Date: Sun, 23 Jun 2024 23:39:50 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Nanyong Sun <sunnanyong@...wei.com>
Cc: David Rientjes <rientjes@...gle.com>, Will Deacon <will@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Matthew Wilcox <willy@...radead.org>, muchun.song@...ux.dev,
	Andrew Morton <akpm@...ux-foundation.org>,
	anshuman.khandual@....com, wangkefeng.wang@...wei.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Yosry Ahmed <yosryahmed@...gle.com>,
	Sourav Panda <souravpanda@...gle.com>
Subject: Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize

On Mon, Mar 25, 2024 at 11:24:34PM +0800, Nanyong Sun wrote:
> On 2024/3/14 7:32, David Rientjes wrote:
> 
> > On Thu, 8 Feb 2024, Will Deacon wrote:
> > 
> > > > How about take a new lock with irq disabled during BBM, like:
> > > > 
> > > > +void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte)
> > > > +{
> > > > +     (NEW_LOCK);
> > > > +    pte_clear(&init_mm, addr, ptep);
> > > > +    flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> > > > +    set_pte_at(&init_mm, addr, ptep, pte);
> > > > +    spin_unlock_irq(NEW_LOCK);
> > > > +}
> > > I really think the only maintainable way to achieve this is to avoid the
> > > possibility of a fault altogether.
> > > 
> > > Will
> > > 
> > > 
> > Nanyong, are you still actively working on making HVO possible on arm64?
> > 
> > This would yield a substantial memory savings on hosts that are largely
> > configured with hugetlbfs.  In our case, the size of this hugetlbfs pool
> > is actually never changed after boot, but it sounds from the thread that
> > there was an idea to make HVO conditional on FEAT_BBM.  Is this being
> > pursued?
> > 
> > If so, any testing help needed?
> I'm afraid that FEAT_BBM may not solve the problem here

I think so too -- I came cross this while working on TAO [1].

[1] https://lore.kernel.org/20240229183436.4110845-4-yuzhao@google.com/

> because from Arm
> ARM,
> I see that FEAT_BBM is only used for changing block size. Therefore, in this
> HVO feature,
> it can work in the split PMD stage, that is, BBM can be avoided in
> vmemmap_split_pmd,
> but in the subsequent vmemmap_remap_pte, the Output address of PTE still
> needs to be
> changed. I'm afraid FEAT_BBM is not competent for this stage. Perhaps my
> understanding
> of ARM FEAT_BBM is wrong, and I hope someone can correct me.
> Actually, the solution I first considered was to use the stop_machine
> method, but we have
> products that rely on /proc/sys/vm/nr_overcommit_hugepages to dynamically
> use hugepages,
> so I have to consider performance issues. If your product does not change
> the amount of huge
> pages after booting, using stop_machine() may be a feasible way.
> So far, I still haven't come up with a good solution.

I do have a patch that's similar to stop_machine() -- it uses NMI IPIs
to pause/resume remote CPUs while the local one is doing BBM.

Note that the problem of updating vmemmap for struct page[], as I see
it, is beyond hugeTLB HVO. I think it impacts virtio-mem and memory
hot removal in general [2]. On arm64, we would need to support BBM on
vmemmap so that we can fix the problem with offlining memory (or to be
precise, unmapping offlined struct page[]), by mapping offlined struct
page[] to a read-only page of dummy struct page[], similar to
ZERO_PAGE(). (Or we would have to make extremely invasive changes to
the reader side, i.e., all speculative PFN walkers.)

In case you are interested in testing my approach, you can swap your
patch 2 with the following:

[2] https://lore.kernel.org/20240621213717.1099079-1-yuzhao@google.com/

diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
index 8ff5f2a2579e..1af1aa34a351 100644
--- a/arch/arm64/include/asm/pgalloc.h
+++ b/arch/arm64/include/asm/pgalloc.h
@@ -12,6 +12,7 @@
 #include <asm/processor.h>
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
+#include <asm/cpu.h>
 
 #define __HAVE_ARCH_PGD_FREE
 #define __HAVE_ARCH_PUD_FREE
@@ -137,4 +138,58 @@ pmd_populate(struct mm_struct *mm, pmd_t *pmdp, pgtable_t ptep)
 	__pmd_populate(pmdp, page_to_phys(ptep), PMD_TYPE_TABLE | PMD_TABLE_PXN);
 }
 
