Re: [PATCH] mm/spase: never partially remove memmap for early section
From: David Hildenbrand
Date: Wed Jun 24 2020 - 04:41:29 EST
On 24.06.20 10:13, Wei Yang wrote:
> On Wed, Jun 24, 2020 at 09:48:43AM +0200, David Hildenbrand wrote:
>> On 23.06.20 17:18, Michal Hocko wrote:
>>> On Tue 23-06-20 17:42:58, Wei Yang wrote:
>>>> For early sections, we assumes its memmap will never be partially
>>>> removed. But current behavior breaks this.
>>>>
>>>> Let's correct it.
>>>>
>>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
>>>
>>> Can a user trigger this or is this a theoretical bug?
>>
>> I tried to reproduce it, but somehow I get unexpected behavior.
>> With a hacked QEMU I can get
>
> David,
>
> Thanks for your effort. Would you mind sharing your qemu command line, so that
> I can have a try at my side?
The following change to QEMU:
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 51b3050d01..c6a78d83c0 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1010,7 +1010,7 @@ void pc_memory_init(PCMachineState *pcms,
}
machine->device_memory->base =
- ROUND_UP(0x100000000ULL + x86ms->above_4g_mem_size, 1 * GiB);
+ 0x100000000ULL + x86ms->above_4g_mem_size;
if (pcmc->enforce_aligned_dimm) {
/* size device region assuming 1G page max alignment per slot */
Then you can use the following QEMU cmdline:
#! /bin/bash
sudo x86_64-softmmu/qemu-system-x86_64 \
--enable-kvm \
-m 4160M,maxmem=20G,slots=1 \
-smp sockets=2,cores=16 \
-numa node,nodeid=0,cpus=0-1 -numa node,nodeid=1,cpus=2-3 \
-machine pc,nvdimm \
-nographic \
-hda /home/dhildenb/git/Fedora-Cloud-Base-31-1.9.x86_64.qcow2 \
-nodefaults \
-net nic -net user \
-chardev stdio,nosignal,id=serial \
-device isa-serial,chardev=serial \
-chardev socket,id=monitor,path=/var/tmp/monitor,server,nowait \
-mon chardev=monitor,mode=readline \
-object memory-backend-ram,id=mem0,size=8G \
-device nvdimm,id=vm0,memdev=mem0,node=0,addr=0x144000000,label-size=128k
>
>>
>> $ cat /proc/iomem
>> [...]
>> 100000000-143ffffff : System RAM
>> 144000000-343dfffff : Persistent Memory
>> 144000000-1441fffff : namespace0.0
>> 144200000-144ffffff : dax0.0
>>
>> After
>> $ ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax --size=16M
>>
>> I get
>>
>> $ cat /proc/iomem
>> [...]
>> 100000000-143ffffff : System RAM
>> 144000000-343dfffff : Persistent Memory
>> 144000000-1441fffff : namespace0.0
>> 144200000-144ffffff : dax0.0
>>
>
> The memory layout seems not changed.
Yeah, sorry, int he first example, dax0.0 should be missing (on a fresh
QEMU instance instead of after a reboot).
>
>> I can trigger remove+re-add via
>> $ ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax --size=16M
>
> Do we need to change the mode to force the reconfig?
No, that's sufficient.
>
>>
>> So we clearly have an overlap between System RAM and dax0.0 within a section.
>> However, I never get early_section() to trigger in
>> section_activate()/section_deactivate() ? That's unexpected
>>
>>
>> Definitely something seems to go wrong after the first "ndctl create-namespace".
>> Using a random page walker:
>>
>> [root@localhost ~]# ./page-types
>
> What is this page-types?
>
>> [ 387.019229] general protection fault, probably for non-canonical address 0xfdfdfdfdfdfdfdfc: 0000 [#1] SMP NOPTI
>> [ 387.020172] CPU: 17 PID: 1314 Comm: page-types Kdump: loaded Tainted: G W 5.8.0-rc2 #20
>> [ 387.021015] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.4
>> [ 387.022056] RIP: 0010:stable_page_flags+0x27/0x3f0
>> [ 387.022519] Code: 00 00 00 66 66 66 66 90 48 85 ff 0f 84 d2 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48f
>> [ 387.024291] RSP: 0018:ffff9f8781057e58 EFLAGS: 00010202
>> [ 387.024775] RAX: fdfdfdfdfdfdfdfc RBX: fdfdfdfdfdfdfdfd RCX: 00007ffc4f4f1f78
>> [ 387.025423] RDX: 0000000000000001 RSI: 0000002000000000 RDI: ffffc590c5100000
>> [ 387.026052] RBP: ffffc590c5100000 R08: 0000000000000001 R09: 0000000000000000
>> [ 387.026696] R10: 0000000000000000 R11: 0000000000000000 R12: 00007ffc4f4f1f80
>> [ 387.027324] R13: 0000000000040000 R14: 00007ffc4f4d1f80 R15: ffffffffa1577ee0
>> [ 387.027974] FS: 00007f91a0f9c580(0000) GS:ffff888b3f7c0000(0000) knlGS:0000000000000000
>> [ 387.028699] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [ 387.029223] CR2: 0000000000449b00 CR3: 000000007a7fc000 CR4: 00000000000006e0
>> [ 387.029864] Call Trace:
>> [ 387.030108] kpageflags_read+0xcc/0x160
>> [ 387.030473] proc_reg_read+0x53/0x80
>> [ 387.030809] vfs_read+0x9d/0x150
>> [ 387.031114] ksys_pread64+0x65/0xa0
>> [ 387.031449] do_syscall_64+0x4d/0x90
>> [ 387.031783] entry_SYSCALL_64_after_hwframe+0x44/0xa9
It's a tool located in Linux tools/vm/page-types.
