Messages in this thread | | | Subject | Re: [PATCH v1 1/1] mm: buddy page accessed before initialized | From | Pavel Tatashin <> | Date | Thu, 2 Nov 2017 09:39:58 -0400 |
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On 11/02/2017 09:32 AM, Michal Hocko wrote: > On Tue 31-10-17 11:50:02, Pavel Tatashin wrote: > [...] >> The problem happens in this path: >> >> page_alloc_init_late >> deferred_init_memmap >> deferred_init_range >> __def_free >> deferred_free_range >> __free_pages_boot_core(page, order) >> __free_pages() >> __free_pages_ok() >> free_one_page() >> __free_one_page(page, pfn, zone, order, migratetype); >> >> deferred_init_range() initializes one page at a time by calling >> __init_single_page(), once it initializes pageblock_nr_pages pages, it >> calls deferred_free_range() to free the initialized pages to the buddy >> allocator. Eventually, we reach __free_one_page(), where we compute buddy >> page: >> buddy_pfn = __find_buddy_pfn(pfn, order); >> buddy = page + (buddy_pfn - pfn); >> >> buddy_pfn is computed as pfn ^ (1 << order), or pfn + pageblock_nr_pages. >> Thefore, buddy page becomes a page one after the range that currently was >> initialized, and we access this page in this function. Also, later when we >> return back to deferred_init_range(), the buddy page is initialized again. >> >> So, in order to avoid this issue, we must initialize the buddy page prior >> to calling deferred_free_range(). > > How come we didn't have this problem previously? I am really confused. >
Hi Michal,
Previously as before my project? That is because memory for all struct pages was always zeroed in memblock, and in __free_one_page() page_is_buddy() was always returning false, thus we never tried to incorrectly remove it from the list:
837 list_del(&buddy->lru);
Now, that memory is not zeroed, page_is_buddy() can return true after kexec when memory is dirty (unfortunately memset(1) with CONFIG_VM_DEBUG does not catch this case). And proceed further to incorrectly remove buddy from the list.
This is why we must initialize the computed buddy page beforehand.
Pasha
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