|page owner: Tracking about who allocated each page
|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
|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 not enabled, then allocation is done
|like as the kernel without page owner. These two unlikely branches should
|not affect to allocation performance, especially if the static keys jump
|label patching functionality is available. 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
|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``.