Re: [PATCH v3] malloc: Remove bin scanning from memalign (bug 30723)

[Florian Weimer <fweimer@redhat.com>]

* Stefan Liebler:

> On 11.08.23 17:48, Florian Weimer via Libc-alpha wrote:
>> On the test workload (mpv --cache=yes with VP9 video decoding), the
>> bin scanning has a very poor success rate (less than 2%).  The tcache
>> scanning has about 50% success rate, so keep that.
>> 
>> Update comments in malloc/tst-memalign-2 to indicate the purpose
>> of the tests.  Even with the scanning removed, the additional
>> merging opportunities since commit 542b1105852568c3ebc712225ae78b
>> ("malloc: Enable merging of remainders in memalign (bug 30723)")
>> are sufficient to pass the existing large bins test.
> Hi,
>
> on s390x, I've observed these test-fails starting with this commit:
> FAIL: malloc/tst-memalign-2
> FAIL: malloc/tst-memalign-2-malloc-hugetlb1
>
> With this output:
> error: tst-memalign-2.c:158: not true: count > LN / 2
> error: 1 test failures
>
> Note, that I only see the fails if /proc/sys/kernel/randomize_va_space
> is set to 0. Then I get count=3; LN=8;
>
> If randomize_va_space is set to 2, the test passes and I get count=5; LN=8;
>
> Is s390x the only architecture where we get this failure?

Does your kernel have a fix for the early sbrk failure?  I suspect what
happens is that we fall over to mmap from sbrk, making the heap
non-contiguous.

The sbrk failure should be visible under strace.

Thanks,
Florian