+#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
+
+#define vmemmap_update_lock vmemmap_update_lock
+static inline void vmemmap_update_lock(void)
+{
+	cpus_read_lock();
+}
+
+#define vmemmap_update_unlock vmemmap_update_unlock
+static inline void vmemmap_update_unlock(void)
+{
+	cpus_read_unlock();
+}
+
+#define vmemmap_update_pte vmemmap_update_pte
+static inline void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte)
+{
+	preempt_disable();
+	pause_remote_cpus();
+
+	pte_clear(&init_mm, addr, ptep);
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+	set_pte_at(&init_mm, addr, ptep, pte);
+
+	resume_remote_cpus();
+	preempt_enable();
+}
+
+#define vmemmap_update_pmd vmemmap_update_pmd
+static inline void vmemmap_update_pmd(unsigned long addr, pmd_t *pmdp, pte_t *ptep)
+{
+	preempt_disable();
+	pause_remote_cpus();
+
+	pmd_clear(pmdp);
+	flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+	pmd_populate_kernel(&init_mm, pmdp, ptep);
+
+	resume_remote_cpus();
+	preempt_enable();
+}
+
+#define vmemmap_flush_tlb_all vmemmap_flush_tlb_all
+static inline void vmemmap_flush_tlb_all(void)
+{
+}
+
+#define vmemmap_flush_tlb_range vmemmap_flush_tlb_range
+static inline void vmemmap_flush_tlb_range(unsigned long start, unsigned long end)
+{
+}
+
+#endif /* CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP */
+
 #endif
diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
index efb13112b408..544b15948b64 100644
--- a/arch/arm64/include/asm/smp.h
+++ b/arch/arm64/include/asm/smp.h
@@ -144,6 +144,9 @@ bool cpus_are_stuck_in_kernel(void);
 extern void crash_smp_send_stop(void);
 extern bool smp_crash_stop_failed(void);
 
+void pause_remote_cpus(void);
+void resume_remote_cpus(void);
+
 #endif /* ifndef __ASSEMBLY__ */
 
 #endif /* ifndef __ASM_SMP_H */
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 31c8b3094dd7..ae0a178db066 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -71,16 +71,25 @@ enum ipi_msg_type {
 	IPI_RESCHEDULE,
 	IPI_CALL_FUNC,
 	IPI_CPU_STOP,
+	IPI_CPU_PAUSE,
+#ifdef CONFIG_KEXEC_CORE
 	IPI_CPU_CRASH_STOP,
+#endif
+#ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
 	IPI_TIMER,
+#endif
+#ifdef CONFIG_IRQ_WORK
 	IPI_IRQ_WORK,
+#endif
 	NR_IPI,
 	/*
 	 * Any enum >= NR_IPI and < MAX_IPI is special and not tracable
 	 * with trace_ipi_*
 	 */
 	IPI_CPU_BACKTRACE = NR_IPI,
+#ifdef CONFIG_KGDB
 	IPI_KGDB_ROUNDUP,
+#endif
 	MAX_IPI
 };
 
@@ -771,9 +780,16 @@ static const char *ipi_types[NR_IPI] __tracepoint_string = {
 	[IPI_RESCHEDULE]	= "Rescheduling interrupts",
 	[IPI_CALL_FUNC]		= "Function call interrupts",
 	[IPI_CPU_STOP]		= "CPU stop interrupts",
+	[IPI_CPU_PAUSE]		= "CPU pause interrupts",
+#ifdef CONFIG_KEXEC_CORE
 	[IPI_CPU_CRASH_STOP]	= "CPU stop (for crash dump) interrupts",
+#endif
+#ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
 	[IPI_TIMER]		= "Timer broadcast interrupts",
+#endif
+#ifdef CONFIG_IRQ_WORK
 	[IPI_IRQ_WORK]		= "IRQ work interrupts",
+#endif
 };
 
 static void smp_cross_call(const struct cpumask *target, unsigned int ipinr);
@@ -832,6 +848,85 @@ void __noreturn panic_smp_self_stop(void)
 	local_cpu_stop();
 }
 
