linux-kernel - Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area

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Message-ID: <ZPmalB+hfaxAZIcw@pc636>
Date:   Thu, 7 Sep 2023 11:40:36 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>, k-hagio-ab@....com,
        lijiang@...hat.com, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        kexec@...ts.infradead.org
Subject: Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree

On Thu, Sep 07, 2023 at 05:38:07PM +0800, Baoquan He wrote:
> On 09/07/23 at 10:17am, Baoquan He wrote:
> > Add Kazu and Lianbo to CC, and kexec mailing list
> > 
> > On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
> > > Store allocated objects in a separate nodes. A va->va_start
> > > address is converted into a correct node where it should
> > > be placed and resided. An addr_to_node() function is used
> > > to do a proper address conversion to determine a node that
> > > contains a VA.
> > > 
> > > Such approach balances VAs across nodes as a result an access
> > > becomes scalable. Number of nodes in a system depends on number
> > > of CPUs divided by two. The density factor in this case is 1/2.
> > > 
> > > Please note:
> > > 
> > > 1. As of now allocated VAs are bound to a node-0. It means the
> > >    patch does not give any difference comparing with a current
> > >    behavior;
> > > 
> > > 2. The global vmap_area_lock, vmap_area_root are removed as there
> > >    is no need in it anymore. The vmap_area_list is still kept and
> > >    is _empty_. It is exported for a kexec only;
> > 
> > I haven't taken a test, while accessing all nodes' busy tree to get
> > va of the lowest address could severely impact kcore reading efficiency
> > on system with many vmap nodes. People doing live debugging via
> > /proc/kcore will get a little surprise.
> > 
> > Empty vmap_area_list will break makedumpfile utility, Crash utility
> > could be impactd too. I checked makedumpfile code, it relys on
> > vmap_area_list to deduce the vmalloc_start value. 
> 
> Except of the empty vmap_area_list, this patch looks good to me.
> 
> We may need think of another way to export the vmalloc_start value or
> deduce it in makedumpfile/Crash utility. And then remove the useless
> vmap_area_list. I am not sure if we should remove vmap_area_list in this
> patch because the empty value will cause breakage anyway. Otherwise,
> 
> Reviewed-by: Baoquan He <bhe@...hat.com>
> 
Thanks for the review!

--
Uladzislau Rezki