linux-kernel - Re: [RFC PATCH 0/4 v2] Define killable version for access_remote

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Date:   Tue, 6 Mar 2018 16:47:22 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     mingo@...nel.org, adobriyan@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC PATCH 0/4 v2] Define killable version for access_remote_vm()
 and use it in fs/proc



On 3/6/18 1:41 PM, Andrew Morton wrote:
> On Tue, 6 Mar 2018 13:17:37 -0800 Yang Shi <yang.shi@...ux.alibaba.com> 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.

Since we already had down_read_killable() APIs available, IMHO, giving 
application a chance to abort at some circumstances sounds not bad.

>
>>> 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?

I'm not quite sure. But, the existing applications may assume munmap is 
atomic?

Thanks,
Yang

>