From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Commit abca7c496 notes that we can not _set_ a page->counters
directly, except when using a real double-cmpxchg. Doing so can
lose updates to ->_count.
That an absolute rule:
You may not *set* page->counters except via a cmpxchg.
Commit abca7c496 fixed this for the folks who have the slub
cmpxchg_double code turned off at compile time, but it left the
bad alone. It can still be reached, and the same bug triggered
in two cases:
1. Turning on slub debugging at runtime, which is available on
the distro kernels that I looked at.
2. On 64-bit CPUs with no CMPXCHG16B (some early AMD x86-64
cpus, evidently)
There are at least 3 ways we could fix this:
1. Take all of the exising calls to cmpxchg_double_slab() and
__cmpxchg_double_slab() and convert them to take an old, new
and target 'struct page'.
2. Do (1), but with the newly-introduced 'slub_data'.
3. Do some magic inside the two cmpxchg...slab() functions to
pull the counters out of new_counters and only set those
fields in page->{inuse,frozen,objects}.
I've done (2) as well, but it's a bunch more code. This patch
is an attempt at (3). This was the most straightforward and
foolproof way that I could think to do this.
This would also technically allow us to get rid of the ugly
#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
in 'struct page', but leaving it alone has the added benefit that
'counters' stays 'unsigned' instead of 'unsigned long', so all
the copies that the slub code does stay a bit smaller.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Matt Mackall <mpm@xxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Pravin B Shelar <pshelar@xxxxxxxxxx>
---
b/mm/slub.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)
diff -puN mm/slub.c~slub-never-set-counters-except-via-cmpxchg mm/slub.c
--- a/mm/slub.c~slub-never-set-counters-except-via-cmpxchg 2014-01-28 14:48:26.420339269 -0800
+++ b/mm/slub.c 2014-01-28 15:09:37.410864843 -0800
@@ -355,6 +355,21 @@ static __always_inline void slab_unlock(
__bit_spin_unlock(PG_locked, &page->flags);
}
+static inline void set_page_slub_counters(struct page *page, unsigned long counters_new)
+{
+ struct page tmp;
+ tmp.counters = counters_new;
+ /*
+ * page->counters can cover frozen/inuse/objects as well
+ * as page->_count. If we assign to ->counters directly
+ * we run the risk of losing updates to page->_count, so
+ * be careful and only assign to the fields we need.
+ */
+ page->frozen = tmp.frozen;
+ page->inuse = tmp.inuse;
+ page->objects = tmp.objects;
+}
+
/* Interrupts must be disabled (for the fallback code to work right) */
static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
void *freelist_old, unsigned long counters_old,
@@ -376,7 +391,7 @@ static inline bool __cmpxchg_double_slab
if (page->freelist == freelist_old &&
page->counters == counters_old) {
page->freelist = freelist_new;
- page->counters = counters_new;
+ set_page_slub_counters(page, counters_new);
slab_unlock(page);
return 1;
}
@@ -415,7 +430,7 @@ static inline bool cmpxchg_double_slab(s
if (page->freelist == freelist_old &&
page->counters == counters_old) {
page->freelist = freelist_new;
- page->counters = counters_new;
+ set_page_slub_counters(page, counters_new);
slab_unlock(page);
local_irq_restore(flags);
return 1;
_
--
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