Documentation: add new page_owner document

page owner: Tracking about who allocated each page

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* 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.