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

[Michal Hocko]

On Tue 26-06-18 18:03:34, Yang Shi wrote:
> 
> 
> On 6/26/18 12:43 AM, Peter Zijlstra wrote:
> > 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.
> 
> Thanks, Peter. Yes, by looking the code and trying two different approaches,
> it looks this approach is the most straight-forward one.

Yes, you just have to be careful about the max vma count limit.
-- 
Michal Hocko
SUSE Labs