Partial page table mappings when allocator runs out of frames during region insertion

[LiuJun5817]

When inserting a new memory region into a MemorySet, the mapping procedure may fail after partially mapping the region if the physical frame allocator runs out of free frames.

The current workflow is:

/// Add a memory region to this set.
pub fn insert(&mut self, region: MemoryRegion<PT::VA>) -> HvResult {
    ...
    self.pt.map(&region)?;
    self.regions.insert(region.start, region);
    Ok(())
}

During map():

fn map(&mut self, region: &MemoryRegion<VA>) -> HvResult {
    ...
    while size > 0 {
        ...
        self.inner
            .map_page(page, paddr, region.flags)
            .map_err(|e: PagingError| {
                error!(
                    "failed to map page: {:#x?}({:?}) -> {:#x?}, {:?}",
                    vaddr, page_size, paddr, e
                );
                e
            })?;
        ...
    }
    Ok(())
}

If the allocator cannot provide enough frames during page table creation (e.g., intermediate page tables), map_page() returns an error and the mapping process aborts.

Problem
This leads to an inconsistent state:

• Some virtual pages in the region are mapped
• Others remain unmapped
• The region is not inserted into self.regions
• Page table state and memory region metadata become inconsistent

[LiuJun5817]

@li041