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Message-ID: <a00a8cca-e435-434f-855c-a3b56f2aece4@lucifer.local>
Date: Wed, 6 Aug 2025 18:18:48 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lokesh Gidra <lokeshgidra@...gle.com>
Cc: akpm@...ux-foundation.org, aarcange@...hat.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, 21cnbao@...il.com, ngeoffray@...gle.com,
Suren Baghdasaryan <surenb@...gle.com>,
Kalesh Singh <kaleshsingh@...gle.com>,
Barry Song <v-songbaohua@...o.com>,
David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2] userfaultfd: opportunistic TLB-flush batching for
present pages in MOVE
Andrew - Could we drop this for now, please, it's splatting and has broken
mm-new.
Lokesh - could you make sure to run the mm self tests with CONFIG_DBUG_VM
set before you submit please? As this is splat is occurring immediately on
uffd-unit-tests.
On Tue, Aug 05, 2025 at 05:14:10AM -0700, Lokesh Gidra wrote:
> MOVE ioctl's runtime is dominated by TLB-flush cost, which is required
> for moving present pages. Mitigate this cost by opportunistically
> batching present contiguous pages for TLB flushing.
>
> Without batching, in our testing on an arm64 Android device with UFFD GC,
> which uses MOVE ioctl for compaction, we observed that out of the total
> time spent in move_pages_pte(), over 40% is in ptep_clear_flush(), and
> ~20% in vm_normal_folio().
>
> With batching, the proportion of vm_normal_folio() increases to over
> 70% of move_pages_pte() without any changes to vm_normal_folio().
> Furthermore, time spent within move_pages_pte() is only ~20%, which
> includes TLB-flush overhead.
>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Kalesh Singh <kaleshsingh@...gle.com>
> Cc: Barry Song <v-songbaohua@...o.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Peter Xu <peterx@...hat.com>
> Signed-off-by: Lokesh Gidra <lokeshgidra@...gle.com>
> ---
> Changes since v1 [1]
> - Removed flush_tlb_batched_pending(), per Barry Song
> - Unified single and multi page case, per Barry Song
Splat, decoded via scripts/decode_stacktrace.sh:
$ sudo ./uffd-unit-tests
Testing UFFDIO_API (with syscall)... done
Testing UFFDIO_API (with /dev/userfaultfd)... done
Testing register-ioctls on anon... done
Testing register-ioctls on shmem... done
Testing register-ioctls on shmem-private... done
Testing register-ioctls on hugetlb... skipped [reason: memory allocation failed]
Testing register-ioctls on hugetlb-private... skipped [reason: memory allocation failed]
Testing zeropage on anon... done
Testing zeropage on shmem... done
Testing zeropage on shmem-private... done
Testing zeropage on hugetlb... skipped [reason: memory allocation failed]
Testing zeropage on hugetlb-private... skipped [reason: memory allocation failed]
Testing move on anon... [ 12.230740] Kernel panic - not syncing: kernel: panic_on_warn set ...
[ 12.231322] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.17.0-1-1 04/01/2014
[ 12.231655] Call Trace:
[ 12.231748] <TASK>
[ 12.231830] dump_stack_lvl (lib/dump_stack.c:122)
[ 12.231963] vpanic (kernel/panic.c:448)
[ 12.232088] panic (kernel/panic.c:312 kernel/panic.c:303)
[ 12.232199] ? move_pages (mm/userfaultfd.c:1964 (discriminator 2))
Appears to be:
VM_WARN_ON_ONCE(err > 0);
[ 12.232341] check_panic_on_warn.cold (kernel/panic.c:327)
[ 12.232502] __warn.cold (kernel/panic.c:839)
[ 12.232628] ? move_pages (mm/userfaultfd.c:1964 (discriminator 2))
[ 12.232764] report_bug (lib/bug.c:176 lib/bug.c:215)
[ 12.232891] handle_bug (arch/x86/kernel/traps.c:338 (discriminator 1))
[ 12.233034] ? move_pages (mm/userfaultfd.c:1964 (discriminator 2))
[ 12.233174] exc_invalid_op (arch/x86/kernel/traps.c:392 (discriminator 3))
[ 12.233312] asm_exc_invalid_op (./arch/x86/include/asm/idtentry.h:621)
[ 12.233460] RIP: 0010:move_pages (mm/userfaultfd.c:1964 (discriminator 2))
