Re: [PATCH v2] userfaultfd: opportunistic TLB-flush batching for present pages in MOVE

[Lokesh Gidra <lokeshgidra@google.com>]

Posted v3 at https://lore.kernel.org/all/20250807103902.2242717-1-lokeshgidra@google.com/

Thanks!

On Wed, Aug 6, 2025 at 10:30 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> On Wed, Aug 6, 2025 at 10:18 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > 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.
>
> Sincere apologies. Will be extra careful from now on.
> >
> > 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@google.com>
> > > Cc: Kalesh Singh <kaleshsingh@google.com>
> > > Cc: Barry Song <v-songbaohua@oppo.com>
> > > Cc: David Hildenbrand <david@redhat.com>
> > > Cc: Peter Xu <peterx@redhat.com>
> > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.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 ... ]---