Eric Dumazet <eric.dumazet@xxxxxxxxx> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>