Commit a0320865 authored Jan 30, 2014 by Dave Hansen Committed by Linus Torvalds Jan 30, 2014

mm/slub.c: fix page->_count corruption (again)



Commit abca7c49 ("mm: fix slab->page _count corruption when using
slub") 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 is an absolute rule:

        You may not *set* page->counters except via a cmpxchg.

Commit abca7c49 fixed this for the folks who have the slub
cmpxchg_double code turned off at compile time, but it left the bad case
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@linux.intel.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 8790c71a

mm/slub.c

+17 −2

Original line number	Diff line number	Diff line
		@@ -355,6 +355,21 @@ static __always_inline void slab_unlock(struct page *page)
		__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(struct kmem_cache s, struct page page
		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(struct kmem_cache s, struct page page,
		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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