linux-kernel - Re: Failing a bio right

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Date:	Tue, 28 Feb 2012 15:56:24 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tomer Margalit <tomermargalit@...il.com>
Cc:	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	scipioenterprises@...oo.com,
	Nezer Zaidenberg <nzaidenberg@....com>,
	"Zaidenberg, Nezer" <Nezer.Zaidenberg@...energy.com>
Subject: Re: Failing a bio right

  Hi,

On Tue 28-02-12 16:34:59, Tomer Margalit wrote:
> I had a bug and that's what probably caused the problems (still need to
> verify).
> 
> However, I still get the I/O error on device warning.
> Is that warning expected? And is this the right way of interrupting a
> disk write?
  The warning is expected from kernel POV because the buffer layer does not
expect writes can be interrupted. From user POV they are unexpected I
guess. But you'd have to modify end_io functions in buffer layer to handle
your new behavior to avoid those messages.

								Honza

> On Mon, Jan 23, 2012 at 10:14 PM, Jan Kara <jack@...e.cz> wrote:
> >
> > On Sun 22-01-12 11:29:44, Tomer Margalit wrote:
> > > Hi Jav,
> > >
> > > Thanks for the reply.
> > >
> > > On Fri, Jan 20, 2012 at 4:01 PM, Jan Kara <jack@...e.cz> wrote:
> > > >  Hello,
> > > >
> > > > On Thu 19-01-12 18:04:19, Tomer Margalit wrote:
> > > >> I have a make_request function that blocks writes (by using
> > > >> wait_event_interruptible on some event).
> > > >> I want the user to be able to stop the function if it takes too long
> > > >> (that's the reason for the interruptible version).
> > > >> So when the call is interrupted I call bio_endio with the EINTR error
> > > >> to signal the interruption.
> > > >> Usually this works fine, but after a lot of writes, the system says
> > > >> "lost page write due to I/O error on device".
> > > >  This is because end_buffer_write_sync() doesn't really distinguish
> > > > errors.  So when some error happens it complains about I/O error.
> > > >
> > > >> At this point the process hangs.
> > > >  That is strange - you should probably collect stack trace of the failing
> > > > process (e.g. via 'echo w >/proc/sysrq-trigger'). That should tell us more.
> > > >
> > >
> > > I cannot get a stack trace of the process since it hangs (probably in
> > > the write) - for instance doing 'gdb -p PID` or `strace -p PID` causes
> > > those to hang as well. The process doesn't segfault either.
> >  That's why I told you to use 'echo w >/proc/sysrq-trigger' and looking
> > at dmesg.
> >
> > > >> Is this the right way to do what I'm trying to do?
> > > >  I'm not sure how is it supposed to work. Writes happen usually in an
> > >
> > > The bdev I am creating is a virtual disk that replicates writes to a
> > > remote location. My intention is that it will behave like a socket -
> > > i.e. block until writes can be done. Actually the bdev is additionally
> > > meant to be semi-synchronous, so that after a buffer is filled, all
> > > writes are blocked until some buffers are sent to the remote end.
> > >
> > > This works in principle, but when I try to cancel a write which is
> > > taking too long (for instance 100MB), it doesn't do anything (since
> > > it's stuck in the kernel).
> > >
> > > > async manner (through page cache and flusher thread) or are you using
> > > > direct IO? Also if a write is interrupted at this point, you just lost the
> > >
> > > All of this behavior happens when I do the final fsync(2) after all
> > > the data has been written.
> > >
> > > > content of the buffer (as it is marked clean and !uptodate). Users usually
> > > > don't like that.
> > > >
> > >
> > > I don't mind about contents lost since the user doesn't want to wait
> > > until the end of the write (if done without flush it may take as long
> > > as it requires, but flushing means wait until writes are done).
> > >
> > > As a side note, I use the fsync since I have also implemented a
> > > marking mechanism for the bdev - and before creating a mark I need to
> > > make sure all previous writes have been flushed.
> >  OK, I see. Let's see what the stack traces of the hung process are.
> >
> >                                                                Honza
> > --
> > Jan Kara <jack@...e.cz>
> > SUSE Labs, CR
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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