[PATCH] mm: Make failslab writable again

[Alexander Atanasov <alexander.atanasov@virtuozzo.com>]

In (060807f841ac mm, slub: make remaining slub_debug related attributes
read-only failslab) it was made RO.
I think it became a collateral victim to the other two options
(sanity_checks and trace) for which the reasons are perfectly valid.
Here is why:
 - sanity_checks and trace are slab internal debug options,
   failslab is used for fault injection.
 - for fault injections, which by presumption are random, it
   does not matter if it is not set atomically. You need to
   set atleast one more option to trigger fault injection.
 - in a testing scenario you may need to change it at runtime
   example: module loading - you test all allocations limited
   by the space option. Then you move to test only your module's
   own slabs.
 - when set by command line flags it effectively disables all
   cache merges.

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Jann Horn <jannh@google.com>
Cc: Vijayanand Jitta <vjitta@codeaurora.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/20200610163135.17364-5-vbabka@suse.cz

Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
---
 Documentation/mm/slub.rst |  2 ++
 mm/slub.c                 | 14 +++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/Documentation/mm/slub.rst b/Documentation/mm/slub.rst
index 43063ade737a..86837073a39e 100644
--- a/Documentation/mm/slub.rst
+++ b/Documentation/mm/slub.rst
@@ -116,6 +116,8 @@ options from the ``slub_debug`` parameter translate to the following files::
 	T	trace
 	A	failslab
 
+failslab file is writable, so writing 1 or 0 will enable or disable
+the option at runtime. Write returns -EINVAL if cache is an alias.
 Careful with tracing: It may spew out lots of information and never stop if
 used on the wrong slab.
 
diff --git a/mm/slub.c b/mm/slub.c
index 862dbd9af4f5..7c15d312e0fb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5617,7 +5617,19 @@ static ssize_t failslab_show(struct kmem_cache *s, char *buf)
 {
 	return sysfs_emit(buf, "%d\n", !!(s->flags & SLAB_FAILSLAB));
 }
-SLAB_ATTR_RO(failslab);
+
+static ssize_t failslab_store(struct kmem_cache *s, const char *buf,
+				size_t length)
+{
+	if (s->refcount > 1)
+		return -EINVAL;
+
+	s->flags &= ~SLAB_FAILSLAB;
+	if (buf[0] == '1')
+		s->flags |= SLAB_FAILSLAB;
+	return length;
+}
+SLAB_ATTR(failslab);
 #endif
 
 static ssize_t shrink_show(struct kmem_cache *s, char *buf)

base-commit: 80e78fcce86de0288793a0ef0f6acf37656ee4cf
-- 
2.31.1