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

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Message-ID: <921bc084-fbde-7975-d6d3-842ee22a38d7@arm.com>
Date:   Fri, 17 Jul 2020 15:21:46 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        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 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()
1. N_MEMORY mask is guaranteed to be initialized at arch_initcall()
2. CMA reservation is available to be called at arch_initcall()