[PATCH v4 36/36] memprofiling: Documentation

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From: Kent Overstreet <kent.overstreet@xxxxxxxxx>

Provide documentation for memory allocation profiling.

Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
 Documentation/mm/allocation-profiling.rst | 86 +++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 Documentation/mm/allocation-profiling.rst

diff --git a/Documentation/mm/allocation-profiling.rst b/Documentation/mm/allocation-profiling.rst
new file mode 100644
index 000000000000..2bcbd9e51fe4
--- /dev/null
+++ b/Documentation/mm/allocation-profiling.rst
@@ -0,0 +1,86 @@
+.. SPDX-License-Identifier: GPL-2.0
+Low overhead (suitable for production) accounting of all memory allocations,
+tracked by file and line number.
+kconfig options:
+   adds warnings for allocations that weren't accounted because of a
+   missing annotation
+Boot parameter:
+  sysctl.vm.mem_profiling=1
+  /proc/sys/vm/mem_profiling
+Runtime info:
+  /proc/allocinfo
+Example output:
+  root@moria-kvm:~# sort -g /proc/allocinfo|tail|numfmt --to=iec
+        2.8M    22648 fs/kernfs/dir.c:615 func:__kernfs_new_node
+        3.8M      953 mm/memory.c:4214 func:alloc_anon_folio
+        4.0M     1010 drivers/staging/ctagmod/ctagmod.c:20 [ctagmod] func:ctagmod_start
+        4.1M        4 net/netfilter/nf_conntrack_core.c:2567 func:nf_ct_alloc_hashtable
+        6.0M     1532 mm/filemap.c:1919 func:__filemap_get_folio
+        8.8M     2785 kernel/fork.c:307 func:alloc_thread_stack_node
+         13M      234 block/blk-mq.c:3421 func:blk_mq_alloc_rqs
+         14M     3520 mm/mm_init.c:2530 func:alloc_large_system_hash
+         15M     3656 mm/readahead.c:247 func:page_cache_ra_unbounded
+         55M     4887 mm/slub.c:2259 func:alloc_slab_page
+        122M    31168 mm/page_ext.c:270 func:alloc_page_ext
+Theory of operation
+Memory allocation profiling builds off of code tagging, which is a library for
+declaring static structs (that typcially describe a file and line number in
+some way, hence code tagging) and then finding and operating on them at runtime
+- i.e. iterating over them to print them in debugfs/procfs.
+To add accounting for an allocation call, we replace it with a macro
+invocation, alloc_hooks(), that
+ - declares a code tag
+ - stashes a pointer to it in task_struct
+ - calls the real allocation function
+ - and finally, restores the task_struct alloc tag pointer to its previous value.
+This allows for alloc_hooks() calls to be nested, with the most recent one
+taking effect. This is important for allocations internal to the mm/ code that
+do not properly belong to the outer allocation context and should be counted
+separately: for example, slab object extension vectors, or when the slab
+allocates pages from the page allocator.
+Thus, proper usage requires determining which function in an allocation call
+stack should be tagged. There are many helper functions that essentially wrap
+e.g. kmalloc() and do a little more work, then are called in multiple places;
+we'll generally want the accounting to happen in the callers of these helpers,
+not in the helpers themselves.
+To fix up a given helper, for example foo(), do the following:
+ - switch its allocation call to the _noprof() version, e.g. kmalloc_noprof()
+ - rename it to foo_noprof()
+ - define a macro version of foo() like so:
+   #define foo(...) alloc_hooks(foo_noprof(__VA_ARGS__))
+It's also possible to stash a pointer to an alloc tag in your own data structures.
+Do this when you're implementing a generic data structure that does allocations
+"on behalf of" some other code - for example, the rhashtable code. This way,
+instead of seeing a large line in /proc/allocinfo for rhashtable.c, we can
+break it out by rhashtable type.
+To do so:
+ - Hook your data structure's init function, like any other allocation function
+ - Within your init function, use the convenience macro alloc_tag_record() to
+   record alloc tag in your data structure.
+ - Then, use the following form for your allocations:
+   alloc_hooks_tag(ht->your_saved_tag, kmalloc_noprof(...))

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