On Thu, Oct 12, 2023 at 5:19 PM Nadav Amit wrote: > > > > On Oct 12, 2023, at 12:54 AM, Linus Torvalds wrote: > > > > !! External Email > > > > On Wed, 11 Oct 2023 at 14:33, Uros Bizjak wrote: > >> > >> Reading the above, it looks to me that we don't want to play games > >> with "const aliased" versions of current_task [1], as proposed by > >> Nadav in his patch series. > > > > Well, maybe I'd like it if I saw what the effect of it was, but that > > patch mentions "sync_mm_rss()" which doesn't actually exist > > (SPLIT_RSS_COUNTING is never defined, the split version is gone and > > hasn't existed since commit f1a7941243c1 "mm: convert mm's rss stats > > into percpu_counter") > > So I added a new version of the current aliasing (well, actually pcpu_hot > in the new version) on top of Uros’s patches, and the effect can be seen > in many functions. I don’t want to bother with many examples so here is > a common and simple one: > > Currently syscall_exit_work() that starts with: > > 0xffffffff8111e120 <+0>: push %rbp > 0xffffffff8111e121 <+1>: mov %rdi,%rbp > 0xffffffff8111e124 <+4>: push %rbx > 0xffffffff8111e125 <+5>: mov %rsi,%rbx > 0xffffffff8111e128 <+8>: and $0x20,%esi > 0xffffffff8111e12b <+11>: je 0xffffffff8111e143 > 0xffffffff8111e12d <+13>: mov %gs:0x2ac80,%rax > 0xffffffff8111e136 <+22>: cmpb $0x0,0x800(%rax) > 0xffffffff8111e13d <+29>: jne 0xffffffff8111e22a > 0xffffffff8111e143 <+35>: mov %gs:0x2ac80,%rax > 0xffffffff8111e14c <+44>: cmpq $0x0,0x7c8(%rax) > > Using the const-alias changes the beginning of syscall_exit_work to: > > 0xffffffff8111cb80 <+0>: push %r12 > 0xffffffff8111cb82 <+2>: mov %gs:0x7ef0e0f6(%rip),%r12 # 0x2ac80 > 0xffffffff8111cb8a <+10>: push %rbp > 0xffffffff8111cb8b <+11>: mov %rdi,%rbp > 0xffffffff8111cb8e <+14>: push %rbx > 0xffffffff8111cb8f <+15>: mov %rsi,%rbx > 0xffffffff8111cb92 <+18>: and $0x20,%esi > 0xffffffff8111cb95 <+21>: je 0xffffffff8111cba6 > 0xffffffff8111cb97 <+23>: cmpb $0x0,0x800(%r12) > 0xffffffff8111cba0 <+32>: jne 0xffffffff8111cc7a > 0xffffffff8111cba6 <+38>: cmpq $0x0,0x7c8(%r12) > > So we both see RIP-relative addressing is being used (hence the instruction is > one byte shorter) and the reload going away. > > Now, I am not a compiler expert as for the rationale, but it googling around > I can see Nick explaining the rationale [1] - if you use “p” your read memory. > BTW: It is related to discussion you had [2], in which you encountered an issue > I also encountered before [3]. My bad for pushing it in. > > Anyhow, I created a similar code on godbolt ( https://godbolt.org/z/dPqKKzPs4 ) > to show this behavior - how compiler barriers cause reload. It seems that this > behavior happens on GCC and CLANG on various versions. > > The idea behind the patch is that it communicates - in the compilation unit > granularity - that current is fixed. There is an issue of whether it works with > LTO, which I have never checked. > > > [1] https://reviews.llvm.org/D145416 > [2] https://lore.kernel.org/lkml/20230306120106.GE1267364@hirez.programming.kicks-ass.net/ > [3] https://lore.kernel.org/all/20190823224424.15296-5-namit@vmware.com/ > > -- > > Here’s the updated patch - but I didn’t really boot a machine with it so new > issues might have come since my last patch-set: Unfortunately, it does not work and dies early in the boot with: [ 4.939358] BUG: kernel NULL pointer dereference, address: 0000000000000000 [ 4.940090] #PF: supervisor write access in kernel mode [ 4.940090] #PF: error_code(0x0002) - not-present