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 stackand 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 analyzethis information.

Although we already have tracepoint for tracing page allocation/free,using it for analyzing who allocate each page is rather complex. We needto enlarge the trace buffer for preventing overlapping until userspaceprogram launched. And, launched program continually dump out the tracebuffer for later analysis and it would change system behaviour with morepossibility rather than just keeping it in memory, so bad for debugging.

page owner can also be used for various purposes. For example, accuratefragmentation statistics can be obtained through gfp flag information ofeach page. It is already implemented and activated if page owner isenabled. Other usages are more than welcome.

It can also be used to show all the stacks and their current number ofallocated base pages, which gives us a quick overview of where the memoryis going without the need to screen through all the pages and match theallocation and free operation. It’s also possible to show only a numericidentifier of all the stacks (without stack traces) and their number ofallocated base pages (faster to read and parse, eg, for monitoring) thatcan be matched with stacks later (show_handles and show_stacks_handles).

page owner is disabled by default. So, if you’d like to use it, you needto add “page_owner=on” to your boot cmdline. If the kernel is builtwith page owner and page owner is disabled in runtime due to not enablingboot option, runtime overhead is marginal. If disabled in runtime, itdoesn’t require memory to store owner information, so there is no runtimememory overhead. And, page owner inserts just two unlikely branches intothe page allocator hotpath and if not enabled, then allocation is donelike as the kernel without page owner. These two unlikely branches shouldnot affect to allocation performance, especially if the static keys jumplabel patching functionality is available. Following is the kernel’s codesize 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. Buildingthe kernel with page owner and turning it on if needed would be greatoption to debug kernel memory problem.

There is one notice that is caused by implementation detail. page ownerstores information into the memory fromstructpage extension. This memoryis initialized some time later than that page allocator starts in sparsememory system, so, until initialization, many pages can be allocated andthey would have no owner information. To fix it up, these early allocatedpages 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 pagesare caught and marked, although they are mostly allocated fromstructpage extension feature. Anyway, after that, no page is left inun-tracking state.

Usage¶

1. Build user-space helper:

   cd tools/mmmake page_owner_sort

2. Enable page owner: add “page_owner=on” to boot cmdline.

3. Do the job that you want to debug.

4. Analyze information from page owner:

   cat /sys/kernel/debug/page_owner_stacks/show_stacks > stacks.txtcat 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+0x4a1/0x820nr_base_pages: 16......echo 7000 > /sys/kernel/debug/page_owner_stacks/count_thresholdcat /sys/kernel/debug/page_owner_stacks/show_stacks> stacks_7000.txtcat 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+0x62/0x6anr_base_pages: 20824...cat /sys/kernel/debug/page_owner_stacks/show_handles > handles_7000.txtcat handles_7000.txthandle: 42nr_base_pages: 20824...cat /sys/kernel/debug/page_owner_stacks/show_stacks_handles > stacks_handles.txtcat stacks_handles.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+0x62/0x6ahandle: 42...cat /sys/kernel/debug/page_owner > page_owner_full.txt./page_owner_sort page_owner_full.txt sorted_page_owner.txt

   The general output ofpage_owner_full.txt is as follows:

       Page allocated via order XXX, ...    PFN XXX ...    // Detailed stack    Page allocated via order XXX, ...    PFN XXX ...    // Detailed stackBy 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);

   Thepage_owner_sort tool ignoresPFN rows, puts the remaining rowsin buf, uses regexp to extract the page order value, counts the timesand pages of buf, and finally sorts them according to the parameter(s).

   See the result about who allocated each pagein thesorted_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,fFilter:        -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 pagesFor --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