[PATCH v1 2/4] arm64: hibernate: Use fixmap for hibernate_map/unmap_page

[Vincent Donnefort]

The default implementation for hibernate_map_page uses the direct map.
This creates an unnecessary limitation for BBML3 enabled systems which
can split linear map blocks and can enable memfd_secret users regardless
of the linear map mapping level.

Block splitting acquires a mutex (pgtable_split_lock) and as a
consequence is incompatible with hibernation. We know hibernation would
only call set_direct_map*() on a PTE-level region since it was
previously split by memfd_secret. But let's avoid special casing
hibernation in split_kernel_leaf_mapping().

On hibernation, do not modify the direct map and instead use a fixmap,
better suited for a transient mapping hibernation needs to copy the page
content. It removes the need for handling atomic context in
split_kernel_leaf_mapping() altogether.

Signed-off-by: Vincent Donnefort <vdonnefort at google.com>
---
 arch/arm64/include/asm/fixmap.h |  3 +++
 arch/arm64/kernel/hibernate.c   | 12 ++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h
index 170c3502d723..c8a65fd8584d 100644
--- a/arch/arm64/include/asm/fixmap.h
+++ b/arch/arm64/include/asm/fixmap.h
@@ -73,6 +73,9 @@ enum fixed_addresses {
 	FIX_ENTRY_TRAMP_TEXT1,
 #define TRAMP_VALIAS		(__fix_to_virt(FIX_ENTRY_TRAMP_TEXT1))
 #endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
+#ifdef CONFIG_HIBERNATION
+	FIX_HIBERNATE,
+#endif
 	__end_of_permanent_fixed_addresses,
 
 	/*
diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
index 7bf117427777..bbd0779081fa 100644
--- a/arch/arm64/kernel/hibernate.c
+++ b/arch/arm64/kernel/hibernate.c
@@ -21,6 +21,7 @@
 #include <asm/cacheflush.h>
 #include <asm/cputype.h>
 #include <asm/daifflags.h>
+#include <asm/fixmap.h>
 #include <asm/irqflags.h>
 #include <asm/kexec.h>
 #include <asm/memory.h>
@@ -213,6 +214,17 @@ static int create_safe_exec_page(void *src_start, size_t length,
 	return 0;
 }
 
+void *hibernate_map_page(struct page *page)
+{
+	set_fixmap(FIX_HIBERNATE, page_to_phys(page));
+	return (void *)__fix_to_virt(FIX_HIBERNATE);
+}
+
+void hibernate_unmap_page(struct page *page)
+{
+	clear_fixmap(FIX_HIBERNATE);
+}
+
 #ifdef CONFIG_ARM64_MTE
 
 static DEFINE_XARRAY(mte_pages);
-- 
2.55.0.1082.g2b9226bbc0-goog