Re: [PATCH] selftests/mm: Simplify byte pattern checking in mremap_test

[David Hildenbrand (Arm)]

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 <sarthak.sharma@xxxxxxx>
> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>

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