linux-kernel - Re: Failing a bio right

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Date:	Sun, 22 Jan 2012 11:29:44 +0200
From:	Tomer Margalit <tomermargalit@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-kernel@...r.kernel.org,
	Nezer Zaidenberg <nzaidenberg@....com>
Subject: Re: Failing a bio right

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.

>> 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.

Thanks,
  Tomer
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