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 by default. So, if you’d like to use it, you need to add “page_owner=on” to your boot cmdline. If the kernel is built with page owner and page owner is disabled in runtime due to not 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.

Although enabling page owner increases kernel size by several kilobytes, most of this code is outside page allocator and its hot path. 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 caught and marked, although they are mostly allocated from struct page extension feature. Anyway, after that, no page is left in un-tracking state.

Usage¶

Build user-space helper:
```
cd tools/mm
make page_owner_sort
```
Enable page owner: add “page_owner=on” to boot cmdline.
Do the job that you want to debug.

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

The general output of page_owner_full.txt is as follows:

    Page allocated via order XXX, ...
    PFN XXX ...
    // Detailed stack

    Page allocated via order XXX, ...
    PFN XXX ...
    // Detailed stack
By default, it will do full pfn dump, to start with a given pfn,
page_owner supports fseek.

FILE *fp = fopen("/sys/kernel/debug/page_owner", "r");
fseek(fp, pfn_start, SEEK_SET);

The page_owner_sort tool ignores PFN rows, puts the remaining rows in buf, uses regexp to extract the page order value, counts the times and pages of buf, and finally sorts them according to the parameter(s).

See the result about who allocated each page in the sorted_page_owner.txt. General output:

XXX times, XXX pages:
Page allocated via order XXX, ...
// Detailed stack

By default, page_owner_sort is sorted according to the times of buf. If you want to sort by the page nums of buf, use the -m parameter. The detailed parameters are:

fundamental function:

Sort:
        -a              Sort by memory allocation time.
        -m              Sort by total memory.
        -p              Sort by pid.
        -P              Sort by tgid.
        -n              Sort by task command name.
        -r              Sort by memory release time.
        -s              Sort by stack trace.
        -t              Sort by times (default).
        --sort <order>  Specify sorting order.  Sorting syntax is [+|-]key[,[+|-]key[,...]].
                        Choose a key from the **STANDARD FORMAT SPECIFIERS** section. The "+" is
                        optional since default direction is increasing numerical or lexicographic
                        order. Mixed use of abbreviated and complete-form of keys is allowed.

        Examples:
                        ./page_owner_sort <input> <output> --sort=n,+pid,-tgid
                        ./page_owner_sort <input> <output> --sort=at

additional function:

Cull:
        --cull <rules>
                        Specify culling rules.Culling syntax is key[,key[,...]].Choose a
                        multi-letter key from the **STANDARD FORMAT SPECIFIERS** section.

        <rules> is a single argument in the form of a comma-separated list,
        which offers a way to specify individual culling rules.  The recognized
        keywords are described in the **STANDARD FORMAT SPECIFIERS** section below.
        <rules> can be specified by the sequence of keys k1,k2, ..., as described in
        the STANDARD SORT KEYS section below. Mixed use of abbreviated and
        complete-form of keys is allowed.

        Examples:
                        ./page_owner_sort <input> <output> --cull=stacktrace
                        ./page_owner_sort <input> <output> --cull=st,pid,name
                        ./page_owner_sort <input> <output> --cull=n,f

Filter:
        -f              Filter out the information of blocks whose memory has been released.

Select:
        --pid <pidlist>         Select by pid. This selects the blocks whose process ID
                                numbers appear in <pidlist>.
        --tgid <tgidlist>       Select by tgid. This selects the blocks whose thread
                                group ID numbers appear in <tgidlist>.
        --name <cmdlist>        Select by task command name. This selects the blocks whose
                                task command name appear in <cmdlist>.

        <pidlist>, <tgidlist>, <cmdlist> are single arguments in the form of a comma-separated list,
        which offers a way to specify individual selecting rules.


        Examples:
                        ./page_owner_sort <input> <output> --pid=1
                        ./page_owner_sort <input> <output> --tgid=1,2,3
                        ./page_owner_sort <input> <output> --name name1,name2

STANDARD FORMAT SPECIFIERS¶

For --sort option:

      KEY             LONG            DESCRIPTION
      p               pid             process ID
      tg              tgid            thread group ID
      n               name            task command name
      st              stacktrace      stack trace of the page allocation
      T               txt             full text of block
      ft              free_ts         timestamp of the page when it was released
      at              alloc_ts        timestamp of the page when it was allocated
      ator            allocator       memory allocator for pages

For --cull option:

      KEY             LONG            DESCRIPTION
      p               pid             process ID
      tg              tgid            thread group ID
      n               name            task command name
      f               free            whether the page has been released or not
      st              stacktrace      stack trace of the page allocation
      ator            allocator       memory allocator for pages