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U2FsdGVkX1+Cks1tzCOyBNCg327mlQbXDbreZ2HnIJhqXIB7Q1HpRlNl3SAAomRRw7Irck9xUQ8ynyhvnCjNU75g2amGK57TxRS1bsRmKcFrHvxjOqB848tv+gVjzAmBVLQx2Mhh06J5kiswcVHd0zhZZbU9n0FVlW1mgxb+LhNLJYaug7vBspVXUeZGJ95HF7j31jZNTypZh5OKrttROghh570UlPMe7fS+fcg792rV9kjVkYl8LxLsrwM8/2L6c0lhydpolDPMYe1lpl6tSGyuazp0lDEiOVV6KbPSmn3PYx4Z1r68Ozayz9NYlDjS21nnwdzM93OX9sNxM6ineNKgmGO2Mey4n+Oekmj7KX09AWNg5Rt7HYvE/QycBViSPE64eyBaqsMI7RELDicFxClsqkTRcAXt3ARGwyKDkpPPvBnNW5AtL3mOBwWb1ugmEBGeWbOw4vyq/rdu95yJEEoi4vGBoHVgIwZTtvYNeMr7Mg0iztbf4u+JIxhggZ1YJTz9YLbYmq84KvXnvcdtCmCT9Bk2RflU2wsI+dveIbPBBmp7Yhj963J4oTdNaWlN8J4sEGF257VXmGTXn+JXg6DZGE2aU3c+d4gDmMQxIkDfmcOV028iou83f9L8e23rIsu9AEMzocPsK1j7hB568HPTsXKenx3IF3DBOh4kF3JxfnWxzZxm79KyHEhqyWj+fK8SGZRKXtyV7twICj5ZV7Rar+d5S3kaiZcPZXLULG1/5Bp9DyOLYw9ArFWnFVl/tvUpTKarsjTE3pWvsTkvTFzV5jTmgdqd68CcK+HvaP62LX0nQvXEeOVMB105XVoLvm54rDbDwT3M6alEvWABl99nPx9bVzG+bBq6ZzYzrGQS97wNeZxCdDiuE04jWM22tAw7j1NtOioF2Pan534ibF8p8loU0ShtSzwRr7Se+MCcu/MWztXQGTQno+1X3b6IQ80c+4G9OCmBGIFdhCE SJ+0vxDx 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2026 at 05:53:38PM +0900, Harry Yoo wrote: > > On Wed, Feb 11, 2026 at 01:07:40PM +0530, Dev Jain wrote: > > > > > > On 10/02/26 9:59 pm, Shakeel Butt wrote: > > > > On Tue, Feb 10, 2026 at 01:08:49PM +0530, Dev Jain wrote: > > > > [...] > > > >>> Oh so it is arm64 specific issue. I tested on x86-64 machine and it solves > > > >>> the little regression it had before. So, on arm64 all this_cpu_ops i.e. without > > > >>> double underscore, uses LL/SC instructions. > > > >>> > > > >>> Need more thought on this. > > > >>> > > > >>>>> Also can you confirm whether my analysis of the regression was correct? > > > >>>>> Because if it was, then this diff looks wrong - AFAIU preempt_disable() > > > >>>>> won't stop an irq handler from interrupting the execution, so this > > > >>>>> will introduce a bug for code paths running in irq context. > > > >>>>> > > > >>>> I was worried about the correctness too, but this_cpu_add() is safe > > > >>>> against IRQs and so the stat will be _eventually_ consistent? > > > >>>> > > > >>>> Ofc it's so confusing! Maybe I'm the one confused. > > > >>> Yeah there is no issue with proposed patch as it is making the function > > > >>> re-entrant safe. > > > >> Ah yes, this_cpu_add() does the addition in one shot without read-modify-write. > > > >> > > > >> I am still puzzled whether the original patch was a bug fix or an optimization. > > > > The original patch was a cleanup patch. The memcg stats update functions > > > > were already irq/nmi safe without disabling irqs and that patch did the > > > > same for the numa stats. Though it seems like that is causing regression > > > > for arm64 as this_cpu* ops are expensive on arm64. > > > > > > > >> The patch description says that node stat updation uses irq unsafe interface. > > > >> Therefore, we had foo() calling __foo() nested with local_irq_save/restore. But > > > >> there were code paths which directly called __foo() - so, your patch fixes a bug right > > > > No, those places were already disabling irqs and should be fine. > > > > > > Please correct me if I am missing something here. Simply putting an > > > if (!irqs_disabled()) -> dump_stack() in __lruvec_stat_mod_folio, before > > > calling __mod_node_page_state, reveals: > > > > > > [ 6.486375] Call trace: > > > [ 6.486376] show_stack+0x20/0x38 (C) > > > [ 6.486379] dump_stack_lvl+0x74/0x90 > > > [ 6.486382] dump_stack+0x18/0x28 > > > [ 6.486383] __lruvec_stat_mod_folio+0x160/0x180 > > > [ 6.486385] folio_add_file_rmap_ptes+0x128/0x480 > > > [ 6.486388] set_pte_range+0xe8/0x320 > > > [ 6.486389] finish_fault+0x260/0x508 > > > [ 6.486390] do_fault+0x2d0/0x598 > > > [ 6.486391] __handle_mm_fault+0x398/0xb60 > > > [ 6.486393] handle_mm_fault+0x15c/0x298 > > > [ 6.486394] __get_user_pages+0x204/0xb88 > > > [ 6.486395] populate_vma_page_range+0xbc/0x1b8 > > > [ 6.486396] __mm_populate+0xcc/0x1e0 > > > [ 6.486397] __arm64_sys_mlockall+0x1d4/0x1f8 > > > [ 6.486398] invoke_syscall+0x50/0x120 > > > [ 6.486399] el0_svc_common.constprop.0+0x48/0xf0 > > > [ 6.486400] do_el0_svc+0x24/0x38 > > > [ 6.486400] el0_svc+0x34/0xf0 > > > [ 6.486402] el0t_64_sync_handler+0xa0/0xe8 > > > [ 6.486404] el0t_64_sync+0x198/0x1a0 > > > > > > Indeed finish_fault() takes a PTL spin lock without irq disablement. > > > > That indeed looks incorrect to me. > > I was assuming __foo() is always called with IRQs disabled! > > Not necessarily. For stats which never get updated in IRQ context, can > be updated using __foo() with just premption disabled. Ah, thanks. I was missing that aspect. -- Cheers, Harry / Hyeonggon