So with two printk's, I can spot:
[ 24.185558] section_deactivate(): depopulate_section_memmap
[ 24.202689] pmem0: detected capacity change from 0 to 16777216
[ 24.229747] section_activate(): early_section, not populating memmap
[ 24.229777] memmap_init_zone_device initialised 3584 pages in 0ms
But nothing actually breaks .... because *drummroll* we use huge pages in the vmemmap,
so the partial depopulate will not actually depopulate anything here. Huge page is 2M,
the memmap of 128MB sections is exactly 2MB == one hugepages. Trying to depopulate a
fraction (e.g., 16MB) of that won't do anything.
Now, forcing a CPU without hugepages - PSE (QEMU: "-cpu host,pse=off)", I can trigger
via ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax --size=16M
[ 18.044973] pmem0: detected capacity change from 0 to 16777216
[ 18.073813] BUG: unable to handle page fault for address: ffffec73c51000b4
[ 18.076236] #PF: supervisor write access in kernel mode
[ 18.077211] #PF: error_code(0x0002) - not-present page
[ 18.077943] PGD 81ff8067 P4D 81ff8067 PUD 81ff7067 PMD 1437cb067 PTE 0
[ 18.078551] Oops: 0002 [#1] SMP NOPTI
[ 18.078915] CPU: 16 PID: 1348 Comm: ndctl Kdump: loaded Tainted: G W 5.8.0-rc2+ #24
[ 18.079718] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.4
[ 18.080750] RIP: 0010:memmap_init_zone+0x154/0x1c2
[ 18.081213] Code: 77 16 f6 40 10 02 74 10 48 03 48 08 48 89 cb 48 c1 eb 0c e9 3a ff ff ff 48 89 df 48 c1 e7 06 48f
[ 18.082902] RSP: 0018:ffffbdc7011a39b0 EFLAGS: 00010282
[ 18.083395] RAX: ffffec73c5100088 RBX: 0000000000144002 RCX: 0000000000144000
[ 18.084057] RDX: 0000000000000004 RSI: 007ffe0000000000 RDI: ffffec73c5100080
[ 18.084704] RBP: 027ffe0000000000 R08: 0000000000000001 R09: ffff9f8d38f6d708
[ 18.085369] R10: ffffec73c0000000 R11: 0000000000000000 R12: 0000000000000004
[ 18.086020] R13: 0000000000000001 R14: 0000000000144200 R15: 0000000000000000
[ 18.086670] FS: 00007efe6b65d780(0000) GS:ffff9f8d3f780000(0000) knlGS:0000000000000000
[ 18.087417] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 18.087965] CR2: ffffec73c51000b4 CR3: 000000007d718000 CR4: 0000000000340ee0
[ 18.088623] Call Trace:
[ 18.088884] move_pfn_range_to_zone+0x128/0x150
[ 18.089313] memremap_pages+0x4e4/0x5a0
[ 18.089681] devm_memremap_pages+0x1e/0x60
[ 18.090081] dev_dax_probe+0x69/0x160 [device_dax]
[ 18.090533] really_probe+0x298/0x3c0
[ 18.090896] driver_probe_device+0xe1/0x150
[ 18.091303] ? driver_allows_async_probing+0x50/0x50
[ 18.091780] bus_for_each_drv+0x7e/0xc0
[ 18.092154] __device_attach+0xdf/0x160
[ 18.092527] bus_probe_device+0x8e/0xa0
[ 18.092916] device_add+0x3b9/0x740
[ 18.093258] __devm_create_dev_dax+0x127/0x1c0
[ 18.093686] __dax_pmem_probe+0x1f2/0x219 [dax_pmem_core]
[ 18.094200] dax_pmem_probe+0xc/0x1b [dax_pmem]
[ 18.094632] nvdimm_bus_probe+0x69/0x1c0 [libnvdimm]
[ 18.095109] really_probe+0x147/0x3c0
[ 18.095470] driver_probe_device+0xe1/0x150
[ 18.095883] device_driver_attach+0x53/0x60
[ 18.096289] bind_store+0xd1/0x110
[ 18.096627] kernfs_fop_write+0xce/0x1b0
[ 18.097017] vfs_write+0xb6/0x1a0
[ 18.097350] ksys_write+0x5f/0xe0
[ 18.097681] do_syscall_64+0x4d/0x90
[ 18.098041] entry_SYSCALL_64_after_hwframe+0x44/0xa9
--
Thanks,
David / dhildenb