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 ��.n������g��� �a��� �&ޖ)���)��h���&������梷���� �Ǟ�m������)������^�����������v���O��zf������