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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Thu, 28 Jan 2021 16:11:52 +0100
To:     Michal Hocko <mhocko@suse.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Daniel Axtens <dja@axtens.net>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <neeraju@codeaurora.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument
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On Mon, Jan 25, 2021 at 05:25:59PM +0100, Uladzislau Rezki wrote:
> On Mon, Jan 25, 2021 at 04:39:43PM +0100, Michal Hocko wrote:
> > On Mon 25-01-21 15:31:50, Uladzislau Rezki wrote:
> > > > On Wed 20-01-21 17:21:46, Uladzislau Rezki (Sony) wrote:
> > > > > For a single argument we can directly request a page from a caller
> > > > > context when a "carry page block" is run out of free spots. Instead
> > > > > of hitting a slow path we can request an extra page by demand and
> > > > > proceed with a fast path.
> > > > > 
> > > > > A single-argument kvfree_rcu() must be invoked in sleepable contexts,
> > > > > and that its fallback is the relatively high latency synchronize_rcu().
> > > > > Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL
> > > > > to allow limited sleeping within the memory allocator.
> > > > 
> > > > __GFP_RETRY_MAYFAIL can be quite heavy. It is effectively the most heavy
> > > > way to allocate without triggering the OOM killer. Is this really what
> > > > you need/want? Is __GFP_NORETRY too weak?
> > > > 
> > > Hm... We agreed to proceed with limited lightwait memory direct reclaim.
> > > Johannes Weiner proposed to go with __GFP_NORETRY flag as a starting
> > > point: https://www.spinics.net/lists/rcu/msg02856.html
> > > 
> > > <snip>
> > >     So I'm inclined to suggest __GFP_NORETRY as a starting point, and make
> > >     further decisions based on instrumentation of the success rates of
> > >     these opportunistic allocations.
> > > <snip>
> > 
> > I completely agree with Johannes here.
> > 
> > > but for some reason, i can't find a tail or head of it, we introduced
> > > __GFP_RETRY_MAYFAIL what is a heavy one from a time consuming point of view.
> > > What we would like to avoid.
> > 
> > Not that I object to this use but I think it would be much better to use
> > it based on actual data. Going along with it right away might become a
> > future burden to make any changes in this aspect later on due to lack of 
> > exact reasoning. General rule of thumb for __GFP_RETRY_MAYFAIL is really
> > try as hard as it can get without being really disruptive (like OOM
> > killing something). And your wording didn't really give me that
> > impression.
> > 
> Initially i proposed just to go with GFP_NOWAIT flag. But later on there
> was a discussion about a fallback path, that uses synchronize_rcu() can be
> slow, thus minimizing its hitting would be great. So, here we go with a
> trade off.
> 
> Doing it hard as __GFP_RETRY_MAYFAIL can do, is not worth(IMHO), but to have some
> light-wait requests would be acceptable. That is why __GFP_NORETRY was proposed.
> 
> There were simple criterias we discussed which we would like to achieve:
> 
> a) minimize a fallback hitting;
> b) avoid of OOM involving;
> c) avoid of dipping into the emergency reserves. See kvfree_rcu: Use __GFP_NOMEMALLOC for single-argument kvfree_rcu()
> 
One question here. Since the code that triggers a page request can be
directly invoked from reclaim context as well as outside of it. We had
a concern about if any recursion is possible, but what i see it is safe.
The context that does it can not enter it twice:

<snip>
    /* Avoid recursion of direct reclaim */
    if (current->flags & PF_MEMALLOC)
        goto nopage;
<snip>

What about any deadlocking in regards to below following flags?

GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN

Thanks!

--
Vlad Rezki