Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping

[Peter Zijlstra]

On Mon, Jun 25, 2018 at 05:06:23PM -0700, Yang Shi wrote:
> By looking this deeper, we may not be able to cover all the unmapping range
> for VM_DEAD, for example, if the start addr is in the middle of a vma. We
> can't set VM_DEAD to that vma since that would trigger SIGSEGV for still
> mapped area.
> 
> splitting can't be done with read mmap_sem held, so maybe just set VM_DEAD
> to non-overlapped vmas. Access to overlapped vmas (first and last) will
> still have undefined behavior.

Acquire mmap_sem for writing, split, mark VM_DEAD, drop mmap_sem. Acquire
mmap_sem for reading, madv_free drop mmap_sem. Acquire mmap_sem for
writing, free everything left, drop mmap_sem.

?

Sure, you acquire the lock 3 times, but both write instances should be
'short', and I suppose you can do a demote between 1 and 2 if you care.