linux-kernel - Re: [PATCH v3] mm/hugetlb: split hugetlb

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Date:   Fri, 17 Jul 2020 10:37:34 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Will Deacon <will@...nel.org>
Cc:     Roman Gushchin <guro@...com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        linux-mm@...ck.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        "H.Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        akpm@...ux-foundation.org, Mike Rapoport <rppt@...ux.ibm.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3] mm/hugetlb: split hugetlb_cma in nodes with memory

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@...cle.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