Re: [PATCH v3] mm/hugetlb: split hugetlb_cma in nodes with memory

From: Mike Kravetz <mike.kravetz@oracle.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	Will Deacon <will@kernel.org>
Cc: Roman Gushchin <guro@fb.com>,
	Barry Song <song.bao.hua@hisilicon.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	linuxarm@huawei.com, linux-mm@kvack.org,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jonathan Cameron <jonathan.cameron@huawei.com>,
	"H.Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
	akpm@linux-foundation.org, Mike Rapoport <rppt@linux.ibm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3] mm/hugetlb: split hugetlb_cma in nodes with memory
Date: Fri, 17 Jul 2020 10:37:34 -0700	[thread overview]
Message-ID: <c2e4d47b-940b-481c-c155-a34c3e853e85@oracle.com> (raw)
In-Reply-To: <921bc084-fbde-7975-d6d3-842ee22a38d7@arm.com>

On 7/17/20 2:51 AM, Anshuman Khandual wrote:
> 
> 
> On 07/17/2020 02:06 PM, Will Deacon wrote:
>> On Fri, Jul 17, 2020 at 10:32:53AM +0530, Anshuman Khandual wrote:
>>>
>>>
>>> On 07/16/2020 11:55 PM, Mike Kravetz wrote:
>>>> >From 17c8f37afbf42fe7412e6eebb3619c6e0b7e1c3c Mon Sep 17 00:00:00 2001
>>>> From: Mike Kravetz <mike.kravetz@oracle.com>
>>>> Date: Tue, 14 Jul 2020 15:54:46 -0700
>>>> Subject: [PATCH] hugetlb: move cma reservation to code setting up gigantic
>>>>  hstate
>>>>
>>>> Instead of calling hugetlb_cma_reserve() directly from arch specific
>>>> code, call from hugetlb_add_hstate when adding a gigantic hstate.
>>>> hugetlb_add_hstate is either called from arch specific huge page setup,
>>>> or as the result of hugetlb command line processing.  In either case,
>>>> this is late enough in the init process that all numa memory information
>>>> should be initialized.  And, it is early enough to still use early
>>>> memory allocator.
>>>
>>> This assumes that hugetlb_add_hstate() is called from the arch code at
>>> the right point in time for the generic HugeTLB to do the required CMA
>>> reservation which is not ideal. I guess it must have been a reason why
>>> CMA reservation should always called by the platform code which knows
>>> the boot sequence timing better.
>>
>> Ha, except we've moved it around two or three times already in the last
>> month or so, so I'd say we don't have a clue when to call it in the arch
>> code.
> 
> The arch dependency is not going way with this change either. Just that
> its getting transferred to hugetlb_add_hstate() which gets called from
> arch_initcall() in every architecture.
> 
> The perfect timing here happens to be because of arch_initcall() instead.
> This is probably fine, as long as
> 
> 0. hugetlb_add_hstate() is always called at arch_initcall()

In another reply, I give reasoning why it would be safe to call even later
at hugetlb command line processing time.

> 1. N_MEMORY mask is guaranteed to be initialized at arch_initcall()

This is a bit more difficult to guarantee.  I find the init sequence hard to
understand.  Looking at the arm code, arch_initcall(hugetlbpage_init)
happens after N_MEMORY mask is setup.  I can't imagine any arch code setting
up huge pages before N_MEMORY.  But, I suppose it is possible and we would
need to somehow guarantee this.

> 2. CMA reservation is available to be called at arch_initcall()

Since I am pretty sure we can delay the reservation until hugetlb command
line processing time, it would be great if it was always done there.
Unfortunately, I can not immediately think of an easy way to do this.
-- 
Mike Kravetz