linux-kernel - Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

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Date:   Mon, 1 Feb 2021 15:44:34 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

On Mon, Feb 01, 2021 at 12:47:55PM +0100, Michal Hocko wrote:
> On Fri 29-01-21 17:35:31, Uladzislau Rezki wrote:
> > On Fri, Jan 29, 2021 at 09:56:29AM +0100, Michal Hocko wrote:
> > > On Thu 28-01-21 19:02:37, Uladzislau Rezki wrote:
> > > [...]
> > > > >From 0bdb8ca1ae62088790e0a452c4acec3821e06989 Mon Sep 17 00:00:00 2001
> > > > From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
> > > > Date: Wed, 20 Jan 2021 17:21:46 +0100
> > > > Subject: [PATCH v2 1/1] kvfree_rcu: Directly allocate page for single-argument
> > > >  case
> > > > 
> > > > Single-argument kvfree_rcu() must be invoked from sleepable contexts,
> > > > so we can directly allocate pages.  Furthermmore, the fallback in case
> > > > of page-allocation failure is the high-latency synchronize_rcu(), so it
> > > > makes sense to do these page allocations from the fastpath, and even to
> > > > permit limited sleeping within the allocator.
> > > > 
> > > > This commit therefore allocates if needed on the fastpath using
> > > > GFP_KERNEL|__GFP_NORETRY.
> > > 
> > > Yes, __GFP_NORETRY as a lightweight allocation mode should be fine. It
> > > is more robust than __GFP_NOWAIT on memory usage spikes.  The caller is
> > > prepared to handle the failure which is likely much less disruptive than
> > > OOM or potentially heavy reclaim __GFP_RETRY_MAYFAIL.
> > > 
> > > I cannot give you ack as I am not familiar with the code but this makes
> > > sense to me.
> > > 
> > No problem, i can separate it. We can have a patch on top of what we have so
> > far. The patch only modifies the gfp_mask passed to __get_free_pages():
> > 
> > >From ec2feaa9b7f55f73b3b17e9ac372151c1aab5ae0 Mon Sep 17 00:00:00 2001
> > From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
> > Date: Fri, 29 Jan 2021 17:16:03 +0100
> > Subject: [PATCH 1/1] kvfree_rcu: replace __GFP_RETRY_MAYFAIL by __GFP_NORETRY
> > 
> > __GFP_RETRY_MAYFAIL is a bit heavy from reclaim process of view,
> > therefore a time consuming. That is not optional and there is
> > no need in doing it so hard, because we have a fallback path.
> > 
> > __GFP_NORETRY in its turn can perform some light-weight reclaim
> > and it rather fails under high memory pressure or low memory
> > condition.
> > 
> > In general there are four simple criterias we we would like to
> > achieve:
> >     a) minimize a fallback hitting;
> >     b) avoid of OOM invoking;
> >     c) do a light-wait page request;
> >     d) avoid of dipping into the emergency reserves.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> 
> Looks good to me. Feel free to add
> Acked-by: Michal Hocko <mhocko@...e.com>
> 
Appreciate it!

--
Vlad Rezki