Re: [PATCH v2] fs/select: add vmalloc fallback for select(2)

[Vlastimil Babka]

On 09/22/2016 06:49 PM, Eric Dumazet wrote:
> 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.

Isn't seq_buf_alloc() similarly exposed? And ipc_alloc()?