[PATCH 0/3] mm/page_alloc: Fix pageblock_order with HUGETLB_PAGE_SIZE_VARIABLE

[Anshuman Khandual]

The following warning gets triggered while trying to boot a 64K page size
without THP config kernel on arm64 platform.

WARNING: CPU: 5 PID: 124 at mm/vmstat.c:1080 __fragmentation_index+0xa4/0xc0
Modules linked in:
CPU: 5 PID: 124 Comm: kswapd0 Not tainted 5.11.0-rc6-00004-ga0ea7d62002 #159
Hardware name: linux,dummy-virt (DT)
[    8.810673] pstate: 20400005 (nzCv daif +PAN -UAO -TCO BTYPE=--)
[    8.811732] pc : __fragmentation_index+0xa4/0xc0
[    8.812555] lr : fragmentation_index+0xf8/0x138
[    8.813360] sp : ffff0000864079b0
[    8.813958] x29: ffff0000864079b0 x28: 0000000000000372
[    8.814901] x27: 0000000000007682 x26: ffff8000135b3948
[    8.815847] x25: 1fffe00010c80f48 x24: 0000000000000000
[    8.816805] x23: 0000000000000000 x22: 000000000000000d
[    8.817764] x21: 0000000000000030 x20: ffff0005ffcb4d58
[    8.818712] x19: 000000000000000b x18: 0000000000000000
[    8.819656] x17: 0000000000000000 x16: 0000000000000000
[    8.820613] x15: 0000000000000000 x14: ffff8000114c6258
[    8.821560] x13: ffff6000bff969ba x12: 1fffe000bff969b9
[    8.822514] x11: 1fffe000bff969b9 x10: ffff6000bff969b9
[    8.823461] x9 : dfff800000000000 x8 : ffff0005ffcb4dcf
[    8.824415] x7 : 0000000000000001 x6 : 0000000041b58ab3
[    8.825359] x5 : ffff600010c80f48 x4 : dfff800000000000
[    8.826313] x3 : ffff8000102be670 x2 : 0000000000000007
[    8.827259] x1 : ffff000086407a60 x0 : 000000000000000d
[    8.828218] Call trace:
[    8.828667]  __fragmentation_index+0xa4/0xc0
[    8.829436]  fragmentation_index+0xf8/0x138
[    8.830194]  compaction_suitable+0x98/0xb8
[    8.830934]  wakeup_kcompactd+0xdc/0x128
[    8.831640]  balance_pgdat+0x71c/0x7a0
[    8.832327]  kswapd+0x31c/0x520
[    8.832902]  kthread+0x224/0x230
[    8.833491]  ret_from_fork+0x10/0x30
[    8.834150] ---[ end trace 472836f79c15516b ]---

This warning comes from __fragmentation_index() when the requested order
is greater than MAX_ORDER.

static int __fragmentation_index(unsigned int order,
				 struct contig_page_info *info)
{
        unsigned long requested = 1UL << order;

        if (WARN_ON_ONCE(order >= MAX_ORDER)) <===== Triggered here
                return 0;

Digging it further reveals that pageblock_order has been assigned a value
which is greater than MAX_ORDER failing the above check. But why this
happened ? Because HUGETLB_PAGE_ORDER for the given config on arm64 is
greater than MAX_ORDER.

The solution involves enabling HUGETLB_PAGE_SIZE_VARIABLE which would make
pageblock_order a variable instead of constant HUGETLB_PAGE_ORDER. But that
change alone also did not really work as pageblock_order still got assigned
as HUGETLB_PAGE_ORDER in set_pageblock_order(). HUGETLB_PAGE_ORDER needs to
be less than MAX_ORDER for its appropriateness as pageblock_order otherwise
just fallback to MAX_ORDER - 1 as before. While here it also fixes a build
problem via type casting MAX_ORDER in rmem_cma_setup().

This series applies on v5.11-rc7 and has been slightly tested on arm64. But
looking for some early feedbacks particularly with respect to concerns in
subscribing HUGETLB_PAGE_SIZE_VARIABLE on a platform where the hugetlb page
size is config dependent but not really a runtime variable. Even though it
appears that HUGETLB_PAGE_SIZE_VARIABLE is used only while computing the
pageblock_order, could there be other implications ?

Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

Changes in V1:

- Rebased on 5.11-rc7
- Dropped the RFC

Changes in RFC:

https://lore.kernel.org/linux-mm/1612422084-30429-1-git-send-email-anshuman.khandual@xxxxxxx/

Anshuman Khandual (3):
  mm/page_alloc: Fix pageblock_order when HUGETLB_PAGE_ORDER >= MAX_ORDER
  arm64/hugetlb: Enable HUGETLB_PAGE_SIZE_VARIABLE
  dma-contiguous: Type cast MAX_ORDER as unsigned int

 arch/arm64/Kconfig      | 4 ++++
 kernel/dma/contiguous.c | 2 +-
 mm/page_alloc.c         | 2 +-
 3 files changed, 6 insertions(+), 2 deletions(-)

-- 
2.20.1