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16a7ade8 JK |
1 | page owner: Tracking about who allocated each page |
2 | ----------------------------------------------------------- | |
3 | ||
4 | * Introduction | |
5 | ||
6 | page owner is for the tracking about who allocated each page. | |
7 | It can be used to debug memory leak or to find a memory hogger. | |
8 | When allocation happens, information about allocation such as call stack | |
9 | and order of pages is stored into certain storage for each page. | |
10 | When we need to know about status of all pages, we can get and analyze | |
11 | this information. | |
12 | ||
13 | Although we already have tracepoint for tracing page allocation/free, | |
14 | using it for analyzing who allocate each page is rather complex. We need | |
15 | to enlarge the trace buffer for preventing overlapping until userspace | |
16 | program launched. And, launched program continually dump out the trace | |
17 | buffer for later analysis and it would change system behviour with more | |
18 | possibility rather than just keeping it in memory, so bad for debugging. | |
19 | ||
20 | page owner can also be used for various purposes. For example, accurate | |
21 | fragmentation statistics can be obtained through gfp flag information of | |
22 | each page. It is already implemented and activated if page owner is | |
23 | enabled. Other usages are more than welcome. | |
24 | ||
25 | page owner is disabled in default. So, if you'd like to use it, you need | |
26 | to add "page_owner=on" into your boot cmdline. If the kernel is built | |
27 | with page owner and page owner is disabled in runtime due to no enabling | |
28 | boot option, runtime overhead is marginal. If disabled in runtime, it | |
29 | doesn't require memory to store owner information, so there is no runtime | |
30 | memory overhead. And, page owner inserts just two unlikely branches into | |
31 | the page allocator hotpath and if it returns false then allocation is | |
32 | done like as the kernel without page owner. These two unlikely branches | |
33 | would not affect to allocation performance. Following is the kernel's | |
34 | code size change due to this facility. | |
35 | ||
36 | - Without page owner | |
37 | text data bss dec hex filename | |
38 | 40662 1493 644 42799 a72f mm/page_alloc.o | |
39 | ||
40 | - With page owner | |
41 | text data bss dec hex filename | |
42 | 40892 1493 644 43029 a815 mm/page_alloc.o | |
43 | 1427 24 8 1459 5b3 mm/page_ext.o | |
44 | 2722 50 0 2772 ad4 mm/page_owner.o | |
45 | ||
46 | Although, roughly, 4 KB code is added in total, page_alloc.o increase by | |
47 | 230 bytes and only half of it is in hotpath. Building the kernel with | |
48 | page owner and turning it on if needed would be great option to debug | |
49 | kernel memory problem. | |
50 | ||
51 | There is one notice that is caused by implementation detail. page owner | |
52 | stores information into the memory from struct page extension. This memory | |
53 | is initialized some time later than that page allocator starts in sparse | |
54 | memory system, so, until initialization, many pages can be allocated and | |
55 | they would have no owner information. To fix it up, these early allocated | |
56 | pages are investigated and marked as allocated in initialization phase. | |
57 | Although it doesn't mean that they have the right owner information, | |
58 | at least, we can tell whether the page is allocated or not, | |
59 | more accurately. On 2GB memory x86-64 VM box, 13343 early allocated pages | |
60 | are catched and marked, although they are mostly allocated from struct | |
61 | page extension feature. Anyway, after that, no page is left in | |
62 | un-tracking state. | |
63 | ||
64 | * Usage | |
65 | ||
66 | 1) Build user-space helper | |
67 | cd tools/vm | |
68 | make page_owner_sort | |
69 | ||
70 | 2) Enable page owner | |
71 | Add "page_owner=on" to boot cmdline. | |
72 | ||
73 | 3) Do the job what you want to debug | |
74 | ||
75 | 4) Analyze information from page owner | |
76 | cat /sys/kernel/debug/page_owner > page_owner_full.txt | |
77 | grep -v ^PFN page_owner_full.txt > page_owner.txt | |
78 | ./page_owner_sort page_owner.txt sorted_page_owner.txt | |
79 | ||
80 | See the result about who allocated each page | |
81 | in the sorted_page_owner.txt. |