vrealloc() shrink TODO

vrealloc() shrink TODO

[Laveesh Bansal]

Hi Danilo,

I came across the TODO you left in vrealloc_node_align_noprof() (mm/vmalloc.c:4317) about shrinking the vm_area when reallocating to a smaller size. I'd like to work on this if it's still relevant.

I wanted to check if this approach makes sense: keep the virtual address range unchanged, unmap pages from the end using vunmap_range(), free them, and update nr_pages and memcg accounting. This would avoid vmap_area allocator complexity while still reclaiming physical memory.

One thing I'm not sure about: for huge page allocations (page_order > 0), partial unmapping seems problematic - should shrink just be skipped in that case?

A few questions:
1. Is this optimization still wanted?
2. Does the approach above make sense, or is there a better way?
3. Any preference on the shrink heuristic?
4. Anything else I should watch out for?

Thanks,
Laveesh