page [ 4.940090] PGD 0 P4D 0 [ 4.940090] Oops: 0002 [#1] PREEMPT SMP PTI [ 4.940090] CPU: 1 PID: 52 Comm: kworker/u4:1 Not tainted 6.6.0-rc6-00365-g0c09c1d70838-dirty #7 [ 4.940090] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-1.fc37 04/01/2014 [ 4.940090] RIP: 0010:begin_new_exec+0x8f2/0xa30 [ 4.940090] Code: 31 f6 e8 c1 49 f9 ff e9 3c fa ff ff 31 f6 4c 89 ef e8 b2 4a f9 ff e9 19 fa ff ff 31 f6 4c 89 ef e8 23 4a f9 ff e9 ea fa ff ff 41 ff 0c 24 0f 85 55 fb ff ff 4c 89 e7 e8 4b 02 df ff e9 48 fb [ 4.940090] RSP: 0000:ffff9c84c01e3d68 EFLAGS: 00010246 [ 4.940090] RAX: 0000000000000000 RBX: ffff9946e30c1f00 RCX: 0000000000000000 [ 4.940090] RDX: 0000000000000000 RSI: ffff9946e2ff0000 RDI: ffff9946e30c2718 [ 4.940090] RBP: ffff9946c03a7c00 R08: 00000000fffffffe R09: 00000000ffffffff [ 4.940090] R10: 000001ffffffffff R11: 0000000000000001 R12: 0000000000000000 [ 4.940090] R13: 0000000000000000 R14: ffff9946e30c2718 R15: ffff9946e2ff0000 [ 4.940090] FS: 0000000000000000(0000) GS:ffff994763f00000(0000) knlGS:0000000000000000 [ 4.940090] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 4.940090] CR2: 0000000000000000 CR3: 00000001003a8000 CR4: 00000000000406f0 [ 4.940090] Call Trace: [ 4.940090] [ 4.940090] ? __die+0x1e/0x60 [ 4.940090] ? page_fault_oops+0x17b/0x470 [ 4.940090] ? search_module_extables+0x14/0x50 [ 4.940090] ? exc_page_fault+0x66/0x140 [ 4.940090] ? asm_exc_page_fault+0x26/0x30 [ 4.940090] ? begin_new_exec+0x8f2/0xa30 [ 4.940090] ? begin_new_exec+0x3ce/0xa30 [ 4.940090] ? load_elf_phdrs+0x67/0xb0 [ 4.940090] load_elf_binary+0x2bb/0x1770 [ 4.940090] ? __kernel_read+0x136/0x2d0 [ 4.940090] bprm_execve+0x277/0x630 [ 4.940090] kernel_execve+0x145/0x1a0 [ 4.940090] call_usermodehelper_exec_async+0xcb/0x180 [ 4.940090] ? __pfx_call_usermodehelper_exec_async+0x10/0x10 [ 4.940090] ret_from_fork+0x2f/0x50 [ 4.940090] ? __pfx_call_usermodehelper_exec_async+0x10/0x10 [ 4.940090] ret_from_fork_asm+0x1b/0x30 [ 4.940090] [ 4.940090] Modules linked in: [ 4.940090] CR2: 0000000000000000 [ 5.017606] ---[ end trace 0000000000000000 ]--- [ 5.018957] RIP: 0010:begin_new_exec+0x8f2/0xa30 [ 5.020299] Code: 31 f6 e8 c1 49 f9 ff e9 3c fa ff ff 31 f6 4c 89 ef e8 b2 4a f9 ff e9 19 fa ff ff 31 f6 4c 89 ef e8 23 4a f9 ff e9 ea fa ff ff 41 ff 0c 24 0f 85 55 fb ff ff 4c 89 e7 e8 4b 02 df ff e9 48 fb [ 5.024765] RSP: 0000:ffff9c84c01e3d68 EFLAGS: 00010246 [ 5.026150] RAX: 0000000000000000 RBX: ffff9946e30c1f00 RCX: 0000000000000000 [ 5.027916] RDX: 0000000000000000 RSI: ffff9946e2ff0000 RDI: ffff9946e30c2718 [ 5.029714] RBP: ffff9946c03a7c00 R08: 00000000fffffffe R09: 00000000ffffffff [ 5.031461] R10: 000001ffffffffff R11: 0000000000000001 R12: 0000000000000000 [ 5.033186] R13: 0000000000000000 R14: ffff9946e30c2718 R15: ffff9946e2ff0000 [ 5.034908] FS: 0000000000000000(0000) GS:ffff994763f00000(0000) knlGS:0000000000000000 [ 5.036907] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 5.038341] CR2: 0000000000000000 CR3: 00000001003a8000 CR4: 00000000000406f0 [ 5.040044] Kernel panic - not syncing: Fatal exception [ 5.040647] Kernel Offset: 0x22e00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff) It looks that aliasing a structure from another namespace is a no-go, since the patch (attached, slightly changed your patch) without __percpu_seg_override decorations bootstraps OK. The working patch (without __percpu_seg_override) is not effective (no effect in syscall_exit_work) and increases the number of current_task reads from 3841 to 4711. Uros.