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Date:   Sat, 19 Dec 2020 09:05:35 -0800
From:   Roman Gushchin <guro@...com>
To:     Wonhyuk Yang <vvghjk1234@...il.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>, <linux-mm@...ck.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Rik van Riel <riel@...riel.com>,
        Michal Hocko <mhocko@...nel.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH v2 2/2] memblock: do not start bottom-up allocations with
 kernel_end

On Sat, Dec 19, 2020 at 11:52:19PM +0900, Wonhyuk Yang wrote:
> Hi Roman,
> 
> On Fri, Dec 18, 2020 at 5:12 AM Roman Gushchin <guro@...com> wrote:
> >
> > With kaslr the kernel image is placed at a random place, so starting
> > the bottom-up allocation with the kernel_end can result in an
> > allocation failure and a warning like this one:
> >
> > [    0.002920] hugetlb_cma: reserve 2048 MiB, up to 2048 MiB per node
> > [    0.002921] ------------[ cut here ]------------
> > [    0.002922] memblock: bottom-up allocation failed, memory hotremove may be affected
> > [    0.002937] WARNING: CPU: 0 PID: 0 at mm/memblock.c:332 memblock_find_in_range_node+0x178/0x25a
> > [    0.002956] Call Trace:
> > [    0.002961]  ? memblock_alloc_range_nid+0x8d/0x11e
> > [    0.002963]  ? cma_declare_contiguous_nid+0x2c4/0x38c
> > [    0.002964]  ? hugetlb_cma_reserve+0xdc/0x128
> > [    0.002968]  ? flush_tlb_one_kernel+0xc/0x20
> > [    0.002969]  ? native_set_fixmap+0x82/0xd0
> > [    0.002971]  ? flat_get_apic_id+0x5/0x10
> > [    0.002973]  ? register_lapic_address+0x8e/0x97
> > [    0.002975]  ? setup_arch+0x8a5/0xc3f
> > [    0.002978]  ? start_kernel+0x66/0x547
> > [    0.002980]  ? load_ucode_bsp+0x4c/0xcd
> > [    0.002982]  ? secondary_startup_64_no_verify+0xb0/0xbb
> > [    0.002986] random: get_random_bytes called from __warn+0xab/0x110 with crng_init=0
> >
> > At the same time, the kernel image is protected with memblock_reserve(),
> > so we can just start searching at PAGE_SIZE. In this case the
> > bottom-up allocation has the same chances to success as a top-down
> > allocation, so there is no reason to fallback in the case of a
> > failure. All together it simplifies the logic.
> 
> I figure out that it was introduced by
> commit 79442ed189acb ("memblock.c: introduce bottom-up allocation mode")
> 
> According to this commit, The purpose of bottom up allocation is to
> allocate memory from the unhotpluggable node.

Hi Wonhyuk,

correct! And it remains this way, we just don't need to skip
all the memory before the kernel_end.

Thanks!

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