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From:   Wonhyuk Yang <vvghjk1234@gmail.com>
Date:   Sat, 19 Dec 2020 23:52:19 +0900
Message-ID: <CAEcHRTqcrEdXcr02OZnSTgxwQ0Por7y4gAXn6uM=Dp=TVq_5kA@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] memblock: do not start bottom-up allocations with kernel_end
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>, linux-mm@kvack.org,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Rik van Riel <riel@surriel.com>,
        Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
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Hi Roman,

On Fri, Dec 18, 2020 at 5:12 AM Roman Gushchin <guro@fb.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.