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

[Vlastimil Babka]

On 09/27/2016 02:01 AM, Andrew Morton wrote:
> On Thu, 22 Sep 2016 18:43:59 +0200 Vlastimil Babka <vbabka@xxxxxxx> 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.
> >
> > Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
> > it doesn't need this kind of fallback.
> >
> > ...
> >
> > --- a/fs/select.c
> > +++ b/fs/select.c
> > @@ -29,6 +29,7 @@
> >  #include <linux/sched/rt.h>
> >  #include <linux/freezer.h>
> >  #include <net/busy_poll.h>
> > +#include <linux/vmalloc.h>
> >
> >  #include <asm/uaccess.h>
> >
> > @@ -558,6 +559,7 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
> >  	struct fdtable *fdt;
> >  	/* Allocate small arguments on the stack to save memory and be faster */
> >  	long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
> > +	unsigned long alloc_size;
> >
> >  	ret = -EINVAL;
> >  	if (n < 0)
> > @@ -580,8 +582,12 @@ int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
> >  	bits = stack_fds;
> >  	if (size > sizeof(stack_fds) / 6) {
> >  		/* Not enough space in on-stack array; must use kmalloc */
> > +		alloc_size = 6 * size;
>
> Well.  `size' is `unsigned'.  The multiplication will be done as 32-bit
> so there was no point in making `alloc_size' unsigned long.

Uh, right. Thanks.

> So can we tighten up the types in this function?  size_t might make
> sense, but vmalloc() takes a ulong.

Let's do size_t then, as the conversion to ulong is safe.