linux-kernel - Re: [PATCH v7 2/7] mm: introduce external memory hinting API

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Date:   Tue, 10 Mar 2020 17:36:46 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, linux-api@...r.kernel.org,
        oleksandr@...hat.com, Suren Baghdasaryan <surenb@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Dias <joaodias@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jann Horn <jannh@...gle.com>,
        alexander.h.duyck@...ux.intel.com, sj38.park@...il.com
Subject: Re: [PATCH v7 2/7] mm: introduce external memory hinting API

On Tue, Mar 10, 2020 at 03:20:08PM -0700, Minchan Kim wrote:
> On Thu, Mar 05, 2020 at 07:15:10PM +0100, Vlastimil Babka wrote:
> > On 3/2/20 8:36 PM, Minchan Kim wrote:
> > > There is usecase that System Management Software(SMS) want to give
> > > a memory hint like MADV_[COLD|PAGEEOUT] to other processes and
> > > in the case of Android, it is the ActivityManagerService.
> > > 
> > > It's similar in spirit to madvise(MADV_WONTNEED), but the information
> > 
> > You mean MADV_DONTNEED?
> 
> Mean to DONT_NEED's future version.
> 
> > 
> > > required to make the reclaim decision is not known to the app.
> > 
> > This seems to be mixing up the differences between MADV_DONTNEED and
> > COLD/PAGEOUT and self-imposed vs external memory hints?
> 
> Sorry, I don't understand what you want here.
> 
> > 
> > > Instead,
> > > it is known to the centralized userspace daemon(ActivityManagerService),
> > > and that daemon must be able to initiate reclaim on its own without
> > > any app involvement.
> > > 
> > > To solve the issue, this patch introduces a new syscall process_madvise(2).
> > > It uses pidfd of an external process to give the hint.
> > > 
> > >  int process_madvise(int pidfd, void *addr, size_t length, int advise,
> > > 			unsigned long flag);
> > 
> > It's more common to call the argument 'flags' not 'flag'? The code seems to call
> > it flags. Also in line with madvise(2), the 'advise' argument should rather be
> > 'advice'. This is more important for the manpage, but let's be consistent.
> 
> Yub, I will send the patch along with your comment in previous patch.

Only place to use *advice* in kernel is comment in madvise_willneed.
Al other places use advise in kernel so I wanted to be more consistent
with other kernel sources. For man page, we could use the term "advice",
no problem.