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diff --git a/Documentation/vm/page_owner.rst b/Documentation/vm/page_owner.rst new file mode 100644 index 000000000..02deac766 --- /dev/null +++ b/Documentation/vm/page_owner.rst @@ -0,0 +1,89 @@ +.. _page_owner: + +================================================== +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 behaviour 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 +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 + ./page_owner_sort page_owner_full.txt sorted_page_owner.txt + + See the result about who allocated each page + in the ``sorted_page_owner.txt``. |