+static DEFINE_SPINLOCK(cpu_pause_lock);
+static cpumask_t paused_cpus;
+static cpumask_t resumed_cpus;
+
+static void pause_local_cpu(void)
+{
+	int cpu = smp_processor_id();
+
+	cpumask_clear_cpu(cpu, &resumed_cpus);
+	/*
+	 * Paired with pause_remote_cpus() to confirm that this CPU not only
+	 * will be paused but also can be reliably resumed.
+	 */
+	smp_wmb();
+	cpumask_set_cpu(cpu, &paused_cpus);
+	/* A typical example for sleep and wake-up functions. */
+	smp_mb();
+	while (!cpumask_test_cpu(cpu, &resumed_cpus)) {
+		wfe();
+		barrier();
+	}
+	barrier();
+	cpumask_clear_cpu(cpu, &paused_cpus);
+}
+
+void pause_remote_cpus(void)
+{
+	cpumask_t cpus_to_pause;
+
+	lockdep_assert_cpus_held();
+	lockdep_assert_preemption_disabled();
+
+	cpumask_copy(&cpus_to_pause, cpu_online_mask);
+	cpumask_clear_cpu(smp_processor_id(), &cpus_to_pause);
+
+	spin_lock(&cpu_pause_lock);
+
+	WARN_ON_ONCE(!cpumask_empty(&paused_cpus));
+
+	smp_cross_call(&cpus_to_pause, IPI_CPU_PAUSE);
+
+	while (!cpumask_equal(&cpus_to_pause, &paused_cpus)) {
+		cpu_relax();
+		barrier();
+	}
+	/*
+	 * Paired pause_local_cpu() to confirm that all CPUs not only will be
+	 * paused but also can be reliably resumed.
+	 */
+	smp_rmb();
+	WARN_ON_ONCE(cpumask_intersects(&cpus_to_pause, &resumed_cpus));
+
+	spin_unlock(&cpu_pause_lock);
+}
+
+void resume_remote_cpus(void)
+{
+	cpumask_t cpus_to_resume;
+
+	lockdep_assert_cpus_held();
+	lockdep_assert_preemption_disabled();
+
+	cpumask_copy(&cpus_to_resume, cpu_online_mask);
+	cpumask_clear_cpu(smp_processor_id(), &cpus_to_resume);
+
+	spin_lock(&cpu_pause_lock);
+
+	cpumask_setall(&resumed_cpus);
+	/* A typical example for sleep and wake-up functions. */
+	smp_mb();
+	while (cpumask_intersects(&cpus_to_resume, &paused_cpus)) {
+		sev();
+		cpu_relax();
+		barrier();
+	}
+
+	spin_unlock(&cpu_pause_lock);
+}
+
 #ifdef CONFIG_KEXEC_CORE
 static atomic_t waiting_for_crash_ipi = ATOMIC_INIT(0);
 #endif
@@ -911,6 +1006,11 @@ static void do_handle_IPI(int ipinr)
 		local_cpu_stop();
 		break;
 
+	case IPI_CPU_PAUSE:
+		pause_local_cpu();
+		break;
+
+#ifdef CONFIG_KEXEC_CORE
 	case IPI_CPU_CRASH_STOP:
 		if (IS_ENABLED(CONFIG_KEXEC_CORE)) {
 			ipi_cpu_crash_stop(cpu, get_irq_regs());
@@ -918,6 +1018,7 @@ static void do_handle_IPI(int ipinr)
 			unreachable();
 		}
 		break;
+#endif
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
 	case IPI_TIMER:
@@ -939,9 +1040,11 @@ static void do_handle_IPI(int ipinr)
 		nmi_cpu_backtrace(get_irq_regs());
 		break;
 
+#ifdef CONFIG_KGDB
 	case IPI_KGDB_ROUNDUP:
 		kgdb_nmicallback(cpu, get_irq_regs());
 		break;
+#endif
 
 	default:
 		pr_crit("CPU%u: Unknown IPI message 0x%x\n", cpu, ipinr);
@@ -971,9 +1074,14 @@ static bool ipi_should_be_nmi(enum ipi_msg_type ipi)
 
 	switch (ipi) {
 	case IPI_CPU_STOP:
+	case IPI_CPU_PAUSE:
+#ifdef CONFIG_KEXEC_CORE
 	case IPI_CPU_CRASH_STOP:
+#endif
 	case IPI_CPU_BACKTRACE:
+#ifdef CONFIG_KGDB
 	case IPI_KGDB_ROUNDUP:
+#endif
 		return true;
 	default:
 		return false;
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 5113753f3ac9..da6f2a7d665e 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -46,6 +46,18 @@ struct vmemmap_remap_walk {
 	unsigned long		flags;
 };
 
+#ifndef vmemmap_update_lock
+static void vmemmap_update_lock(void)
+{
+}
+#endif
+
+#ifndef vmemmap_update_unlock
+static void vmemmap_update_unlock(void)
+{
+}
+#endif
+
 #ifndef vmemmap_update_pmd
 static inline void vmemmap_update_pmd(unsigned long addr,
 				      pmd_t *pmdp, pte_t *ptep)
@@ -194,10 +206,12 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end,
 
 	VM_BUG_ON(!PAGE_ALIGNED(start | end));
 
+	vmemmap_update_lock();
 	mmap_read_lock(&init_mm);
 	ret = walk_page_range_novma(&init_mm, start, end, &vmemmap_remap_ops,
 				    NULL, walk);
 	mmap_read_unlock(&init_mm);
+	vmemmap_update_unlock();
 	if (ret)
 		return ret;
 

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