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

[Shakeel Butt <shakeel.butt@linux.dev>]

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.

> 
> > > I am working on adding batched stats update functionality in the hope
> > > that will fix the regression.
> > 
> > Thanks! FYI, I have zeroed in the issue on to preempt_disable(). Dropping this
> > from _pcpu_protect_return solves the regression.
> 
> That's interesting, why is the cost of preempt disable/enable so high?
> 

What made you (Dev) so convinced that preempt_disable is that expensive.

> > Unlike x86, arm64 does a preempt_disable
> > when doing this_cpu_*. On a cursory look it seems like this is unnecessary - since we
> > are doing preempt_enable() immediately after reading the pointer, CPU migration is
> > possible anyways, so there is nothing to be gained by reading pcpu pointer with
> > preemption disabled. I am investigating whether we can simply drop this in general.
> 
[...]
> 
> ... so, removing preempt disable _in general_ is probably not a good idea.
> 

Yup, I agree here.

> [1] https://lore.kernel.org/all/20190311164837.GD24275@lakrids.cambridge.arm.com
> 
> -- 
> Cheers,
> Harry / Hyeonggon
>