Re: [RFC 6/7] mm: extend process_madvise syscall to support vector arrary

From: Michal Hocko <mhocko@kernel.org>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Tim Murray <timmurray@google.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Daniel Colascione <dancol@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	Sonny Rao <sonnyrao@google.com>,
	Brian Geffon <bgeffon@google.com>,
	linux-api@vger.kernel.org
Subject: Re: [RFC 6/7] mm: extend process_madvise syscall to support vector arrary
Date: Tue, 21 May 2019 12:37:26 +0200	[thread overview]
Message-ID: <20190521103726.GM32329@dhcp22.suse.cz> (raw)
In-Reply-To: <20190521102613.GC219653@google.com>

On Tue 21-05-19 19:26:13, Minchan Kim wrote:
> On Tue, May 21, 2019 at 08:24:21AM +0200, Michal Hocko wrote:
> > On Tue 21-05-19 11:48:20, Minchan Kim wrote:
> > > On Mon, May 20, 2019 at 11:22:58AM +0200, Michal Hocko wrote:
> > > > [Cc linux-api]
> > > > 
> > > > On Mon 20-05-19 12:52:53, Minchan Kim wrote:
> > > > > Currently, process_madvise syscall works for only one address range
> > > > > so user should call the syscall several times to give hints to
> > > > > multiple address range.
> > > > 
> > > > Is that a problem? How big of a problem? Any numbers?
> > > 
> > > We easily have 2000+ vma so it's not trivial overhead. I will come up
> > > with number in the description at respin.
> > 
> > Does this really have to be a fast operation? I would expect the monitor
> > is by no means a fast path. The system call overhead is not what it used
> > to be, sigh, but still for something that is not a hot path it should be
> > tolerable, especially when the whole operation is quite expensive on its
> > own (wrt. the syscall entry/exit).
> 
> What's different with process_vm_[readv|writev] and vmsplice?
> If the range needed to be covered is a lot, vector operation makes senese
> to me.

I am not saying that the vector API is wrong. All I am trying to say is
that the benefit is not really clear so far. If you want to push it
through then you should better get some supporting data.
-- 
Michal Hocko
SUSE Labs