This enables validation of slab.  Validation means that all objects are
checked to see if there are redzone violations, if padding has been
overwritten or any pointers have been corrupted.  Also checks the consistency
of slab counters.

Validation enables the detection of metadata corruption without the kernel
having to execute code that actually uses (allocs/frees) and object.  It
allows one to make sure that the slab metainformation and the guard values
around an object have not been compromised.

A single slabcache can be checked by writing a 1 to the "validate" file.

i.e.

echo 1 >/sys/slab/kmalloc-128/validate

or use the slabinfo tool to check all slabs

slabinfo -v

Error messages will show up in the syslog.

Note that validation can only reach slabs that are on a list.  This means that
we are usually restricted to partial slabs and active slabs unless
SLAB_STORE_USER is active which will build a full slab list and allows
validation of slabs that are fully in use.  Booting with "slub_debug" set will
enable SLAB_STORE_USER and then full diagnostic are available.

Note that we attempt to push cpu slabs back to the lists when we start the
check.  If the cpu slab is reactivated before we get to it (another processor
grabs it before we get to it) then it cannot be checked.
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>