[ 12.233615] Code: 5e 41 5f c3 cc cc cc cc 49 89 c5 e9 e1 fe ff ff eb c4 e9 6d ff ff ff 90 0f 0b 90 45 31 ff eb cf 90 0f 0b 90 48 85 d2 7e c6 90 <0f> 0b 90 eb b9 90 0f 0b 90 f7 c1 ff 0f 00 00 0f 84 4e fe ff ff 90
All code
========
0: 5e pop %rsi
1: 41 5f pop %r15
3: c3 ret
4: cc int3
5: cc int3
6: cc int3
7: cc int3
8: 49 89 c5 mov %rax,%r13
b: e9 e1 fe ff ff jmp 0xfffffffffffffef1
10: eb c4 jmp 0xffffffffffffffd6
12: e9 6d ff ff ff jmp 0xffffffffffffff84
17: 90 nop
18: 0f 0b ud2
1a: 90 nop
1b: 45 31 ff xor %r15d,%r15d
1e: eb cf jmp 0xffffffffffffffef
20: 90 nop
21: 0f 0b ud2
23: 90 nop
24: 48 85 d2 test %rdx,%rdx
27: 7e c6 jle 0xffffffffffffffef
29: 90 nop
2a:* 0f 0b ud2 <-- trapping instruction
2c: 90 nop
2d: eb b9 jmp 0xffffffffffffffe8
2f: 90 nop
30: 0f 0b ud2
32: 90 nop
33: f7 c1 ff 0f 00 00 test $0xfff,%ecx
39: 0f 84 4e fe ff ff je 0xfffffffffffffe8d
3f: 90 nop
Code starting with the faulting instruction
===========================================
0: 0f 0b ud2
2: 90 nop
3: eb b9 jmp 0xffffffffffffffbe
5: 90 nop
6: 0f 0b ud2
8: 90 nop
9: f7 c1 ff 0f 00 00 test $0xfff,%ecx
f: 0f 84 4e fe ff ff je 0xfffffffffffffe63
15: 90 nop
[ 12.234294] RSP: 0018:ffffafeb00483d70 EFLAGS: 00010206
[ 12.234484] RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000002
[ 12.234738] RDX: 0000000000001000 RSI: ffff90afbb433078 RDI: ffff90afbc7f0080
[ 12.234997] RBP: ffff90afbc7f0000 R08: ffff90b037cae540 R09: 0000000000000001
[ 12.235255] R10: ffffffffffffffff R11: 0000000000000003 R12: ffff90af01acb980
[ 12.235508] R13: ffff90afbc7f0240 R14: ffffefc386e80b00 R15: 0000000000001000
[ 12.235764] userfaultfd_ioctl (fs/userfaultfd.c:1925 fs/userfaultfd.c:2046)
[ 12.235917] __x64_sys_ioctl (fs/ioctl.c:51 fs/ioctl.c:598 fs/ioctl.c:584 fs/ioctl.c:584)
[ 12.236065] ? ksys_read (./include/linux/file.h:63 ./include/linux/file.h:80 ./include/linux/file.h:85 fs/read_write.c:706)
[ 12.236202] do_syscall_64 (arch/x86/entry/syscall_64.c:63 (discriminator 1) arch/x86/entry/syscall_64.c:94 (discriminator 1))
[ 12.236345] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
[ 12.236524] RIP: 0033:0x7f457600fecd
[ 12.236658] Code: 04 25 28 00 00 00 48 89 45 c8 31 c0 48 8d 45 10 c7 45 b0 10 00 00 00 48 89 45 b8 48 8d 45 d0 48 89 45 c0 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1a 48 8b 45 c8 64 48 2b 04 25 28 00 00 00
All code
========
0: 04 25 add $0x25,%al
2: 28 00 sub %al,(%rax)
4: 00 00 add %al,(%rax)
6: 48 89 45 c8 mov %rax,-0x38(%rbp)
a: 31 c0 xor %eax,%eax
c: 48 8d 45 10 lea 0x10(%rbp),%rax
10: c7 45 b0 10 00 00 00 movl $0x10,-0x50(%rbp)
17: 48 89 45 b8 mov %rax,-0x48(%rbp)
1b: 48 8d 45 d0 lea -0x30(%rbp),%rax
1f: 48 89 45 c0 mov %rax,-0x40(%rbp)
23: b8 10 00 00 00 mov $0x10,%eax
28: 0f 05 syscall
2a:* 89 c2 mov %eax,%edx <-- trapping instruction
2c: 3d 00 f0 ff ff cmp $0xfffff000,%eax
31: 77 1a ja 0x4d
33: 48 8b 45 c8 mov -0x38(%rbp),%rax
37: 64 48 2b 04 25 28 00 sub %fs:0x28,%rax
3e: 00 00
Code starting with the faulting instruction
===========================================
0: 89 c2 mov %eax,%edx
2: 3d 00 f0 ff ff cmp $0xfffff000,%eax
7: 77 1a ja 0x23
9: 48 8b 45 c8 mov -0x38(%rbp),%rax
d: 64 48 2b 04 25 28 00 sub %fs:0x28,%rax
14: 00 00
[ 12.237333] RSP: 002b:00007f4569dfed20 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[ 12.237599] RAX: ffffffffffffffda RBX: 0000000000001000 RCX: 00007f457600fecd
[ 12.237854] RDX: 00007f4569dfeda0 RSI: 00000000c028aa05 RDI: 0000000000000009
[ 12.238129] RBP: 00007f4569dfed70 R08: 0000000000000000 R09: 0000000000000000
[ 12.238384] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fffb974ba60
[ 12.238636] R13: 00007fffb974b830 R14: 00007f4569dffcdc R15: 00007fffb974b937
[ 12.238890] </TASK>
[ 12.239094] Kernel Offset: 0x33000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[ 12.239480] ---[ end Kernel panic - not syncing: kernel: panic_on_warn set ... ]---
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