Re: [PATCH] mm/mm_init.c: remove spinlock in early_pfn_to_nid()

["Yajun Deng" <yajun.deng@linux.dev>]

June 14, 2023 7:09 PM, "Greg KH" <gregkh@linuxfoundation.org> wrote:

> On Wed, Jun 14, 2023 at 07:03:24PM +0800, Yajun Deng wrote:
> 
>> When the system boots, only one cpu is enabled before smp_init().
>> So the spinlock is not needed in most cases, remove it.
>> 
>> Add spinlock in get_nid_for_pfn() because it is after smp_init().
> 
> So this is two different things at once in the same patch?
> 
> Or are they the same problem and both need to go in to solve it?
> 
> And if a spinlock is not needed at early boot, is it really causing any
> problems?
> 

They are the same problem.
I added pr_info in early_pfn_to_nid(), found get_nid_for_pfn() is the only
case need to add spinlock.
This patch tested on my x86 system.


>> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
>> ---
>> drivers/base/node.c | 11 +++++++++--
>> mm/mm_init.c | 18 +++---------------
>> 2 files changed, 12 insertions(+), 17 deletions(-)
>> 
>> diff --git a/drivers/base/node.c b/drivers/base/node.c
>> index 9de524e56307..844102570ff2 100644
>> --- a/drivers/base/node.c
>> +++ b/drivers/base/node.c
>> @@ -748,8 +748,15 @@ int unregister_cpu_under_node(unsigned int cpu, unsigned int nid)
>> static int __ref get_nid_for_pfn(unsigned long pfn)
>> {
>> #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> - if (system_state < SYSTEM_RUNNING)
>> - return early_pfn_to_nid(pfn);
>> + static DEFINE_SPINLOCK(early_pfn_lock);
>> + int nid;
>> +
>> + if (system_state < SYSTEM_RUNNING) {
>> + spin_lock(&early_pfn_lock);
>> + nid = early_pfn_to_nid(pfn);
>> + spin_unlock(&early_pfn_lock);
> 
> Adding an external lock for when you call a function is VERY dangerous
> as you did not document this anywhere, and there's no way to enforce it
> properly at all.
> 

I should add a comment before early_pfn_to_nid().

> Does your change actually result in any boot time changes? How was this
> tested?
> 

Just a bit.

> thanks,
> 
> greg k-h