linux-kernel - Re: [RFC 4/7] mm: factor out madvise's core functionality

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Message-ID: <20190521113254.GU32329@dhcp22.suse.cz>
Date:   Tue, 21 May 2019 13:32:54 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Oleksandr Natalenko <oleksandr@...hat.com>,
        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>
Subject: Re: [RFC 4/7] mm: factor out madvise's core functionality

On Tue 21-05-19 20:24:23, Minchan Kim wrote:
> On Tue, May 21, 2019 at 01:00:30PM +0200, Michal Hocko wrote:
> > On Tue 21-05-19 19:52:56, Minchan Kim wrote:
> > > On Tue, May 21, 2019 at 09:06:38AM +0200, Oleksandr Natalenko wrote:
> > > > Hi.
> > > > 
> > > > On Tue, May 21, 2019 at 08:50:00AM +0200, Michal Hocko wrote:
> > > > > On Tue 21-05-19 08:36:28, Oleksandr Natalenko wrote:
> > > > > [...]
> > > > > > Regarding restricting the hints, I'm definitely interested in having
> > > > > > remote MADV_MERGEABLE/MADV_UNMERGEABLE. But, OTOH, doing it via remote
> > > > > > madvise() introduces another issue with traversing remote VMAs reliably.
> > > > > > IIUC, one can do this via userspace by parsing [s]maps file only, which
> > > > > > is not very consistent, and once some range is parsed, and then it is
> > > > > > immediately gone, a wrong hint will be sent.
> > > > > > 
> > > > > > Isn't this a problem we should worry about?
> > > > > 
> > > > > See http://lkml.kernel.org/r/20190520091829.GY6836@dhcp22.suse.cz
> > > > 
> > > > Oh, thanks for the pointer.
> > > > 
> > > > Indeed, for my specific task with remote KSM I'd go with map_files
> > > > instead. This doesn't solve the task completely in case of traversal
> > > > through all the VMAs in one pass, but makes it easier comparing to a
> > > > remote syscall.
> > > 
> > > I'm wondering how map_files can solve your concern exactly if you have
> > > a concern about the race of vma unmap/remap even there are anonymous
> > > vma which map_files doesn't support.
> > 
> > See http://lkml.kernel.org/r/20190521105503.GQ32329@dhcp22.suse.cz
> 
> Question is how it works for anonymous vma which don't have backing
> file.

We would have to export map_files like interface for anonymous vmas
and have a way to invalidate on the VMA removal (reference counting or
something similar), no question this will be some additional work to do.
-- 
Michal Hocko
SUSE Labs