linux-kernel - Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200930152517.GA1470428@google.com>
Date:   Wed, 30 Sep 2020 11:25:17 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

On Fri, Sep 25, 2020 at 05:47:41PM +0200, Michal Hocko wrote:
> On Fri 25-09-20 17:31:29, Uladzislau Rezki wrote:
> > > > > > 
> > > > > > All good points!
> > > > > > 
> > > > > > On the other hand, duplicating a portion of the allocator functionality
> > > > > > within RCU increases the amount of reserved memory, and needlessly most
> > > > > > of the time.
> > > > > > 
> > > > > 
> > > > > But it's very similar to what mempools are for.
> > > > > 
> > > > As for dynamic caching or mempools. It requires extra logic on top of RCU
> > > > to move things forward and it might be not efficient way. As a side
> > > > effect, maintaining of the bulk arrays in the separate worker thread
> > > > will introduce other drawbacks:
> > > 
> > > This is true but it is also true that it is RCU to require this special
> > > logic and we can expect that we might need to fine tune this logic
> > > depending on the RCU usage. We definitely do not want to tune the
> > > generic page allocator for a very specific usecase, do we?
> > > 
> > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability
> > to provide a memory service for contexts which are not allowed to
> > sleep, RCU is part of them. Both flags used to provide such ability
> > before but not anymore.
> > 
> > Do you agree with it?
> 
> Yes this sucks. But this is something that we likely really want to live
> with. We have to explicitly _document_ that really atomic contexts in RT
> cannot use the allocator. From the past discussions we've had this is
> likely the most reasonable way forward because we do not really want to
> encourage anybody to do something like that and there should be ways
> around that. The same is btw. true also for !RT. The allocator is not
> NMI safe and while we should be able to make it compatible I am not
> convinced we really want to.
> 
> Would something like this be helpful wrt documentation?
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 67a0774e080b..9fcd47606493 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -238,7 +238,9 @@ struct vm_area_struct;
>   * %__GFP_FOO flags as necessary.
>   *
>   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
> - * watermark is applied to allow access to "atomic reserves"
> + * watermark is applied to allow access to "atomic reserves".
> + * The current implementation doesn't support NMI and other non-preemptive context
> + * (e.g. raw_spin_lock).

I think documenting is useful.

Could it be more explicit in what the issue is? Something like:

* Even with GFP_ATOMIC, calls to the allocator can sleep on PREEMPT_RT
systems. Therefore, the current low-level allocator implementation does not
support being called from special contexts that are atomic on RT - such as
NMI and raw_spin_lock. Due to these constraints and considering calling code
usually has no control over the PREEMPT_RT configuration, callers of the
allocator should avoid calling the allocator from these cotnexts even in
non-RT systems.

thanks!

 - Joel


>   *
>   * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
>   * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> 
> [...]
> -- 
> Michal Hocko
> SUSE Labs