[PATCH 0/3] sparc32 SRMMU fixes for SMP

[PATCH 0/3] sparc32 SRMMU fixes for SMP

[Will Deacon]

Hi folks,

Enabling SMP for sparc32 uncovered some issues in the SRMMU page-table
allocation code. One of these was introduced by me, but the other two
seem to have been there a while and are probably just exposed more
easily by my recent changes.

Tested on QEMU. I'm assuming these will go via David's tree.

Cheers,

Will

--->8

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: kernel-team@android.com

Will Deacon (3):
  sparc32: mm: Don't try to free page-table pages if ctor() fails
  sparc32: mm: Disable SPLIT_PTLOCK_CPUS
  sparc32: mm: Only call ctor()/dtor() functions for first and last user

 arch/sparc/mm/srmmu.c | 15 +++++++++++----
 mm/Kconfig            |  4 ++++
 2 files changed, 15 insertions(+), 4 deletions(-)

-- 
2.27.0.rc0.183.gde8f92d652-goog

[PATCH 1/3] sparc32: mm: Don't try to free page-table pages if ctor() fails

[Will Deacon]

The pages backing page-table allocations for SRMMU are allocated via
memblock as part of the "nocache" region initialisation during
srmmu_paging_init() and should not be freed even if a later call to
pgtable_pte_page_ctor() fails.

Remove the broken call to __free_page().

Cc: David S. Miller <davem@davemloft.net>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Fixes: 1ae9ae5f7df7 ("sparc: handle pgtable_page_ctor() fail")
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/sparc/mm/srmmu.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/sparc/mm/srmmu.c b/arch/sparc/mm/srmmu.c
index c861c0f0df73..589370a21b12 100644
--- a/arch/sparc/mm/srmmu.c
+++ b/arch/sparc/mm/srmmu.c
@@ -365,7 +365,6 @@ pgtable_t pte_alloc_one(struct mm_struct *mm)
 		return NULL;
 	page = pfn_to_page(__nocache_pa((unsigned long)ptep) >> PAGE_SHIFT);
 	if (!pgtable_pte_page_ctor(page)) {
-		__free_page(page);
 		return NULL;
 	}
 	return ptep;
-- 
2.27.0.rc0.183.gde8f92d652-goog

[PATCH 2/3] sparc32: mm: Disable SPLIT_PTLOCK_CPUS

[Will Deacon]

The SRMMU page-table allocator is not compatible with SPLIT_PTLOCK_CPUS
for two major reasons:

  1. Pages are allocated via memblock, and therefore the ptl is not
     cleared by prep_new_page(), which is expected by ptlock_init()

  2. Multiple PTE tables can exist in a single page, causing them to
     share the same ptl and deadlock when attempting to take the same
     lock twice (e.g. as part of copy_page_range()).

Ensure that SPLIT_PTLOCK_CPUS is not selected for SPARC32.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Will Deacon <will@kernel.org>
---
 mm/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

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
-- 
2.27.0.rc0.183.gde8f92d652-goog

[PATCH 3/3] sparc32: mm: Only call ctor()/dtor() functions for first and last user

[Will Deacon]

The SRMMU page-table allocator allocates multiple PTE tables per page,
since they are only 1K in size. However, this means that calls to
pgtable_pte_page_{ctor,dtor}() must be serialised and performed only by
the first and last page-table allocation for the page respectively.

Use the page reference count to track how many PTE tables we have
allocated for a given page returned by the SRMMU allocator and only
call the ctor()/dtor() functions for the first and last user respectively.

Cc: David S. Miller <davem@davemloft.net>
Fixes: 8c8f3156dd40 ("sparc32: mm: Reduce allocation size for PMD and PTE tables")
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/sparc/mm/srmmu.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/sparc/mm/srmmu.c b/arch/sparc/mm/srmmu.c
index 589370a21b12..116d19a390f2 100644
--- a/arch/sparc/mm/srmmu.c
+++ b/arch/sparc/mm/srmmu.c
@@ -364,9 +364,13 @@ 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)) {
-		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;
 }
 
@@ -375,7 +379,11 @@ 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);
 }
 
-- 
2.27.0.rc0.183.gde8f92d652-goog

Re: [PATCH 0/3] sparc32 SRMMU fixes for SMP

[David Miller]

From: Will Deacon <will@kernel.org>
Date: Tue, 26 May 2020 18:32:59 +0100

> Hi folks,
> 
> Enabling SMP for sparc32 uncovered some issues in the SRMMU page-table
> allocation code. One of these was introduced by me, but the other two
> seem to have been there a while and are probably just exposed more
> easily by my recent changes.
> 
> Tested on QEMU. I'm assuming these will go via David's tree.

Series applied, thanks Will.