linux-kernel - Re: [RFC PATCH 4/4] x86/mm: remove bottom-up allocation style for x86

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Date:   Tue, 8 Jan 2019 09:32:59 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     x86@...nel.org, linux-acpi@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] x86/mm: remove bottom-up allocation style for
 x86_64

On 1/7/19 10:13 PM, Pingfan Liu wrote:
> On Tue, Jan 8, 2019 at 1:42 AM Dave Hansen <dave.hansen@...el.com> wrote:
>> Why is this 0x100000 open-coded?  Why is this needed *now*?
>>
> 
> Memory under 1MB should be used by BIOS. For x86_64, after
> e820__memblock_setup(), the memblock allocator has already been ready
> to work. But there are two factors to in order to
> set_alloc_range(0x100000, end). The major one is to be compatible with
> x86_32, please refer to alloc_low_pages->memblock_find_in_range() uses
> [min_pfn_mapped, max_pfn_mapped] to limit the range, which is ready to
> be allocated from. The minor one is to prevent unexpected allocation
> from memblock allocator through allow_low_pages() at very early stage.

Wow, that's a ton of critical information which was neither commented
upon or referenced in the changelog.  Can you fix this up in the next
version, please?