Short users guide for SLUB -------------------------- The basic philosophy of SLUB is very different from SLAB. SLAB requires rebuilding the kernel to activate debug options for all slab caches. SLUB always includes full debugging but it is off by default. SLUB can enable debugging only for selected slabs in order to avoid an impact on overall system performance which may make a bug more difficult to find. In order to switch debugging on one can add a option "slub_debug" to the kernel command line. That will enable full debugging for all slabs. Typically one would then use the "slabinfo" command to get statistical data and perform operation on the slabs. By default slabinfo only lists slabs that have data in them. See "slabinfo -h" for more options when running the command. slabinfo can be compiled with gcc -o slabinfo Documentation/vm/slabinfo.c Some of the modes of operation of slabinfo require that slub debugging be enabled on the command line. F.e. no tracking information will be available without debugging on and validation can only partially be performed if debugging was not switched on. Some more sophisticated uses of slub_debug: ------------------------------------------- Parameters may be given to slub_debug. If none is specified then full debugging is enabled. Format: slub_debug= Enable options for all slabs slub_debug=, Enable options only for select slabs Possible debug options are F Sanity checks on (enables SLAB_DEBUG_FREE. Sorry SLAB legacy issues) Z Red zoning P Poisoning (object and padding) U User tracking (free and alloc) T Trace (please only use on single slabs) F.e. in order to boot just with sanity checks and red zoning one would specify: slub_debug=FZ Trying to find an issue in the dentry cache? Try slub_debug=,dentry_cache to only enable debugging on the dentry cache. Red zoning and tracking may realign the slab. We can just apply sanity checks to the dentry cache with slub_debug=F,dentry_cache In case you forgot to enable debugging on the kernel command line: It is possible to enable debugging manually when the kernel is up. Look at the contents of: /sys/slab// Look at the writable files. Writing 1 to them will enable the corresponding debug option. All options can be set on a slab that does not contain objects. If the slab already contains objects then sanity checks and tracing may only be enabled. The other options may cause the realignment of objects. Careful with tracing: It may spew out lots of information and never stop if used on the wrong slab. Slab merging ------------ If no debug options are specified then SLUB may merge similar slabs together in order to reduce overhead and increase cache hotness of objects. slabinfo -a displays which slabs were merged together. Slab validation --------------- SLUB can validate all object if the kernel was booted with slub_debug. In order to do so you must have the slabinfo tool. Then you can do slabinfo -v which will test all objects. Output will be generated to the syslog. This also works in a more limited way if boot was without slab debug. In that case slabinfo -v simply tests all reachable objects. Usually these are in the cpu slabs and the partial slabs. Full slabs are not tracked by SLUB in a non debug situation. Getting more performance ------------------------ To some degree SLUB's performance is limited by the need to take the list_lock once in a while to deal with partial slabs. That overhead is governed by the order of the allocation for each slab. The allocations can be influenced by kernel parameters: slub_min_objects=x (default 4) slub_min_order=x (default 0) slub_max_order=x (default 1) slub_min_objects allows to specify how many objects must at least fit into one slab in order for the allocation order to be acceptable. In general slub will be able to perform this number of allocations on a slab without consulting centralized resources (list_lock) where contention may occur. slub_min_order specifies a minim order of slabs. A similar effect like slub_min_objects. slub_max_order specified the order at which slub_min_objects should no longer be checked. This is useful to avoid SLUB trying to generate super large order pages to fit slub_min_objects of a slab cache with large object sizes into one high order page. SLUB Debug output ----------------- Here is a sample of slub debug output: *** SLUB kmalloc-8: Redzone Active@0xc90f6d20 slab 0xc528c530 offset=3360 flags=0x400000c3 inuse=61 freelist=0xc90f6d58 Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005 Redzone 0xc90f6d28: 00 cc cc cc . FreePointer 0xc90f6d2c -> 0xc90f6d58 Last alloc: get_modalias+0x61/0xf5 jiffies_ago=53 cpu=1 pid=554 Filler 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ [] dump_trace+0x63/0x1eb [] show_trace_log_lvl+0x1a/0x2f [] show_trace+0x12/0x14 [] dump_stack+0x16/0x18 [] object_err+0x143/0x14b [] check_object+0x66/0x234 [] __slab_free+0x239/0x384 [] kfree+0xa6/0xc6 [] get_modalias+0xb9/0xf5 [] dmi_dev_uevent+0x27/0x3c [] dev_uevent+0x1ad/0x1da [] kobject_uevent_env+0x20a/0x45b [] kobject_uevent+0xa/0xf [] store_uevent+0x4f/0x58 [] dev_attr_store+0x29/0x2f [] sysfs_write_file+0x16e/0x19c [] vfs_write+0xd1/0x15a [] sys_write+0x3d/0x72 [] sysenter_past_esp+0x5f/0x99 [] 0xb7f7b410 ======================= @@@ SLUB kmalloc-8: Restoring redzone (0xcc) from 0xc90f6d28-0xc90f6d2b If SLUB encounters a corrupted object then it will perform the following actions: 1. Isolation and report of the issue This will be a message in the system log starting with *** SLUB : @ offset= flags= inuse= freelist= 2. Report on how the problem was dealt with in order to ensure the continued operation of the system. These are messages in the system log beginning with @@@ SLUB : In the above sample SLUB found that the Redzone of an active object has been overwritten. Here a string of 8 characters was written into a slab that has the length of 8 characters. However, a 8 character string needs a terminating 0. That zero has overwritten the first byte of the Redzone field. After reporting the details of the issue encountered the @@@ SLUB message tell us that SLUB has restored the redzone to its proper value and then system operations continue. Various types of lines can follow the @@@ SLUB line: Bytes b4
: Show a few bytes before the object where the problem was detected. Can be useful if the corruption does not stop with the start of the object. Object
: The bytes of the object. If the object is inactive then the bytes typically contain poisoning values. Any non-poison value shows a corruption by a write after free. Redzone
: The redzone following the object. The redzone is used to detect writes after the object. All bytes should always have the same value. If there is any deviation then it is due to a write after the object boundary. Freepointer The pointer to the next free object in the slab. May become corrupted if overwriting continues after the red zone. Last alloc: Last free: Shows the address from which the object was allocated/freed last. We note the pid, the time and the CPU that did so. This is usually the most useful information to figure out where things went wrong. Here get_modalias() did an kmalloc(8) instead of a kmalloc(9). Filler
: Unused data to fill up the space in order to get the next object properly aligned. In the debug case we make sure that there are at least 4 bytes of filler. This allow for the detection of writes before the object. Following the filler will be a stackdump. That stackdump describes the location where the error was detected. The cause of the corruption is more likely to be found by looking at the information about the last alloc / free. Christoph Lameter, , May 23, 2007