Official docs:
Some headers from C library were deprecated in C++ and are no longer
welcome in C++ codebases. Some have no effect in C++. For more details
refer to the C++ 14 Standard [depr.c.headers] section. This check
replaces C standard library headers with their C++ alternatives and
removes redundant ones.
qemu does not support MADV_DONTNEED, and by not support it simply ignore
it (i.e. return 0 -- no error).
This issue has been "fixed" in #15590, however it just
terminates the process, and completely breaks clickhouse under qemu
(see also #15174).
But there is no need in such strong protection, we can stop using
madvise in jemalloc if MADV_DONTNEED does not work properly.
And this is what #18169 was tried to do (by override madvise), however
this will break sanitizers, at least TSAN and UBSAN.
The problem there is that sanitizers initialization code uses madvise
(and there is no way to turn this off with TSAN_OPTIONS) and overwritten
madvise function will have sanitizers traits (__tsan_func_entry), while
TSAN is not ready for this, and eventually it SIGSEGV.
Interesting thing is that in the recent clang-12, madvise was replaced
with direct syscall [1].
[1]: 9f8c4039f2
But it is better to make clickhouse compatible with clang < 12 too, so
instead of override madvise completely, the runtime check was moved into
the jemalloc code [2].
[2]: https://github.com/ClickHouse-Extras/jemalloc/pull/1
jemalloc relies on working MADV_DONTNEED (that fact that after
madvise(MADV_DONTNEED) returns success, after subsequent access to those
pages they will be zeroed).
However qemu does not support this, yet [1], and you can get very tricky
assert if you will run clickhouse-server under qemu:
<jemalloc>: ../contrib/jemalloc/src/extent.c:1195: Failed assertion: "p[i] == 0"
[1]: https://patchwork.kernel.org/patch/10576637/
But after this patch you will get pretty error:
$ qemu-x86_64-static programs/clickhouse
MADV_DONTNEED does not zeroed page. jemalloc will be broken
