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

[Pavel Tatashin]

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