LinuxKPI: Add kvrealloc to linux/slab.h

[wulf]

Summary

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week

[bz]

sys/compat/linuxkpi/common/include/linux/slab.h

198

This should really use a kvrealloc (which we don't have yet).
See 8e4b8e9d807aa379d2a1c3aaac2537ba7d6bf0bf for the discussion. And I can say that the hypothetical case in that commit message is actually coming.
So this implementation as-is would likely be wrong in the future.

Alternatively please leave a comment /* XXX-BZ realloc */ here.

[emaste]

sys/compat/linuxkpi/common/include/linux/slab.h

198

In particular the comment at https://reviews.freebsd.org/D45181#1031077.

[wulf]

sys/compat/linuxkpi/common/include/linux/slab.h

198

Why do we need re-implementation of kv* functions?
According to https://www.kernel.org/doc/html/v5.0/core-api/memory-allocation.html kvalloc()/kvrealloc()/kvfree() is exactly what FreeBSD implements as malloc()/realloc()/free(). It combines both slab-based and vm page-based allocators.
Linux`s kmalloc() and vmalloc() is just stripped down of kvalloc(). The first one only for slab and the second one only for vm page allocations. FreeBSD combines both of them in single malloc().

[wulf]

Implement kvrealloc via kvmalloc/kvfree pair

[wulf]

sys/compat/linuxkpi/common/include/linux/slab.h

198

After glance over code kvmalloc appeared to be a different thing. It tries to allocate pages contiguously and fall-backs to noncontig allocations