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.

It can also be used to show all the stacks and their current number of allocated base pages, which gives us a quick overview of where the memory is going without the need to screen through all the pages and match the allocation and free operation.

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
    48392   2333     644   51369    c8a9 mm/page_alloc.o
    
  • With page owner:

    text    data     bss     dec     hex filename
    48800   2445     644   51889    cab1 mm/page_alloc.o
    6662     108      29    6799    1a8f mm/page_owner.o
    1025       8       8    1041     411 mm/page_ext.o
    

Although, roughly, 8 KB code is added in total, page_alloc.o increase by 520 bytes and less than 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_stacks/show_stacks > stacks.txt
    cat stacks.txt
     post_alloc_hook+0x177/0x1a0
     get_page_from_freelist+0xd01/0xd80
     __alloc_pages+0x39e/0x7e0
     allocate_slab+0xbc/0x3f0
     ___slab_alloc+0x528/0x8a0
     kmem_cache_alloc+0x224/0x3b0
     sk_prot_alloc+0x58/0x1a0
     sk_alloc+0x32/0x4f0
     inet_create+0x427/0xb50
     __sock_create+0x2e4/0x650
     inet_ctl_sock_create+0x30/0x180
     igmp_net_init+0xc1/0x130
     ops_init+0x167/0x410
     setup_net+0x304/0xa60
     copy_net_ns+0x29b/0x4a0
     create_new_namespaces+0x4a1/0x820
    nr_base_pages: 16
    ...
    ...
    echo 7000 > /sys/kernel/debug/page_owner_stacks/count_threshold
    cat /sys/kernel/debug/page_owner_stacks/show_stacks> stacks_7000.txt
    cat stacks_7000.txt
     post_alloc_hook+0x177/0x1a0
     get_page_from_freelist+0xd01/0xd80
     __alloc_pages+0x39e/0x7e0
     alloc_pages_mpol+0x22e/0x490
     folio_alloc+0xd5/0x110
     filemap_alloc_folio+0x78/0x230
     page_cache_ra_order+0x287/0x6f0
     filemap_get_pages+0x517/0x1160
     filemap_read+0x304/0x9f0
     xfs_file_buffered_read+0xe6/0x1d0 [xfs]
     xfs_file_read_iter+0x1f0/0x380 [xfs]
     __kernel_read+0x3b9/0x730
     kernel_read_file+0x309/0x4d0
     __do_sys_finit_module+0x381/0x730
     do_syscall_64+0x8d/0x150
     entry_SYSCALL_64_after_hwframe+0x62/0x6a
    nr_base_pages: 20824
    ...
    
    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.