[PATCH v3 8/8] Documentation: add new page_owner document

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

page owner is for the tracking about who allocated each page.
This document explains what is the page owner feature and what is
the merit of it. And, simple HOW-TO is also explained. See the document
for detailed information.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
---
 Documentation/vm/page_owner.txt |   81 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 Documentation/vm/page_owner.txt

diff --git a/Documentation/vm/page_owner.txt b/Documentation/vm/page_owner.txt
new file mode 100644
index 0000000..8f3ce9b
--- /dev/null
+++ b/Documentation/vm/page_owner.txt
@@ -0,0 +1,81 @@
+page owner: Tracking about who allocated each page
+-----------------------------------------------------------
+
+* Introduction
+
+page owner is for the tracking about who allocated each page.
+It can be used to debug memory leak or to find a memory hogger.
+When allocation happens, information about allocation such as call stack
+and order of pages is stored into certain storage for each page.
+When we need to know about status of all pages, we can get and analyze
+this information.
+
+Although we already have tracepoint for tracing page allocation/free,
+using it for analyzing who allocate each page is rather complex. We need
+to enlarge the trace buffer for preventing overlapping until userspace
+program launched. And, launched program continually dump out the trace
+buffer for later analysis and it would change system behviour with more
+possibility rather than just keeping it in memory, so bad for debugging.
+
+page owner can also be used for various purposes. For example, accurate
+fragmentation statistics can be obtained through gfp flag information of
+each page. It is already implemented and activated if page owner is
+enabled. Other usages are more than welcome.
+
+page owner is disabled in default. So, if you'd like to use it, you need
+to add "page_owner=on" into your boot cmdline. If the kernel is built
+with page owner and page owner is disabled in runtime due to no enabling
+boot option, runtime overhead is marginal. If disabled in runtime, it
+doesn't require memory to store owner information, so there is no runtime
+memory overhead. And, page owner inserts just two unlikely branches into
+the page allocator hotpath and if it returns false then allocation is
+done like as the kernel without page owner. These two unlikely branches
+would not affect to allocation performance. Following is the kernel's
+code size change due to this facility.
+
+- Without page owner
+   text    data     bss     dec     hex filename
+  40662    1493     644   42799    a72f mm/page_alloc.o
+
+- With page owner
+   text    data     bss     dec     hex filename
+  40892    1493     644   43029    a815 mm/page_alloc.o
+   1427      24       8    1459     5b3 mm/page_ext.o
+   2722      50       0    2772     ad4 mm/page_owner.o
+
+Although, roughly, 4 KB code is added in total, page_alloc.o increase by
+230 bytes and only half of it is in hotpath. Building the kernel with
+page owner and turning it on if needed would be great option to debug
+kernel memory problem.
+
+There is one notice that is caused by implementation detail. page owner
+stores information into the memory from struct page extension. This memory
+is initialized some time later than that page allocator starts in sparse
+memory system, so, until initialization, many pages can be allocated and
+they would have no owner information. To fix it up, these early allocated
+pages are investigated and marked as allocated in initialization phase.
+Although it doesn't mean that they have the right owner information,
+at least, we can tell whether the page is allocated or not,
+more accurately. On 2GB memory x86-64 VM box, 13343 early allocated pages
+are catched and marked, although they are mostly allocated from struct
+page extension feature. Anyway, after that, no page is left in
+un-tracking state.
+
+* Usage
+
+1) Build user-space helper
+	cd tools/vm
+	make page_owner_sort
+
+2) Enable page owner
+	Add "page_owner=on" to boot cmdline.
+
+3) Do the job what you want to debug
+
+4) Analyze information from page owner
+	cat /sys/kernel/debug/page_owner > page_owner_full.txt
+	grep -v ^PFN page_owner_full.txt > page_owner.txt
+	./page_owner_sort page_owner.txt sorted_page_owner.txt
+
+	See the result about who allocated each page
+	in the sorted_page_owner.txt.
-- 
1.7.9.5

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>