lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option-fix

Update the comment to be clearer, and account for the improvement
to MMU_LAZY_TLB_REFCOUNT comment.

Link: https://lkml.kernel.org/r/1623121901.mszkmmum0n.astroid@bobo.none
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

diff --git a/arch/Kconfig b/arch/Kconfig
index 57477e18e5a9965f66b2a15f26c74bf20a9889b0..1baac7bfdd4413ac9592bf8511df41ad50182083 100644 (file)
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -440,15 +440,16 @@ config MMU_LAZY_TLB_REFCOUNT
        def_bool y
        depends on !MMU_LAZY_TLB_SHOOTDOWN
 
-# Instead of refcounting the lazy mm struct for kernel thread references
-# (which can cause contention with multi-threaded apps on large multiprocessor
-# systems), this option causes __mmdrop to IPI all CPUs in the mm_cpumask and
-# switch to init_mm if they were using the to-be-freed mm as the lazy tlb. To
-# implement this, architectures must use _lazy_tlb variants of mm refcounting
-# when releasing kernel thread mm references, and mm_cpumask must include at
-# least all possible CPUs in which the mm might be lazy, at the time of the
-# final mmdrop. mmgrab/mmdrop in arch/ code must be switched to _lazy_tlb
-# postfix as necessary.
+# This option allows MMU_LAZY_TLB_REFCOUNT=n. It ensures no CPUs are using an
+# mm as a lazy tlb beyond its last reference count, by shooting down these
+# users before the mm is deallocated. __mmdrop() first IPIs all CPUs that may
+# be using the mm as a lazy tlb, so that they may switch themselves to using
+# init_mm for their active mm. mm_cpumask(mm) is used to determine which CPUs
+# may be using mm as a lazy tlb mm.
+#
+# To implement this, an arch must ensure mm_cpumask(mm) contains at least all
+# possible CPUs in which the mm is lazy, and it must meet the requirements for
+# MMU_LAZY_TLB_REFCOUNT=n (see above).
 config MMU_LAZY_TLB_SHOOTDOWN
        bool