mm/highmem: Provide kmap_local*

Now that the kmap atomic index is stored in task struct provide a
preemptible variant. On context switch the maps of an outgoing task are
removed and the map of the incoming task are restored. That's obviously
slow, but highmem is slow anyway.

The kmap_local.*() functions can be invoked from both preemptible and
atomic context. kmap local sections disable migration to keep the resulting
virtual mapping address correct, but disable neither pagefaults nor
preemption.

A wholesale conversion of kmap_atomic to be fully preemptible is not
possible because some of the usage sites might rely on the preemption
disable for serialization or on the implicit pagefault disable. Needs to be
done on a case by case basis.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204007.468533059@linutronix.de

1 files changed, 6 insertions, 0 deletions

diff --git a/mm/highmem.c b/mm/highmem.c
index d1ef06aa6de6..83f9660f168f 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -453,6 +453,11 @@ void *__kmap_local_pfn_prot(unsigned long pfn, pgprot_t prot)
 	unsigned long vaddr;
 	int idx;
 
+	/*
+	 * Disable migration so resulting virtual address is stable
+	 * accross preemption.
+	 */
+	migrate_disable();
 	preempt_disable();
 	idx = arch_kmap_local_map_idx(kmap_local_idx_push(), pfn);
 	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
@@ -522,6 +527,7 @@ void kunmap_local_indexed(void *vaddr)
 	current->kmap_ctrl.pteval[kmap_local_idx()] = __pte(0);
 	kmap_local_idx_pop();
 	preempt_enable();
+	migrate_enable();
 }
 EXPORT_SYMBOL(kunmap_local_indexed);