by: Dev Jain <[email protected]>
Fixes: 7033c6cc9620 ("selftests/mm: mremap_test: optimize execution time
from minutes to seconds using chunkwise memcmp")
?
> ---
> 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(-)
I mean, it certainly looks like a nice cleanup.
--
Cheers,
David
