Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

From: Michal Hocko <mhocko@suse.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
	Minchan Kim <minchan@kernel.org>,
	Christian Brauner <christian@brauner.io>,
	Christoph Hellwig <hch@infradead.org>,
	Oleg Nesterov <oleg@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	Jann Horn <jannh@google.com>, Shakeel Butt <shakeelb@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Florian Weimer <fweimer@redhat.com>,
	Jan Engelhardt <jengelh@inai.de>,
	Linux API <linux-api@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap
Date: Mon, 1 Nov 2021 09:37:52 +0100	[thread overview]
Message-ID: <YX+nYGlZBOAljoeF@dhcp22.suse.cz> (raw)
In-Reply-To: <CAJuCfpHBoMGPOUvB2ZWQ=TxbFuWBRF++UaKJZDCrQV4mzb5kMA@mail.gmail.com>

On Fri 29-10-21 09:07:39, Suren Baghdasaryan wrote:
> On Fri, Oct 29, 2021 at 6:03 AM Michal Hocko <mhocko@suse.com> wrote:
[...]
> > Well, I still do not see why that is a problem. This syscall is meant to
> > release the address space not to do it fast.
> 
> It's the same problem for a userspace memory reaper as for the
> oom-reaper. The goal is to release the memory of the victim and to
> quickly move on to the next one if needed.

The purpose of the oom_reaper is to _guarantee_ a forward progress. It
doesn't have to be quick or optimized for speed.

[...]

> > Btw. the above code will not really tell you much on a larger machine
> > unless you manage to trigger mmap_sem contection. Otherwise you are
> > measuring the mmap_sem writelock fast path and that should be really
> > within a noise comparing to the whole address space destruction time. If
> > that is not the case then we have a real problem with the locking...
> 
> My understanding of that discussion is that the concern was that even
> taking uncontended mmap_sem writelock would regress the exit path.
> That was what I wanted to confirm. Am I misreading it?

No, your reading match my recollection. I just think that code
robustness in exchange of a rw semaphore write lock fast path is a
reasonable price to pay even if that has some effect on micro
benchmarks.
-- 
Michal Hocko
SUSE Labs