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

From: Eric Dumazet
Date: Thu Sep 22 2016 - 13:07:58 EST

On Thu, 2016-09-22 at 18:56 +0200, Vlastimil Babka wrote:
> 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()?


We don't have a library function (attempting kmalloc(), fallback to
vmalloc() presumably to avoid abuses, but I guess some patches were
accepted without thinking about this.