kasan: docs: update shadow memory section

Update the "Shadow memory" section in KASAN documentation:

 - Rearrange the introduction paragraph do it doesn't give a
   "KASAN has an issue" impression.

 - Update the list of architectures with vmalloc support.

 - Punctuation, readability, and other minor clean-ups.

Link: https://lkml.kernel.org/r/00f8c38b0fd5290a3f4dced04eaba41383e67e14.1615559068.git.andreyknvl@google.comSigned-off-by: NAndrey Konovalov <andreyknvl@google.com>
Reviewed-by: NMarco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/kasan.rst

@@ -313,14 +313,11 @@ checking gets disabled.
 Shadow memory
 -------------
 
-The kernel maps memory in a number of different parts of the address
-space. This poses something of a problem for KASAN, which requires
-that all addresses accessed by instrumented code have a valid shadow
-region.
-
-The range of kernel virtual addresses is large: there is not enough
-real memory to support a real shadow region for every address that
-could be accessed by the kernel.
+The kernel maps memory in several different parts of the address space.
+The range of kernel virtual addresses is large: there is not enough real
+memory to support a real shadow region for every address that could be
+accessed by the kernel. Therefore, KASAN only maps real shadow for certain
+parts of the address space.
 
 Default behaviour
 ~~~~~~~~~~~~~~~~~
@@ -332,10 +329,9 @@ page is mapped over the shadow area. This read-only shadow page
 declares all memory accesses as permitted.
 
 This presents a problem for modules: they do not live in the linear
-mapping, but in a dedicated module space. By hooking in to the module
-allocator, KASAN can temporarily map real shadow memory to cover
-them. This allows detection of invalid accesses to module globals, for
-example.
+mapping but in a dedicated module space. By hooking into the module
+allocator, KASAN temporarily maps real shadow memory to cover them.
+This allows detection of invalid accesses to module globals, for example.
 
 This also creates an incompatibility with ``VMAP_STACK``: if the stack
 lives in vmalloc space, it will be shadowed by the read-only page, and
@@ -346,9 +342,10 @@ CONFIG_KASAN_VMALLOC
 ~~~~~~~~~~~~~~~~~~~~
 
 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
-cost of greater memory usage. Currently this is only supported on x86.
+cost of greater memory usage. Currently, this is supported on x86,
+riscv, s390, and powerpc.
 
-This works by hooking into vmalloc and vmap, and dynamically
+This works by hooking into vmalloc and vmap and dynamically
 allocating real shadow memory to back the mappings.
 
 Most mappings in vmalloc space are small, requiring less than a full
@@ -367,10 +364,10 @@ memory.
 
 To avoid the difficulties around swapping mappings around, KASAN expects
 that the part of the shadow region that covers the vmalloc space will
-not be covered by the early shadow page, but will be left
-unmapped. This will require changes in arch-specific code.
+not be covered by the early shadow page but will be left unmapped.
+This will require changes in arch-specific code.
 
-This allows ``VMAP_STACK`` support on x86, and can simplify support of
+This allows ``VMAP_STACK`` support on x86 and can simplify support of
 architectures that do not have a fixed module region.
 
 For developers