Re: [PATCH 1/4] memcg: use mod_node_page_state to update stats

[Harry Yoo]

On Wed, Feb 11, 2026 at 01:24:35AM -0800, Shakeel Butt wrote:
> On Wed, Feb 11, 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