Re: vmemmap alloc failure in hot_add_req()

From: Nathan Chancellor
Date: Wed Jun 16 2021 - 21:08:18 EST

Hi David,

On 6/14/2021 12:38 AM, David Hildenbrand wrote:
On 12.06.21 04:11, Hillf Danton wrote:
On Fri, 11 Jun 2021 12:48:26 -0700 Nathan Chancellor wrote:
Hi all,

I am occasionally seeing a kernel warning when running virtual machines
in Hyper-V, which usually happens a minute or so after boot. It does not
happen on every boot and it is reproducible on at least v5.10. I think
it might have something to do with constant reboots, which I do when
testing various kernels.

The stack trace is as follows:

[   49.215291] kworker/0:1: vmemmap alloc failure: order:9, mode:0x4cc0(GFP_KERNEL|__GFP_RETRY_MAYFAIL), nodemask=(null),cpuset=/,mems_allowed=0
[   49.215299] CPU: 0 PID: 18 Comm: kworker/0:1 Not tainted 5.13.0-rc5 #1
[   49.215301] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.0 11/01/2019
[   49.215302] Workqueue: events hot_add_req [hv_balloon]

Apart from order:9 (mm Cced), events_unbound is the right workqueue instead
because the report shows the risk that hot_add_req could block other pending
events longer than thought. Any special reason for the events wq?

[   49.215307] Call Trace:
[   49.215310]  dump_stack+0x76/0x94
[   49.215314]  warn_alloc.cold+0x78/0xdc
[   49.215316]  ? __alloc_pages+0x200/0x230
[   49.215319]  vmemmap_alloc_block+0x86/0xdc
[   49.215323]  vmemmap_populate+0x10e/0x31c
[   49.215324]  __populate_section_memmap+0x38/0x4e
[   49.215326]  sparse_add_section+0x12c/0x1cf
[   49.215329]  __add_pages+0xa9/0x130
[   49.215330]  add_pages+0x12/0x60
[   49.215333]  add_memory_resource+0x180/0x300
[   49.215335]  __add_memory+0x3b/0x80
[   49.215336]  add_memory+0x2e/0x50
[   49.215337]  hot_add_req+0x3fc/0x5a0 [hv_balloon]
[   49.215340]  process_one_work+0x214/0x3e0
[   49.215342]  worker_thread+0x4d/0x3d0
[   49.215344]  ? process_one_work+0x3e0/0x3e0
[   49.215345]  kthread+0x133/0x150
[   49.215347]  ? kthread_associate_blkcg+0xc0/0xc0
[   49.215348]  ret_from_fork+0x22/0x30
[   49.215351] Mem-Info:
[   49.215352] active_anon:251 inactive_anon:140868 isolated_anon:0
                 active_file:47497 inactive_file:88505 isolated_file:0
                 unevictable:8 dirty:14 writeback:0
                 slab_reclaimable:12013 slab_unreclaimable:11403
                 mapped:131701 shmem:12671 pagetables:3140 bounce:0
                 free:41388 free_pcp:37 free_cma:0
[   49.215355] Node 0 active_anon:1004kB inactive_anon:563472kB active_file:189988kB inactive_file:354020kB unevictable:32kB isolated(anon):0kB isolated(file):0kB mapped:526804kB dirty:56kB writeback:0kB shmem:50684kB shmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 0kB writeback_tmp:0kB kernel_stack:5904kB pagetables:12560kB all_unreclaimable? no
[   49.215358] Node 0 DMA free:6496kB min:480kB low:600kB high:720kB reserved_highatomic:0KB active_anon:0kB inactive_anon:3120kB active_file:2584kB inactive_file:2792kB unevictable:0kB writepending:0kB present:15996kB managed:15360kB mlocked:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
[   49.215361] lowmem_reserve[]: 0 1384 1384 1384 1384
[   49.215364] Node 0 DMA32 free:159056kB min:44572kB low:55712kB high:66852kB reserved_highatomic:0KB active_anon:1004kB inactive_anon:560352kB active_file:187004kB inactive_file:350864kB unevictable:32kB writepending:56kB present:1555760kB managed:1432388kB mlocked:32kB bounce:0kB free_pcp:172kB local_pcp:0kB free_cma:0kB
[   49.215367] lowmem_reserve[]: 0 0 0 0 0
[   49.215369] Node 0 DMA: 17*4kB (UM) 13*8kB (M) 10*16kB (M) 3*32kB (ME) 3*64kB (UME) 4*128kB (UME) 1*256kB (E) 2*512kB (UE) 2*1024kB (ME) 1*2048kB (E) 0*4096kB = 6508kB
[   49.215377] Node 0 DMA32: 8061*4kB (UME) 5892*8kB (UME) 2449*16kB (UME) 604*32kB (UME) 207*64kB (UME) 49*128kB (UM) 7*256kB (M) 1*512kB (M) 0*1024kB 0*2048kB 0*4096kB = 159716kB
[   49.215388] 148696 total pagecache pages
[   49.215388] 0 pages in swap cache
[   49.215389] Swap cache stats: add 0, delete 0, find 0/0
[   49.215390] Free swap  = 0kB
[   49.215390] Total swap = 0kB
[   49.215391] 392939 pages RAM
[   49.215391] 0 pages HighMem/MovableOnly
[   49.215391] 31002 pages reserved
[   49.215392] 0 pages cma reserved
[   49.215393] 0 pages hwpoisoned

Is this a known issue and/or am I doing something wrong? I only noticed
this because there are times when I am compiling something intensive in
the VM such as LLVM and the VM runs out of memory even though I have
plenty of free memory on the host but I am not sure if this warning is
related to that issue.

Hi,

Is hotplugged memory getting onlined automatically (either from user space via a udev script or via the kernel, for example, with CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE)?

It does look like this kernel configuration has CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y.

If it's not getting onlined, you easily sport after hotplug e.g., via "lsmem" that there are quite some offline memory blocks.

Note that x86_64 code will fallback from populating huge pages to populating base pages for the vmemmap; this can happen easily when under memory pressure.

Not sure if it is relevant or not but this warning can show up within a minute of startup without me doing anything in particular.

If adding memory would fail completely, you'd see another "hot_add memory failed error is ..." error message from hyper-v in the kernel log. If that doesn't show up, it's simply suboptimal, but hotplugging memory still succeeded.

I did notice that from the code in hv_balloon.c but I do not think I have ever seen that message in my logs.

Note: we could support "memmap_on_memory" in some cases (e.g., no memory holes in hotadded range) when hotplugging memory blocks via hyper-v, which would result in this warning less trigger less frequently.

Cheers,
Nathan