memblock: do not start bottom-up allocations with kernel_end

memblock: do not start bottom-up allocations with kernel_end

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
[    0.002937] Modules linked in:
[    0.002939] CPU: 0 PID: 0 Comm: swapper Not tainted 5.10.0+ #1169
[    0.002940] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-1.fc33 04/01/2014
[    0.002942] RIP: 0010:memblock_find_in_range_node+0x178/0x25a
[    0.002944] Code: e9 6d ff ff ff 48 85 c0 0f 85 da 00 00 00 80 3d 9b 35 df 00 00 75 15 48 c7 c7 c0 75 59 88 c6 05 8b 35 df 00 01 e8 25 8a fa ff <0f> 0b 48 c7 44 24 20 ff ff ff ff 44 89 e6 44 89 ea 48 c7 c1 70 5c
[    0.002945] RSP: 0000:ffffffff88803d18 EFLAGS: 00010086 ORIG_RAX: 0000000000000000
[    0.002947] RAX: 0000000000000000 RBX: 0000000240000000 RCX: 00000000ffffdfff
[    0.002948] RDX: 00000000ffffdfff RSI: 00000000ffffffea RDI: 0000000000000046
[    0.002948] RBP: 0000000100000000 R08: ffffffff88922788 R09: 0000000000009ffb
[    0.002949] R10: 00000000ffffe000 R11: 3fffffffffffffff R12: 0000000000000000
[    0.002950] R13: 0000000000000000 R14: 0000000080000000 R15: 00000001fb42c000
[    0.002952] FS:  0000000000000000(0000) GS:ffffffff88f71000(0000) knlGS:0000000000000000
[    0.002953] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.002954] CR2: ffffa080fb401000 CR3: 00000001fa80a000 CR4: 00000000000406b0
[    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
[    0.002988] ---[ end trace f151227d0b39be70 ]---

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.

Link: https://lkml.kernel.org/r/20201217201214.3414100-2-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Wonhyuk Yang <vvghjk1234@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>