If a page fault happens, you might need to swap pages out in order to satisfy the request for an additional page. You might think you could just use GFP_ATOMIC allocation for this, but the pages have to be contiguous (which might involve memory motion and swapping on its own), and if a lot of processes all need extra stack at once you'll run short on the free (-> normally wasted) memory available for GFP_ATOMIC allocations.
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