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SubjectRe: [PATCH v1 1/1] mm: buddy page accessed before initialized
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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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