linux-kernel - [PATCH 2/3] sparc32: mm: Disable SPLIT_PTLOCK

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Date:   Tue, 26 May 2020 18:33:01 +0100
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     sparclinux@...r.kernel.org, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "David S . Miller" <davem@...emloft.net>,
        Guenter Roeck <linux@...ck-us.net>,
        Mike Rapoport <rppt@...nel.org>, kernel-team@...roid.com
Subject: [PATCH 2/3] sparc32: mm: Disable SPLIT_PTLOCK_CPUS

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@...emloft.net>
Signed-off-by: Will Deacon <will@...nel.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