[User-mode-linux-devel] Re: host os 2.3.51 and vm_file

[Jeff D. <jd...@ka...>]

> [wstearns@sparrow uml]$ touch vm_file 
> [wstearns@sparrow uml]$ ./linux-2.3.49 devfs=nomount 
> open: File exists 

The one thing that's important about vm_file is that it not exist beforehand.  
That's the file that holds the kernel's physical memory.  It is opened (and 
created) and then unlinked.

What seems to be the problem is that your /dev/ttyp* files aren't usable by 
anyone besides root.  Loosening up protections would help a lot.  Mine look 
like this:
	crw-rw-rw-   1 root     tty        3,   0 May  5  1998 /dev/ttyp0

Which reminds me that I'd better switch everything over to use /dev/pts 
sometime...

Those diffs are innocuous.  Here's what they mean:

> -fstat(4, {st_mode=S_IFREG|0775, st_size=16777217, ...}) = 0
> -mprotect(0x100c8000, 8192, PROT_READ|PROT_WRITE|PROT_EXEC) = 0
> +fstat(4, {st_mode=S_IFREG|0755, st_size=16777217, ...}) = 0
> +mprotect(0x100d6000, 8192, PROT_READ|PROT_WRITE|PROT_EXEC) = 0 

I don't know what the fstat is doing.  The mprotect is changing the 
permissions on the two pages containing init_task so that they are suitable 
for use as a stack.  The address is different because init_task is in the 
executable and it changed size from .49 to .51.

> -rt_sigaction(SIGUSR1, {0x100779dc, [], SA_RESTART}, {SIG_DFL}, 8) = 0
> -rt_sigaction(SIGIO, {0x100778b8, [], SA_RESTART}, {SIG_DFL}, 8) = 0
> -select(1, [], NULL, NULL, NULL)         = ? ERESTARTNOHAND (To be
> restarted) 
> ---- SIGUSR1 (User defined signal 1) --- 
> -kill(5671, SIGUSR1)                     = 0 
> -sigreturn()    = ? (mask now []) 
> -select(2, [0], NULL, NULL, NULL)        = ? ERESTARTNOHAND (To be restarted) 
> -+++ killed by SIGKILL +++ 
> +pipe([7, 8])                            = 0 
> +fn=0x411, child_stack=0x50001ff4, flags=0x7, args=0x1000000clone()
>                                = 6429 
> +rt_sigaction(SIGIO, {0x10084688, [], SA_RESTART}, {SIG_DFL}, 8) = 0
> +select(9, [7], NULL, NULL, NULL)        = 1 (in [7]) 
> +read(7, "\2\0\0\0\0\0\0\0\210\264\10\20\4\0\0\0\277\0\0\0000\n\0"..., 
> 		140) = 140 
> +select(9, [0 7], NULL, NULL, NULL 

I don't know what the SIGIO stuff is about here, but the rest of it is my 
redoing the mechanism by which other threads make requests of the input thread.

Its main job is to select on file descriptors that are important to the rest 
of the kernel and let it know when input has arrived.  Since it has the only 
user stack in the whole kernel and is the only thread that can call fork, I 
also gave it the job of firing off xterms for the virtual consoles.

I used to give it jobs by sending it a SIGUSR1 (hence the SIGUSR1 
disappearing) and have it figure out what's going on by having it read some 
global data.  Now it makes a pipe, listens on the read end, and any thread 
that needs something asks nicely on the write end.  This automatically 
serializes the requests and eliminates the need for the locking I was doing 
before.

The trace above is catching the input thread in the act of getting a request 
from the console to select on stdin (the read end of the pipe is fd 7, which 
gets input, and as a result of that input, the next select has fd 0 added to 
the read mask).
				Jeff