xen-tools: Revamp mmap allocation in QEMU

[temerkhanov]

Change the algorithm of mmap-based memory allocation to reduce the amount of PTE changes

Signed-off-by: Sergey Temerkhanov [email protected]

[rvs]

Instead of calling mmap() twice, we do mmap(...PROT_READ|PROT_WRITE...) + mprotect(PROT_NONE) to get the guard pages

Right @temerkhanov -- I get this -- but what's the benefit of this way of doing things?

[temerkhanov]

Instead of calling mmap() twice, we do mmap(...PROT_READ|PROT_WRITE...) + mprotect(PROT_NONE) to get the guard pages

Right @temerkhanov -- I get this -- but what's the benefit of this way of doing things?

At least it reduces amount of noise in memory profilers, and, apparently, the VSZ

[rouming]

The patch calls mprotect() on a memory region instead of a second mmap(), expecting, that this will reduce amount of PTE changes, which is not the case: mprotect splits VMA and updates PTE range, so performance benefits are not clear. @eriknordmark let's close this.

[eriknordmark]

Closing based on the comment from @rouming above.