Re: [PATCH v5 04/18] sparc32: mm: Reduce allocation size for PMD and PTE tables

[Will Deacon]

On Tue, May 26, 2020 at 02:26:35PM +0100, Will Deacon wrote:
> On Sun, May 24, 2020 at 03:32:56PM +0300, Mike Rapoport wrote:
> > On Thu, May 21, 2020 at 04:02:11PM -0700, Guenter Roeck wrote:
> > > On 5/20/20 12:51 PM, Mike Rapoport wrote:
> > > > On Wed, May 20, 2020 at 12:03:31PM -0700, Guenter Roeck wrote:
> > > >> With above patch applied on top of Ira's patch, I get:
> > > >>
> > > >> BUG: spinlock recursion on CPU#0, S01syslogd/139
> > > >>  lock: 0xf5448350, .magic: dead4ead, .owner: S01syslogd/139, .owner_cpu: 0
> > > >> CPU: 0 PID: 139 Comm: S01syslogd Not tainted 5.7.0-rc6-next-20200518-00002-gb178d2d56f29-dirty #1
> > > >> [f0067a64 :
> > > >> do_raw_spin_lock+0xa8/0xd8 ]
> > > >> [f00d5034 :
> > > >> copy_page_range+0x328/0x804 ]
> > > >> [f0025be4 :
> > > >> dup_mm+0x334/0x434 ]
> > > >> [f0027124 :
> > > >> copy_process+0x1224/0x12b0 ]
> > > >> [f0027344 :
> > > >> _do_fork+0x54/0x30c ]
> > > >> [f0027670 :
> > > >> do_fork+0x5c/0x6c ]
> > > >> [f000de44 :
> > > >> sparc_do_fork+0x18/0x38 ]
> > > >> [f000b7f4 :
> > > >> do_syscall+0x34/0x40 ]
> > > >> [5010cd4c :
> > > >> 0x5010cd4c ]
> > > >>
> > > >> Looks like yet another problem.
> > > > 
> > > > I've checked the patch above on top of the mmots which already has Ira's
> > > > patches and it booted fine. I've used sparc32_defconfig to build the
> > > > kernel and qemu-system-sparc with default machine and CPU. 
> > > > 
> > > 
> > > Try sparc32_defconfig+SMP.
> >  
> > I see a differernt problem, but this could be related:
> > 
> > INIT: version 2.86 booting
> > rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> > 	(detected by 0, t=5252 jiffies, g=-935, q=3)
> > rcu: All QSes seen, last rcu_sched kthread activity 5252 (-68674--73926), jiffies_till_next_fqs=1, root ->qsmask 0x0
> > rcu: rcu_sched kthread starved for 5252 jiffies! g-935 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
> > rcu: 	Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
> > rcu: RCU grace-period kthread stack dump:
> > rcu_sched       R  running task        0    10      2 0x00000000
> > 
> > I'm running a bit old debian [1] with qemu-img-sparc.
> > 
> > My bisect pointed at commit 8c8f3156dd40 ("sparc32: mm: Reduce
> > allocation size for PMD and PTE tables"). The commit ID is valid for
> > next-20200522.
> 
> Can you try the diff below please?

Actually, that's racy. New version below!

Will

--->8

diff --git a/arch/sparc/mm/srmmu.c b/arch/sparc/mm/srmmu.c
index c861c0f0df73..068029471aa4 100644
--- a/arch/sparc/mm/srmmu.c
+++ b/arch/sparc/mm/srmmu.c
@@ -363,11 +363,16 @@ pgtable_t pte_alloc_one(struct mm_struct *mm)
 
 	if ((ptep = pte_alloc_one_kernel(mm)) == 0)
 		return NULL;
+
 	page = pfn_to_page(__nocache_pa((unsigned long)ptep) >> PAGE_SHIFT);
-	if (!pgtable_pte_page_ctor(page)) {
-		__free_page(page);
-		return NULL;
+
+	spin_lock(&mm->page_table_lock);
+	if (page_ref_inc_return(page) == 2 && !pgtable_pte_page_ctor(page)) {
+		page_ref_dec(page);
+		ptep = NULL;
 	}
+	spin_unlock(&mm->page_table_lock);
+
 	return ptep;
 }
 
@@ -376,7 +381,12 @@ void pte_free(struct mm_struct *mm, pgtable_t ptep)
 	struct page *page;
 
 	page = pfn_to_page(__nocache_pa((unsigned long)ptep) >> PAGE_SHIFT);
-	pgtable_pte_page_dtor(page);
+
+	spin_lock(&mm->page_table_lock);
+	if (page_ref_dec_return(page) == 1)
+		pgtable_pte_page_dtor(page);
+	spin_unlock(&mm->page_table_lock);
+
 	srmmu_free_nocache(ptep, SRMMU_PTE_TABLE_SIZE);
 }
 
diff --git a/mm/Kconfig b/mm/Kconfig
index c1acc34c1c35..97458119cce8 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -192,6 +192,9 @@ config MEMORY_HOTREMOVE
 # Default to 4 for wider testing, though 8 might be more appropriate.
 # ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock.
 # PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes.
+# SPARC32 allocates multiple pte tables within a single page, and therefore
+# a per-page lock leads to problems when multiple tables need to be locked
+# at the same time (e.g. copy_page_range()).
 # DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC spinlock_t also enlarge struct page.
 #
 config SPLIT_PTLOCK_CPUS
@@ -199,6 +202,7 @@ config SPLIT_PTLOCK_CPUS
 	default "999999" if !MMU
 	default "999999" if ARM && !CPU_CACHE_VIPT
 	default "999999" if PARISC && !PA20
+	default "999999" if SPARC32
 	default "4"
 
 config ARCH_ENABLE_SPLIT_PMD_PTLOCK