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.
