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Subject: Re: [PATCH v2 6/6] powerpc: Don't flush caches when adding memory
To:     Alastair D'Silva <alastair@au1.ibm.com>, alastair@d-silva.org
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, Qian Cai <cai@lca.pw>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Nicholas Piggin <npiggin@gmail.com>,
        Allison Randal <allison@lohutok.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <20190903052407.16638-1-alastair@au1.ibm.com>
 <20190903052407.16638-7-alastair@au1.ibm.com>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <e6713f8a-6465-f9fe-40e9-91d52aa06195@c-s.fr>
Date:   Tue, 3 Sep 2019 08:23:28 +0200
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Le 03/09/2019 à 07:24, Alastair D'Silva a écrit :
> From: Alastair D'Silva <alastair@d-silva.org>
> 
> This operation takes a significant amount of time when hotplugging
> large amounts of memory (~50 seconds with 890GB of persistent memory).
> 
> This was orignally in commit fb5924fddf9e
> ("powerpc/mm: Flush cache on memory hot(un)plug") to support memtrace,
> but the flush on add is not needed as it is flushed on remove.
> 
> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> ---
>   arch/powerpc/mm/mem.c | 7 -------
>   1 file changed, 7 deletions(-)
> 
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index 854aaea2c6ae..2a14b5b93e19 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -111,7 +111,6 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
>   {
>   	unsigned long start_pfn = start >> PAGE_SHIFT;
>   	unsigned long nr_pages = size >> PAGE_SHIFT;
> -	u64 i;
>   	int rc;
>   
>   	resize_hpt_for_hotplug(memblock_phys_mem_size());
> @@ -124,12 +123,6 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
>   		return -EFAULT;
>   	}
>   
> -	for (i = 0; i < size; i += FLUSH_CHUNK_SIZE) {
> -		flush_dcache_range(start + i,
> -				   min(start + size, start + i + FLUSH_CHUNK_SIZE));
> -		cond_resched();
> -	}
> -

So you are removing the code you added in patch 4. Why not move this one 
before patch 4 ?

>   	return __add_pages(nid, start_pfn, nr_pages, restrictions);
>   }
>   
> 

Christophe