linux-kernel - Re: BUG in VFS or block layer

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Date:	Thu, 7 Aug 2008 12:12:58 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	linux-kernel@...r.kernel.org, ospite@...denti.unina.it,
	Matthew Wilcox <matthew@....cx>
Subject: Re: BUG in VFS or block layer

On Thursday 07 August 2008 07:28, Andrew Morton wrote:
[...]

I'll have a go at reproducing it if I can.


> Aside: why does this code in do_generic_file_read() return -EIO when it
> got a signal?
>
>
> page_not_up_to_date:
>                 /* Get exclusive access to the page ... */
>                 if (lock_page_killable(page))
>                         goto readpage_eio;

I wondered about that too, but the error never gets back to userspace, so
we just want something that is going to return quickly.


> One possible problem is here:
>
> readpage:
> 		/* Start the actual read. The read will unlock the page. */
> 		error = mapping->a_ops->readpage(filp, page);
>
> 		if (unlikely(error)) {
> 			if (error == AOP_TRUNCATED_PAGE) {
> 				page_cache_release(page);
> 				goto find_page;
> 			}
> 			goto readpage_error;
> 		}
>
> the VFS layer assumes that if ->readpage() returned a synchronous error
> then the page was already unlocked within ->readpage().  Usually this
> means that the driver layer had to run the BIO completion callback to
> do that unlocking.  It is possible that the USB code forgot to do this.
> This would explain what you're seeing.
>
> So...  would you be able to verify that the USB, layer is correctly
> calling bio->bi_end_io() for the offending requests?

That seems likely.


> Aside2: why does this code:
>
> readpage:
> 		/* Start the actual read. The read will unlock the page. */
> 		error = mapping->a_ops->readpage(filp, page);
>
> 		if (unlikely(error)) {
> 			if (error == AOP_TRUNCATED_PAGE) {
> 				page_cache_release(page);
> 				goto find_page;
> 			}
> 			goto readpage_error;
> 		}
>
> 		if (!PageUptodate(page)) {
> 			if (lock_page_killable(page))
> 				goto readpage_eio;
>
> return EIO if lock_page_killable() saw a signal?

Same as above.
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