Re: [PATCH 0/4] Was: deferring __fput()

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Mon, 2012-07-02 at 13:02 +0100, Al Viro wrote:
> On Mon, Jul 02, 2012 at 07:49:50AM -0400, Mimi Zohar wrote:
> 
> > > > > pid=1 uid=0 d_name=init comm=swapper/0 dev="rootfs" mode=100775
> > > > > pid=1 uid=0 d_name=bash comm=swapper/0 dev="rootfs" mode=100755
> > > > 
> > > > OK...  Here's what I suspect is going on:
> > > > 	* populating initramfs writes binaries there.  We open files (for write) from
> > > > the kernel thread (there's nothing other than kernel threads at that point), write to
> > > > them, then close().  Final fput() gets delayed.
> > > > 	* Then we proceed to execve().  Which means mapping the binary with MAP_DENYWRITE.
> > > > Which fails, since there's a struct file still opened for write on that sucker.
> > > > 
> > > > Your patch did not delay those fput() - they were done without ->mmap_sem held.  So
> > > > it survived.  Booting without initramfs always survives; booting with initramfs may
> > > > or may not survive, depending on the timings - if that scheduled work manages to
> > > > run by the time we do those execve(), we win.  Note that async_synchronize_full()
> > > > done in init_post() might easily affect that, depending on config.
> > > > 
> > > > As a quick test, could you try slapping a delay somewhere around the beginning
> > > > of init_post() and see if it rescues the system?
> > > 
> > > Ho-hum...  How about this (modulo missing documentation of the whole sad mess):
> > 
> > Sorry, neither adding the delay or this patch helped.
> 
> Really odd.  Could you print the error returned by kernel_execve() in run_init_process()?
> At least that way we'll get some indication of what's going on there.  Another thing:
> could you slap matching printks into the nested if() in fput() and the loop in
> delayed_fput(), just to see if we do get __fput() done on all the right struct file?
> Just "fput: %p", file and "delayed_fput: %p", file would probably be enough.

ok, it calls delayed_fput(), but never makes it into the delayed_fput()
while statement.  I added an additional printk to make sure
delayed_fput() was being called.

delayed_fput:
fput: xxxx
run_init_process: -26
failed to execute /init

run_init_process: -2
run_init_process: -2
run_init_process: -2

delayed_fput:
fput: xxxx
run_init_process: -26

> I'm assuming that I hadn't misparsed what you wrote and that __fput() in nested
> if() in fput() was enough to get the thing working.  Could you confirm that?

Yes, everything is exactly as you described.

Mimi