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

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Date:   Tue, 21 May 2019 12:37:26 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>, linux-api@...r.kernel.org
Subject: Re: [RFC 6/7] mm: extend process_madvise syscall to support vector
 arrary

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