arm64: mm: Simplify check in arch_kfence_init_pool()

TL;DR: checking force_pte_mapping() in arch_kfence_init_pool() is
sufficient

Commit ce2b3a50ad92 ("arm64: mm: Don't sleep in
split_kernel_leaf_mapping() when in atomic context") recently added
an arm64 implementation of arch_kfence_init_pool() to ensure that
the KFENCE pool is PTE-mapped. Assuming that the pool was not
initialised early, block splitting is necessary if the linear
mapping is not fully PTE-mapped, in other words if
force_pte_mapping() is false.

arch_kfence_init_pool() currently makes another check: whether
BBML2-noabort is supported, i.e. whether we are *able* to split
block mappings. This check is however unnecessary, because
force_pte_mapping() is always true if KFENCE is enabled and
BBML2-noabort is not supported. This must be the case by design,
since KFENCE requires PTE-mapped pages in all cases. We can
therefore remove that check.

The situation is different in split_kernel_leaf_mapping(), as that
function is called unconditionally regardless of the configuration.
If BBML2-noabort is not supported, it cannot do anything and bails
out. If force_pte_mapping() is true, there is nothing to do and it
also bails out, but these are independent checks.

Commit 53357f14f924 ("arm64: mm: Tidy up force_pte_mapping()")
grouped these checks into a helper, split_leaf_mapping_possible().
This isn't so helpful as only split_kernel_leaf_mapping() should
check both. Revert the parts of that commit that introduced the
helper, reintroducing the more accurate comments in
split_kernel_leaf_mapping().

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 9ae7ce00a7ef280944950a6d0ba01ddf2503b5f7..8e1d80a7033e34388c05da4e4902f14d75f4f37c 100644 (file)
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -767,18 +767,6 @@ static inline bool force_pte_mapping(void)
        return rodata_full || arm64_kfence_can_set_direct_map() || is_realm_world();
 }
 
-static inline bool split_leaf_mapping_possible(void)
-{
-       /*
-        * !BBML2_NOABORT systems should never run into scenarios where we would
-        * have to split. So exit early and let calling code detect it and raise
-        * a warning.
-        */
-       if (!system_supports_bbml2_noabort())
-               return false;
-       return !force_pte_mapping();
-}
-
 static DEFINE_MUTEX(pgtable_split_lock);
 
 int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
@@ -786,11 +774,22 @@ int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
        int ret;
 
        /*
-        * Exit early if the region is within a pte-mapped area or if we can't
-        * split. For the latter case, the permission change code will raise a
-        * warning if not already pte-mapped.
+        * !BBML2_NOABORT systems should not be trying to change permissions on
+        * anything that is not pte-mapped in the first place. Just return early
+        * and let the permission change code raise a warning if not already
+        * pte-mapped.
         */
-       if (!split_leaf_mapping_possible() || is_kfence_address((void *)start))
+       if (!system_supports_bbml2_noabort())
+               return 0;
+
+       /*
+        * If the region is within a pte-mapped area, there is no need to try to
+        * split. Additionally, CONFIG_DEBUG_PAGEALLOC and CONFIG_KFENCE may
+        * change permissions from atomic context so for those cases (which are
+        * always pte-mapped), we must not go any further because taking the
+        * mutex below may sleep.
+        */
+       if (force_pte_mapping() || is_kfence_address((void *)start))
                return 0;
 
        /*
@@ -1089,7 +1088,7 @@ bool arch_kfence_init_pool(void)
        int ret;
 
        /* Exit early if we know the linear map is already pte-mapped. */
-       if (!split_leaf_mapping_possible())
+       if (force_pte_mapping())
                return true;
 
        /* Kfence pool is already pte-mapped for the early init case. */