linux-kernel - Re: [PATCH] mm/mm_init.c: remove spinlock in early_pfn_to

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Date:   Wed, 14 Jun 2023 14:53:39 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Yajun Deng <yajun.deng@...ux.dev>
Cc:     Greg KH <gregkh@...uxfoundation.org>, rafael@...nel.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm/mm_init.c: remove spinlock in early_pfn_to_nid()

Hi,

On Wed, Jun 14, 2023 at 11:28:32AM +0000, Yajun Deng wrote:
> June 14, 2023 7:09 PM, "Greg KH" <gregkh@...uxfoundation.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.
 
Are you sure it'll work on !x86?
 
> >> Signed-off-by: Yajun Deng <yajun.deng@...ux.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.
 
Just a bit tested? Or just a bit of boot time changes?
For the latter, do you have numbers?

-- 
Sincerely yours,
Mike.