90 lines
3.7 KiB
ReStructuredText
90 lines
3.7 KiB
ReStructuredText
.. _page_owner:
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==================================================
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page owner: Tracking about who allocated each page
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==================================================
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Introduction
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============
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page owner is for the tracking about who allocated each page.
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It can be used to debug memory leak or to find a memory hogger.
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When allocation happens, information about allocation such as call stack
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and order of pages is stored into certain storage for each page.
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When we need to know about status of all pages, we can get and analyze
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this information.
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Although we already have tracepoint for tracing page allocation/free,
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using it for analyzing who allocate each page is rather complex. We need
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to enlarge the trace buffer for preventing overlapping until userspace
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program launched. And, launched program continually dump out the trace
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buffer for later analysis and it would change system behaviour with more
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possibility rather than just keeping it in memory, so bad for debugging.
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page owner can also be used for various purposes. For example, accurate
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fragmentation statistics can be obtained through gfp flag information of
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each page. It is already implemented and activated if page owner is
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enabled. Other usages are more than welcome.
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page owner is disabled in default. So, if you'd like to use it, you need
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to add "page_owner=on" into your boot cmdline. If the kernel is built
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with page owner and page owner is disabled in runtime due to no enabling
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boot option, runtime overhead is marginal. If disabled in runtime, it
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doesn't require memory to store owner information, so there is no runtime
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memory overhead. And, page owner inserts just two unlikely branches into
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the page allocator hotpath and if not enabled, then allocation is done
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like as the kernel without page owner. These two unlikely branches should
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not affect to allocation performance, especially if the static keys jump
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label patching functionality is available. Following is the kernel's code
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size change due to this facility.
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- Without page owner::
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text data bss dec hex filename
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48392 2333 644 51369 c8a9 mm/page_alloc.o
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- With page owner::
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text data bss dec hex filename
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48800 2445 644 51889 cab1 mm/page_alloc.o
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6662 108 29 6799 1a8f mm/page_owner.o
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1025 8 8 1041 411 mm/page_ext.o
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Although, roughly, 8 KB code is added in total, page_alloc.o increase by
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520 bytes and less than half of it is in hotpath. Building the kernel with
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page owner and turning it on if needed would be great option to debug
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kernel memory problem.
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There is one notice that is caused by implementation detail. page owner
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stores information into the memory from struct page extension. This memory
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is initialized some time later than that page allocator starts in sparse
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memory system, so, until initialization, many pages can be allocated and
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they would have no owner information. To fix it up, these early allocated
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pages are investigated and marked as allocated in initialization phase.
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Although it doesn't mean that they have the right owner information,
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at least, we can tell whether the page is allocated or not,
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more accurately. On 2GB memory x86-64 VM box, 13343 early allocated pages
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are catched and marked, although they are mostly allocated from struct
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page extension feature. Anyway, after that, no page is left in
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un-tracking state.
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Usage
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=====
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1) Build user-space helper::
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cd tools/vm
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make page_owner_sort
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2) Enable page owner: add "page_owner=on" to boot cmdline.
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3) Do the job what you want to debug
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4) Analyze information from page owner::
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cat /sys/kernel/debug/page_owner > page_owner_full.txt
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./page_owner_sort page_owner_full.txt sorted_page_owner.txt
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See the result about who allocated each page
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in the ``sorted_page_owner.txt``.
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