Re: [mm/sl[au]b] 3c4cafa313: canonical_address#:#[##]

Hyeonggon Yoo
2022-09-09T14:32:54Z

On Fri, Sep 09, 2022 at 03:44:19PM +0200, Vlastimil Babka wrote:
> On 9/9/22 13:05, Hyeonggon Yoo wrote:
> >> ----8<----
> >> From d6f9fbb33b908eb8162cc1f6ce7f7c970d0f285f Mon Sep 17 00:00:00 2001
> >> From: Vlastimil Babka <vbabka@suse.cz>
> >> Date: Fri, 9 Sep 2022 12:03:10 +0200
> >> Subject: [PATCH 2/3] mm/migrate: make isolate_movable_page() skip slab pages
> >>
> >> In the next commit we want to rearrange struct slab fields to allow a
> >> larger rcu_head. Afterwards, the page->mapping field will overlap
> >> with SLUB's "struct list_head slab_list", where the value of prev
> >> pointer can become LIST_POISON2, which is 0x122 + POISON_POINTER_DELTA.
> >> Unfortunately the bit 1 being set can confuse PageMovable() to be a
> >> false positive and cause a GPF as reported by lkp [1].
> >>
> >> To fix this, make isolate_movable_page() skip pages with the PageSlab
> >> flag set. This is a bit tricky as we need to add memory barriers to SLAB
> >> and SLUB's page allocation and freeing, and their counterparts to
> >> isolate_movable_page().
> >
> > Hello, I just took a quick grasp,
> > Is this approach okay with folio_test_anon()?
>
> Not if used on a completely random page as compaction scanners can, but
> relies on those being first tested for PageLRU or coming from a page table
> lookup etc.
> Not ideal huh. Well I could improve also by switching 'next' and 'slabs'
> field and relying on the fact that the value of LIST_POISON2 doesn't include
> 0x1, just 0x2.

What about swapping counters and freelist?
freelist should be always aligned.

diff --git a/mm/slab.h b/mm/slab.h
index 2c248864ea91..7d4762a39065 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -27,17 +27,7 @@ struct slab {
struct kmem_cache *slab_cache;
union {
struct {
- union {
- struct list_head slab_list;
-#ifdef CONFIG_SLUB_CPU_PARTIAL
- struct {
- struct slab *next;
- int slabs; /* Nr of slabs left */
- };
-#endif
- };
/* Double-word boundary */
- void *freelist; /* first free object */
union {
unsigned long counters;
struct {
@@ -46,6 +36,16 @@ struct slab {
unsigned frozen:1;
};
};
+ void *freelist; /* first free object */
+ union {
+ struct list_head slab_list;
+#ifdef CONFIG_SLUB_CPU_PARTIAL
+ struct {
+ struct slab *next;
+ int slabs; /* Nr of slabs left */
+ };
+#endif
+ };
};
struct rcu_head rcu_head;
};
@@ -81,10 +81,14 @@ SLAB_MATCH(_refcount, __page_refcount);
#ifdef CONFIG_MEMCG
SLAB_MATCH(memcg_data, memcg_data);
#endif
+#ifdef CONFIG_SLUB
+SLAB_MATCH(mapping, freelist);
+#endif
+
#undef SLAB_MATCH
static_assert(sizeof(struct slab) <= sizeof(struct page));
#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && defined(CONFIG_SLUB)
-static_assert(IS_ALIGNED(offsetof(struct slab, freelist), 16));
+static_assert(IS_ALIGNED(offsetof(struct slab, counters), 16));
#endif

/**
diff --git a/mm/slub.c b/mm/slub.c
index 2f9cb6e67de3..0c9595c63e33 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -487,9 +487,9 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab
#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
if (s->flags & __CMPXCHG_DOUBLE) {
- if (cmpxchg_double(&slab->freelist, &slab->counters,
- freelist_old, counters_old,
- freelist_new, counters_new))
+ if (cmpxchg_double(&slab->counters, &slab->freelist,
+ counters_old, freelist_old,
+ counters_new, freelist_new))
return true;
} else
#endif
@@ -526,9 +526,9 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
if (s->flags & __CMPXCHG_DOUBLE) {
- if (cmpxchg_double(&slab->freelist, &slab->counters,
- freelist_old, counters_old,
- freelist_new, counters_new))
+ if (cmpxchg_double(&slab->counters, &slab->freelist,
+ counters_old, freelist_old,
+ counters_new, freelist_new))
return true;
} else
#endif

--
Thanks,
Hyeonggon