Re: AW: Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

[Ming Lei]

On Wed, Dec 11, 2019 at 11:07:45AM -0500, Theodore Y. Ts'o wrote:
> On Wed, Dec 11, 2019 at 12:00:58PM +0800, Ming Lei wrote:
> > I didn't reproduce the issue in my test environment, and follows
> > Andrea's test commands[1]:
> > 
> >   mount UUID=$uuid /mnt/pendrive 2>&1 |tee -a $logfile
> >   SECONDS=0
> >   cp $testfile /mnt/pendrive 2>&1 |tee -a $logfile
> >   umount /mnt/pendrive 2>&1 |tee -a $logfile
> > 
> > The 'cp' command supposes to open/close the file just once, however
> > ext4_release_file() & write pages is observed to run for 4358 times
> > when executing the above 'cp' test.
> 
> Why are we sure the ext4_release_file() / _fput() is coming from the
> cp command, as opposed to something else that might be running on the
> system under test?  _fput() is called by the kernel when the last

Please see the log:

https://lore.kernel.org/linux-scsi/3af3666920e7d46f8f0c6d88612f143ffabc743c.camel@xxxxxxxx/2-log_ming.zip

Which is collected by:

#!/bin/sh
MAJ=$1
MIN=$2
MAJ=$(( $MAJ << 20 ))
DEV=$(( $MAJ | $MIN ))

/usr/share/bcc/tools/trace -t -C \
    't:block:block_rq_issue (args->dev == '$DEV') "%s %d %d", args->rwbs, args->sector, args->nr_sector' \
    't:block:block_rq_insert (args->dev == '$DEV') "%s %d %d", args->rwbs, args->sector, args->nr_sector'

$MAJ:$MIN points to the USB storage disk.

>From the above IO trace, there are two write paths, one is from cp,
another is from writeback wq.

The stackcount trace[1] is consistent with the IO trace log since it
only shows two IO paths, that is why I concluded that the write done via
ext4_release_file() is from 'cp'.

[1] https://lore.kernel.org/linux-scsi/320b315b9c87543d4fb919ecbdf841596c8fbcea.camel@xxxxxxxx/2-log_ming_20191129_150609.zip

> reference to a struct file is released.  (Specifically, if you have a
> fd which is dup'ed, it's only when the last fd corresponding to the
> struct file is closed, and the struct file is about to be released,
> does the file system's f_ops->release function get called.)
> 
> So the first question I'd ask is whether there is anything else going
> on the system, and whether the writes are happening to the USB thumb
> drive, or to some other storage device.  And if there is something
> else which is writing to the pendrive, maybe that's why no one else
> has been able to reproduce the OP's complaint....

OK, we can ask Andrea to confirm that via the following trace, which
will add pid/comm info in the stack trace:

/usr/share/bcc/tools/stackcount  blk_mq_sched_request_inserted

Andrew, could you collect the above log again when running new/bad
kernel for confirming if the write done by ext4_release_file() is from
the 'cp' process?

Thanks,
Ming