[patch] mm/slab.c: fix early init assumption

Ingo Molnar
2006-06-02T19:45:04Z

Subject: mm/slab.c: fix early init assumption
From: Ingo Molnar <mingo@elte.hu>

the SLAB bootstrap code assumes that the first two kmalloc caches
created (the INDEX_AC and INDEX_L3 kmalloc caches) wont be off-slab.
But due to AC and L3 structure size increase in lockdep, one of them
ended up being off-slab, and subsequently crashing with:

Unable to handle kernel NULL pointer dereference at 0000000000000000 RIP:
[<ffffffff80267478>] kmem_cache_alloc+0x26/0x7d

the fix is to introduce a bootstrap flag and to use it to prevent
off-slab caches being created so early during bootup.

(the calculation for off-slab caches is quite complex so i didnt want
to complicate things with introducing yet another INDEX_ calculation,
the flag approach is simpler and smaller.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
mm/slab.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

Index: linux/mm/slab.c
===================================================================
--- linux.orig/mm/slab.c
+++ linux/mm/slab.c
@@ -333,6 +333,8 @@ static __always_inline int index_of(cons
return 0;
}

+static int slab_early_init = 1;
+
#define INDEX_AC index_of(sizeof(struct arraycache_init))
#define INDEX_L3 index_of(sizeof(struct kmem_list3))

@@ -1377,6 +1379,8 @@ void __init kmem_cache_init(void)
NULL, NULL);
}

+ slab_early_init = 0;
+
while (sizes->cs_size != ULONG_MAX) {
/*
* For performance, all the general caches are L1 aligned.
@@ -2128,8 +2132,12 @@ kmem_cache_create (const char *name, siz
#endif
#endif

- /* Determine if the slab management is 'on' or 'off' slab. */
- if (size >= (PAGE_SIZE >> 3))
+ /*
+ * Determine if the slab management is 'on' or 'off' slab.
+ * (bootstrapping cannot cope with offslab caches so dont do
+ * it too early on.)
+ */
+ if ((size >= (PAGE_SIZE >> 3)) && !slab_early_init)
/*
* Size is large, assume best to place the slab management obj
* off-slab (should allow better packing of objs).