Re: [RFC v4 0/3] mm: zap pages with read mmap_sem in munmap for large mapping

From: Yang Shi <yang.shi@linux.alibaba.com>
To: Michal Hocko <mhocko@kernel.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>
Cc: willy@infradead.org, ldufour@linux.vnet.ibm.com,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC v4 0/3] mm: zap pages with read mmap_sem in munmap for large mapping
Date: Wed, 11 Jul 2018 10:08:52 -0700	[thread overview]
Message-ID: <4076e0b4-f3a8-6d71-2a98-e7b8cc6986d4@linux.alibaba.com> (raw)
In-Reply-To: <20180711115332.GM20050@dhcp22.suse.cz>

On 7/11/18 4:53 AM, Michal Hocko wrote:
> On Wed 11-07-18 14:13:12, Kirill A. Shutemov wrote:
>> On Wed, Jul 11, 2018 at 12:33:12PM +0200, Michal Hocko wrote:
>>> this is not a small change for something that could be achieved
>>> from the userspace trivially (just call madvise before munmap - library
>>> can hide this). Most workloads will even not care about races because
>>> they simply do not play tricks with mmaps and userspace MM. So why do we
>>> want to put the additional complexity into the kernel?
>> As I said before, kernel latency issues have to be addressed in kernel.
>> We cannot rely on userspace being kind here.
> Those who really care and create really large mappings will know how to
> do this properly. Most others just do not care enough. So I am not
> really sure this alone is a sufficient argument.
>
> I personally like the in kernel auto tuning but as I've said the
> changelog should be really clear why all the complications are
> justified. This would be a lot easier to argue about if it was a simple
> 	if (len > THARSHOLD)
> 		do_madvise(DONTNEED)
> 	munmap().

The main difference AFAICS, is it can't deal with the parallel faults 
and those special mappings. Someone may not care about it, but someone may.

Yang

> approach. But if we really have to care about parallel faults and munmap
> consitency this will always be tricky