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

[Dev Jain <dev.jain@arm.com>]

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.

>
> 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. 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.