No copy on write system call?

How is it that a write (man 2 write) can be passed a buffer
that exceeds most of the physical and virtual memory on a
system and work successfully?

I wrote a simple program that allocates a large buffer on
the heap, zeros the space and then calls write
(man 2 write). Even though the buffer is almost as large
as physical memory + page space the call completes without
error. In my example I allocated 705MB on a system that
has 256MB physical and 512MB virtual. When it completes
there is a 705MB file on disk as expected.

So is it in the case of the sys_write that the memory is
accessed and allowed to page fault as needed while running
in the context of the kernel? Most of the kernel books
talk about using copy_from_user on system calls before
using memory from the user. Strace shows one write
call with the entire length.

So what is going on to allow this to happen?

Thanks,
Tony

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