s390/mm: improve description of mapping permissions of prefix pages

Slightly improve the description which explains why the first prefix
page must be mapped executable when the BEAR-enhancement facility is
not installed.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

diff --git a/arch/s390/mm/vmem.c b/arch/s390/mm/vmem.c
index 9b31dc565bc5ea1a1e41607b89aed7918a5120ad..a1e01542790b22fe9afbb6dde21be459ee28aba0 100644 (file)
--- a/arch/s390/mm/vmem.c
+++ b/arch/s390/mm/vmem.c
@@ -659,8 +659,11 @@ void __init vmem_map_init(void)
                       (unsigned long)(_einittext - _sinittext) >> PAGE_SHIFT);
        set_memory_rox((unsigned long)__stext_amode31,
                       (unsigned long)(__etext_amode31 - __stext_amode31) >> PAGE_SHIFT);
-
-       /* lowcore must be executable for LPSWE */
+       /*
+        * If the BEAR-enhancement facility is not installed the first
+        * prefix page is used to return to the previous context with
+        * an LPSWE instruction and therefore must be executable.
+        */
        if (!static_key_enabled(&cpu_has_bear))
                set_memory_x(0, 1);
        if (debug_pagealloc_enabled()) {