Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree

Uladzislau Rezki
2023-09-07T15:33:18Z

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@redhat.com>
>
Thanks for the review!

--
Uladzislau Rezki