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

[Vlastimil Babka <vbabka@suse.cz>]

On 09/23/2016 11:42 AM, David Laight wrote:
> 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.

Interesting... so it was added without actually being true in the kernel
code?

>> 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.

Sure but IIUC we need 6 bits per file. That means up to almost 42k
files, we should fit into order-3 allocation, which effectively cannot
fail right now.

> I suspect that fdt->max_fds is an upper bound for the highest fd the
> process has open - not the RLIMIT_NOFILE value.

I gathered that the highest fd effectively limits the number of files,
so it's the same. I might be wrong.

> select() shouldn't be silently ignoring large values of 'n' unless
> the fd_set bits are zero.

Yeah that doesn't seem to conform to the manpage.

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

True.

> 	David
>