On Thu, 2016-09-22 at 18:43 +0200, Vlastimil Babka wrote:

The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
with the number of fds passed. We had a customer report page allocation
failures of order-4 for this allocation. This is a costly order, so it might
easily fail, as the VM expects such allocation to have a lower-order fallback.

Such trivial fallback is vmalloc(), as the memory doesn't have to be
physically contiguous. Also the allocation is temporary for the duration of the
syscall, so it's unlikely to stress vmalloc too much.

vmalloc() uses a vmap_area_lock spinlock, and TLB flushes.

So I guess allowing vmalloc() being called from an innocent application
doing a select() might be dangerous, especially if this select() happens
thousands of time per second.





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