Re: dma_unmap causing issues with __get_free_pages

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Aug 15, 2013 at 02:35:59AM -0500, Joel Fernandes wrote:
> Hi,
> 
> I'm having some trouble with using the dma_map/unmap API.
> 
> On unmapping a particular page using dma_unmap, it seems that the
> PG_dcache_clean flag is set in the page->flags. This is set by the
> following statement in __dma_page_dev_to_cpu function in
> arch/arm/mm/dma-mapping.c
>                 set_bit(PG_dcache_clean, &page->flags);
> 
> Due to this, on any subsequent page allocations using __get_free_pages,
> the following BUG gets triggered.

Are you calling dma_unmap() after the page has been freed?

> What is correct way to fix this? Why does the page allocator think its a
> BAD page descriptor after the unmap?

Well, on free, this is done:

        if (page->flags & PAGE_FLAGS_CHECK_AT_PREP)
                page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;

which clears PG_arch_1.  On allocation:

        if (unlikely(page_mapcount(page) |
                (page->mapping != NULL)  |
                (atomic_read(&page->_count) != 0)  |
                (page->flags & PAGE_FLAGS_CHECK_AT_PREP) |
                (mem_cgroup_bad_page_check(page)))) {
                bad_page(page);
                return 1;
        }

As PG_arch_1 is part of the PAGE_FLAGS_CHECK_AT_PREP mask, this means that
when a page is freed, it has PG_arch_1 cleared.  Therefore, if on allocation
the page now has this bit set, it means that something touched the page
after it was freed.  Quite simply, the page was freed while still being
in use.  That's very bad and needs fixing.