docs/vm: rename documentation files to .rst

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 361 insertions, 0 deletions

diff --git a/Documentation/vm/slub.rst b/Documentation/vm/slub.rst
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+.. _slub:
+
+==========================
+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 an 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 tools/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=<Debug-Options>
+	Enable options for all slabs
+slub_debug=<Debug-Options>,<slab name>
+	Enable options only for select slabs
+
+
+Possible debug options are::
+
+	F		Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS
+			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)
+	A		Toggle failslab filter mark for the cache
+	O		Switch debugging off for caches that would have
+			caused higher minimum slab orders
+	-		Switch all debugging off (useful if the kernel is
+			configured with CONFIG_SLUB_DEBUG_ON)
+
+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
+
+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
+
+Debugging options may require the minimum possible slab order to increase as
+a result of storing the metadata (for example, caches with PAGE_SIZE object
+sizes).  This has a higher liklihood of resulting in slab allocation errors
+in low memory situations or if there's high fragmentation of memory.  To
+switch off debugging for such caches by default, use::
+
+	slub_debug=O
+
+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/kernel/slab/<slab name>/
+
+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 3 (PAGE_ALLOC_COSTLY_ORDER))
+
+``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. Setting command line parameter
+	``debug_guardpage_minorder=N`` (N > 0), forces setting
+	``slub_max_order`` to 0, what cause minimum possible order of
+	slabs allocation.
+
+SLUB Debug output
+=================
+
+Here is a sample of slub debug output::
+
+ ====================================================================
+ BUG kmalloc-8: Redzone overwritten
+ --------------------------------------------------------------------
+
+ INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
+ INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
+ INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
+ INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
+
+ 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                                     .
+  Padding 0xc90f6d50:  5a 5a 5a 5a 5a 5a 5a 5a                         ZZZZZZZZ
+
+   [<c010523d>] dump_trace+0x63/0x1eb
+   [<c01053df>] show_trace_log_lvl+0x1a/0x2f
+   [<c010601d>] show_trace+0x12/0x14
+   [<c0106035>] dump_stack+0x16/0x18
+   [<c017e0fa>] object_err+0x143/0x14b
+   [<c017e2cc>] check_object+0x66/0x234
+   [<c017eb43>] __slab_free+0x239/0x384
+   [<c017f446>] kfree+0xa6/0xc6
+   [<c02e2335>] get_modalias+0xb9/0xf5
+   [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
+   [<c027866a>] dev_uevent+0x1ad/0x1da
+   [<c0205024>] kobject_uevent_env+0x20a/0x45b
+   [<c020527f>] kobject_uevent+0xa/0xf
+   [<c02779f1>] store_uevent+0x4f/0x58
+   [<c027758e>] dev_attr_store+0x29/0x2f
+   [<c01bec4f>] sysfs_write_file+0x16e/0x19c
+   [<c0183ba7>] vfs_write+0xd1/0x15a
+   [<c01841d7>] sys_write+0x3d/0x72
+   [<c0104112>] sysenter_past_esp+0x5f/0x99
+   [<b7f7b410>] 0xb7f7b410
+   =======================
+
+ FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
+
+If SLUB encounters a corrupted object (full detection requires the kernel
+to be booted with slub_debug) then the following output will be dumped
+into the syslog:
+
+1. Description of the problem encountered
+
+   This will be a message in the system log starting with::
+
+     ===============================================
+     BUG <slab cache affected>: <What went wrong>
+     -----------------------------------------------
+
+     INFO: <corruption start>-<corruption_end> <more info>
+     INFO: Slab <address> <slab information>
+     INFO: Object <address> <object information>
+     INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
+	cpu> pid=<pid of the process>
+     INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
+	pid=<pid of the process>
+
+   (Object allocation / free information is only available if SLAB_STORE_USER is
+   set for the slab. slub_debug sets that option)
+
+2. The object contents if an object was involved.
+
+   Various types of lines can follow the BUG SLUB line:
+
+   Bytes b4 <address> : <bytes>
+	Shows 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 <address> : <bytes>
+	The bytes of the object. If the object is inactive then the bytes
+	typically contain poison values. Any non-poison value shows a
+	corruption by a write after free.
+
+   Redzone <address> : <bytes>
+	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.
