[RFC] mm: Add MADV_NONZERO to skip redundant zeroing in MADV_DONTNEED

[Ayaan Mirza Baig <ayaanmirzabaig85@gmail.com>]

Hi Andrew,

I've been looking at MADV_DONTNEED and noticed we always zero pages, 
even when
it might not be necessary for cases like:

- Media buffers (video/audio processing)
- Temporary compute workloads
- Pre-zeroed memory pools

This causes measurable overhead - in some synthetic tests I'm seeing 
~40% extra
latency from the zeroing. I'm thinking of adding a MADV_NONZERO flag 
that would
let us skip the zeroing when:

1. It's explicitly requested (MADV_DONTNEED|MADV_NONZERO)
2. We're confident it's safe (VM_IO, VM_HUGETLB, etc)
3. The user has explicitly marked the region as non-sensitive

Before I go deeper on patches, I wanted to check:
- Does this sound reasonable?
- Any obvious security holes I'm missing?
- Would you prefer a new flag or a separate advice type?

Early prototype numbers look promising, but I want to make sure I'm not 
missing
something fundamental. If the concept seems viable, I'll work up proper 
benchmarks
and formal patches.

Thanks for your time!
- Ayaan Mirza Baig