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SubjectRe: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer dereference
Date
Rafael Aquini <aquini@redhat.com> writes:

>>
>> If there's a race, we should fix the race. But the code path for
>> swapcache insertion is,
>>
>> add_to_swap()
>> get_swap_page() /* Return if fails to allocate */
>> add_to_swap_cache()
>> SetPageSwapCache()
>>
>> While the code path to split THP is,
>>
>> split_huge_page_to_list()
>> if PageSwapCache()
>> split_swap_cluster()
>>
>> Both code paths are protected by the page lock. So there should be some
>> other reasons to trigger the bug.
>
> As mentioned above, no they seem to not be protected (at least, not the
> same page, depending on the case). While add_to_swap() will assure a
> page_lock on the compound head, split_huge_page_to_list() does not.
>

int split_huge_page_to_list(struct page *page, struct list_head *list)
{
struct page *head = compound_head(page);
struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
struct deferred_split *ds_queue = get_deferred_split_queue(head);
struct anon_vma *anon_vma = NULL;
struct address_space *mapping = NULL;
int count, mapcount, extra_pins, ret;
unsigned long flags;
pgoff_t end;

VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
VM_BUG_ON_PAGE(!PageLocked(head), head);

I found there's page lock checking in split_huge_page_to_list().

Best Regards,
Huang, Ying

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