+
+	(Redzone information is only available if SLAB_RED_ZONE is set.
+	slub_debug sets that option)
+
+   Padding <address> : <bytes>
+	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 padding. This allows the detection of writes
+	before the object.
+
+3. A stackdump
+
+   The stackdump describes the location where the error was detected. The cause
+   of the corruption is may be more likely found by looking at the function that
+   allocated or freed the object.
+
+4. 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::
+
+	FIX <slab cache affected>: <corrective action taken>
+
+   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 FIX SLUB message
+   tells us that SLUB has restored the Redzone to its proper value and then
+   system operations continue.
+
+Emergency operations
+====================
+
+Minimal debugging (sanity checks alone) can be enabled by booting with::
+
+	slub_debug=F
+
+This will be generally be enough to enable the resiliency features of slub
+which will keep the system running even if a bad kernel component will
+keep corrupting objects. This may be important for production systems.
+Performance will be impacted by the sanity checks and there will be a
+continual stream of error messages to the syslog but no additional memory
+will be used (unlike full debugging).
+
+No guarantees. The kernel component still needs to be fixed. Performance
+may be optimized further by locating the slab that experiences corruption
+and enabling debugging only for that cache
+
+I.e.::
+
+	slub_debug=F,dentry
+
+If the corruption occurs by writing after the end of the object then it
+may be advisable to enable a Redzone to avoid corrupting the beginning
+of other objects::
+
+	slub_debug=FZ,dentry
+
+Extended slabinfo mode and plotting
+===================================
+
+The ``slabinfo`` tool has a special 'extended' ('-X') mode that includes:
+ - Slabcache Totals
+ - Slabs sorted by size (up to -N <num> slabs, default 1)
+ - Slabs sorted by loss (up to -N <num> slabs, default 1)
+
+Additionally, in this mode ``slabinfo`` does not dynamically scale
+sizes (G/M/K) and reports everything in bytes (this functionality is
+also available to other slabinfo modes via '-B' option) which makes
+reporting more precise and accurate. Moreover, in some sense the `-X'
+mode also simplifies the analysis of slabs' behaviour, because its
+output can be plotted using the ``slabinfo-gnuplot.sh`` script. So it
+pushes the analysis from looking through the numbers (tons of numbers)
+to something easier -- visual analysis.
+
+To generate plots:
+
+a) collect slabinfo extended records, for example::
+
+	while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done
+
+b) pass stats file(-s) to ``slabinfo-gnuplot.sh`` script::
+
+	slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]
+
+   The ``slabinfo-gnuplot.sh`` script will pre-processes the collected records
+   and generates 3 png files (and 3 pre-processing cache files) per STATS
+   file:
+   - Slabcache Totals: FOO_STATS-totals.png
+   - Slabs sorted by size: FOO_STATS-slabs-by-size.png
+   - Slabs sorted by loss: FOO_STATS-slabs-by-loss.png
+
+Another use case, when ``slabinfo-gnuplot.sh`` can be useful, is when you
+need to compare slabs' behaviour "prior to" and "after" some code
+modification.  To help you out there, ``slabinfo-gnuplot.sh`` script
+can 'merge' the `Slabcache Totals` sections from different
+measurements. To visually compare N plots:
+
+a) Collect as many STATS1, STATS2, .. STATSN files as you need::
+
+	while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
+
+b) Pre-process those STATS files::
+
+	slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN
+
+c) Execute ``slabinfo-gnuplot.sh`` in '-t' mode, passing all of the
+   generated pre-processed \*-totals::
+
+	slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals
+
+   This will produce a single plot (png file).
+
+   Plots, expectedly, can be large so some fluctuations or small spikes
+   can go unnoticed. To deal with that, ``slabinfo-gnuplot.sh`` has two
+   options to 'zoom-in'/'zoom-out':
+
+   a) ``-s %d,%d`` -- overwrites the default image width and heigh
+   b) ``-r %d,%d`` -- specifies a range of samples to use (for example,
+      in ``slabinfo -X >> FOO_STATS; sleep 1;`` case, using a ``-r
+      40,60`` range will plot only samples collected between 40th and
+      60th seconds).
+
+Christoph Lameter, May 30, 2007
+Sergey Senozhatsky, October 23, 2015