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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Jul 02, 2012 at 04:43:10AM +0100, Al Viro wrote:
> On Sun, Jul 01, 2012 at 09:46:31PM -0400, Mimi Zohar wrote:
> > On Sun, 2012-07-01 at 21:57 +0100, Al Viro wrote:
> > > On Sun, Jul 01, 2012 at 03:50:02PM -0400, Mimi Zohar wrote:
> > > > Replacing it with a call to __fput(), the system boots.
> > > 
> > > "it" being just the part under that if (unlikely(...)))?  Very interesting...  If so, we
> > > have some kernel thread ending up with delayed __fput() which somehow makes dracut (assuimg
> > > you are using fedora initramfs to go with fedora config) unhappy.  With your own patch,
> > > doing async __fput() in a lot of cases when this one doesn't delay past the return to
> > > userland managing to survive the boot...  I wonder which files end up triggering that fun
> > > and which kernel thread is responsible...  Could you slap a printk() in there, showing
> > > file->f_dentry->d_inode->i_mode (octal) and at least file->f_dentry->d_name.name?
> > > Along with the current->comm[], all under that inner if ().  And see which ones end up
> > > going that way by the time execve() of /sbin/init fails.
> > 
> > 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):

diff --git a/fs/file_table.c b/fs/file_table.c
index 470da0b..00fd849 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -284,6 +284,11 @@ static void ____fput(struct callback_head *work)
 	__fput(container_of(work, struct file, f_u.fu_rcuhead));
 }
 
+void flush_delayed_fput(void)
+{
+	delayed_fput(NULL);
+}
+
 static DECLARE_WORK(delayed_fput_work, delayed_fput);
 
 void fput(struct file *file)
diff --git a/include/linux/file.h b/include/linux/file.h
index 58bf158..d9a4f5a 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -39,4 +39,6 @@ extern void put_unused_fd(unsigned int fd);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
+extern void flush_delayed_fput(void);
+
 #endif /* __LINUX_FILE_H */
diff --git a/init/main.c b/init/main.c
index b5cc0a7..3f151f6 100644
--- a/init/main.c
+++ b/init/main.c
@@ -68,6 +68,7 @@
 #include <linux/shmem_fs.h>
 #include <linux/slab.h>
 #include <linux/perf_event.h>
+#include <linux/file.h>
 
 #include <asm/io.h>
 #include <asm/bugs.h>
@@ -804,8 +805,8 @@ static noinline int init_post(void)
 	system_state = SYSTEM_RUNNING;
 	numa_default_policy();
 
-
 	current->signal->flags |= SIGNAL_UNKILLABLE;
+	flush_delayed_fput();
 
 	if (ramdisk_execute_command) {
 		run_init_process(ramdisk_execute_command);