Mapping of ZONE_HIGHMEM in kernel address space in 32bit x86

[Valdis.Kletnieks at vt.edu]

On Mon, 13 May 2013 14:11:22 -0500, Sergio Andr said:

> 2. When user applications allocates memory, the kernel must allocate
> virtual memory and physical memory, right?

Wrong. If userspace allocates (say) 15M of memory, the kernel has every right
to overcommit and not actually allocate either physical memory or backing page
space for all 15M.  It instead maps it as a non-existent virtual address, and
if/when the application actually touches the page, it generates a page fault,
and *then* the kernel does the allocating of physical memory and maybe swap
space.

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