Re: [RFC PATCH 0/4 v2] Define killable version for access_remote_vm() and use it in fs/proc

[Andrew Morton]

On Tue, 6 Mar 2018 13:17:37 -0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:

> 
> 
> It just mitigates the hung task warning, can't resolve the mmap_sem 
> scalability issue. Furthermore, waiting on pure uninterruptible state 
> for reading /proc sounds unnecessary. It doesn't wait for I/O completion.

OK.

> >
> > Where the heck are we holding mmap_sem for so long?  Can that be fixed?
> 
> The mmap_sem is held for unmapping a large map which has every single 
> page mapped. This is not a issue in real production code. Just found it 
> by running vm-scalability on a machine with ~600GB memory.
> 
> AFAIK, I don't see any easy fix for the mmap_sem scalability issue. I 
> saw range locking patches (https://lwn.net/Articles/723648/) were 
> floating around. But, it may not help too much on the case that a large 
> map with every single page mapped.

Well it sounds fairly simple to mitigate?  Simplistically: don't unmap
600G in a single hit; do it 1G at a time, dropping mmap_sem each time. 
A smarter version might only come up for air if there are mmap_sem
waiters and if it has already done some work.  I don't think we have
any particular atomicity requirements when unmapping?