linux-kernel - Re: [PATCH 14/15] x86/numa: remove redundant iteration over memblock.reserved

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Message-ID: <20200728142348.GE10792@MiWiFi-R3L-srv>
Date:   Tue, 28 Jul 2020 22:23:48 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Hellwig <hch@....de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Max Filippov <jcmvbkbc@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Simek <monstr@...str.eu>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Russell King <linux@...linux.org.uk>,
        Stafford Horne <shorne@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        clang-built-linux@...glegroups.com,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-c6x-dev@...ux-c6x.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, linuxppc-dev@...ts.ozlabs.org,
        openrisc@...ts.librecores.org, sparclinux@...r.kernel.org,
        uclinux-h8-devel@...ts.sourceforge.jp, x86@...nel.org
Subject: Re: [PATCH 14/15] x86/numa: remove redundant iteration over
 memblock.reserved

On 07/28/20 at 05:15pm, Mike Rapoport wrote:
> On Tue, Jul 28, 2020 at 07:02:54PM +0800, Baoquan He wrote:
> > On 07/28/20 at 08:11am, Mike Rapoport wrote:
> > > From: Mike Rapoport <rppt@...ux.ibm.com>
> > > 
> > > numa_clear_kernel_node_hotplug() function first traverses numa_meminfo
> > > regions to set node ID in memblock.reserved and than traverses
> > > memblock.reserved to update reserved_nodemask to include node IDs that were
> > > set in the first loop.
> > > 
> > > Remove redundant traversal over memblock.reserved and update
> > > reserved_nodemask while iterating over numa_meminfo.
> > > 
> > > Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> > > ---
> > >  arch/x86/mm/numa.c | 26 ++++++++++----------------
> > >  1 file changed, 10 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> > > index 8ee952038c80..4078abd33938 100644
> > > --- a/arch/x86/mm/numa.c
> > > +++ b/arch/x86/mm/numa.c
> > > @@ -498,31 +498,25 @@ static void __init numa_clear_kernel_node_hotplug(void)
> > >  	 * and use those ranges to set the nid in memblock.reserved.
> > >  	 * This will split up the memblock regions along node
> > >  	 * boundaries and will set the node IDs as well.
> > > +	 *
> > > +	 * The nid will also be set in reserved_nodemask which is later
> > > +	 * used to clear MEMBLOCK_HOTPLUG flag.
> > > +	 *
> > > +	 * [ Note, when booting with mem=nn[kMG] or in a kdump kernel,
> > > +	 *   numa_meminfo might not include all memblock.reserved
> > > +	 *   memory ranges, because quirks such as trim_snb_memory()
> > > +	 *   reserve specific pages for Sandy Bridge graphics.
> > > +	 *   These ranges will remain with nid == MAX_NUMNODES. ]
> > >  	 */
> > >  	for (i = 0; i < numa_meminfo.nr_blks; i++) {
> > >  		struct numa_memblk *mb = numa_meminfo.blk + i;
> > >  		int ret;
> > >  
> > >  		ret = memblock_set_node(mb->start, mb->end - mb->start, &memblock.reserved, mb->nid);
> > > +		node_set(mb->nid, reserved_nodemask);
> > 
> > Really? This will set all node id into reserved_nodemask. But in the
> > current code, it's setting nid into memblock reserved region which
> > interleaves with numa_memoinfo, then get those nid and set it in
> > reserved_nodemask. This is so different, with my understanding. Please
> > correct me if I am wrong.
> 
> You are right, I've missed the intersections of numa_meminfo with
> memblock.reserved.
> 
> x86 interaction with membock is so, hmm, interesting...

Yeah, numa_clear_kernel_node_hotplug() intends to find out any memory node
which has reserved memory, then make it as unmovable. Setting all node
id into reserved_nodemask will break the use case of hot removing hotpluggable
boot memory after system bootup.