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

[David Laight <David.Laight@ACULAB.COM>]

From: Vlastimil Babka
> Sent: 22 September 2016 18:55
...
> So in the case of select() it seems like the memory we need 6 bits per file
> descriptor, multiplied by the highest possible file descriptor (nfds) as passed
> to the syscall. According to the man page of select:
> 
>         EINVAL nfds is negative or exceeds the RLIMIT_NOFILE resource limit (see
> getrlimit(2)).

That second clause is relatively recent.

> The code actually seems to silently cap the value instead of returning EINVAL
> though? (IIUC):
> 
>         /* max_fds can increase, so grab it once to avoid race */
>          rcu_read_lock();
>          fdt = files_fdtable(current->files);
>          max_fds = fdt->max_fds;
>          rcu_read_unlock();
>          if (n > max_fds)
>                  n = max_fds;
> 
> The default for this cap seems to be 1024 where I checked (again, IIUC, it's
> what ulimit -n returns?). I wasn't able to change it to more than 2048, which
> makes the bitmaps still below PAGE_SIZE.
> 
> So if I get that right, the system admin would have to allow really large
> RLIMIT_NOFILE to even make vmalloc() possible here. So I don't see it as a large
> concern?

4k open files isn't that many.
Especially for programs that are using pipes to emulate windows events.

I suspect that fdt->max_fds is an upper bound for the highest fd the
process has open - not the RLIMIT_NOFILE value.
select() shouldn't be silently ignoring large values of 'n' unless
the fd_set bits are zero.

Of course, select does scale well for high numbered fds
and neither poll nor select scale well for large numbers of fds.

	David