6a97a3f756
a hint. Right now, for non-fixed mmap(2) calls, addr is de-facto interpreted as the absolute minimal address of the range where the mapping is created. The VA allocator only allocates in the range [addr, VM_MAXUSER_ADDRESS]. This is too restrictive, the mmap(2) call might unduly fail if there is no free addresses above addr but a lot of usable space below it. Lift this implementation limitation by allocating VA in two passes. First, try to allocate above addr, as before. If that fails, do the second pass with less restrictive constraints for the start of allocation by specifying minimal allocation address at the max bss end, if this limit is less than addr. One important case where this change makes a difference is the allocation of the stacks for new threads in libthr. Under some configuration conditions, libthr tries to hint kernel to reuse the main thread stack grow area for the new stacks. This cannot work by design now after grow area is converted to stack, and there is no unallocated VA above the main stack. Interpreting requested stack base address as the hint provides compatibility with old libthr and with (mis-)configured current libthr. Reviewed by: alc Tested by: dim (previous version) Sponsored by: The FreeBSD Foundation MFC after: 1 week
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