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Message-ID: <CA+EESO40g7ikjxEPYd1U9f_8HTvVudrWRQXXCfNJofpgiPaMjQ@mail.gmail.com> Date: Thu, 7 Aug 2025 03:42:00 -0700 From: Lokesh Gidra <lokeshgidra@...gle.com> To: akpm@...ux-foundation.org, Barry Song <v-songbaohua@...o.com> Cc: aarcange@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 21cnbao@...il.com, ngeoffray@...gle.com, Suren Baghdasaryan <surenb@...gle.com>, Kalesh Singh <kaleshsingh@...gle.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 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@...gle.com> wrote: > > On Wed, Aug 6, 2025 at 10:18 AM Lorenzo Stoakes > <lorenzo.stoakes@...cle.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@...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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