Eric Dumazet <eric.duma...@gmail.com> writes: > 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.
Yes it seems like a bad idea because of all the scaling problems here. The right solution would be to fix select to use multiple non virtually contiguous pages. -Andi
