From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Subject: Re: [Bug 12829] kernel complains on ENOSPC
Date: Mon, 9 Mar 2009 10:46:45 +0530
Message-ID: <20090309051645.GB26853@skywalker>
References: <bug-12829-13602@http.bugzilla.kernel.org/> <20090306230222.2732110800D@picon.linux-foundation.org>
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On Fri, Mar 06, 2009 at 03:02:21PM -0800, bugme-daemon@bugzilla.kernel.org wrote:
> 
> 
> ------- Comment #1 from tytso@mit.edu  2009-03-06 15:02 -------
> What are the precise reproduction details?   We're supposed to keep track of
> how many delayed allocation blocks are outstanding, so that we return ENOSPC
> *before* we get to this stage.

Yes. We should not get the ENOSPC during writeback. That would imply the
block reservation is going wrong.

> 
> I'm not sure we do the right thing if we mmap into an unallocated region of
> file; when do we actually track delayed allocation blocks?   The right answer
> would be at mmap() time, but OTOH that means if we mmap a 2GB region, do we
> immediately take a 2GB charge because the process might write into this region?
>  And does free space returned by 'df' immediately drop by 2GB?


For mmap block reservation is doing during page_mkwrite.

> 
> Do you know if there was any allocation by mmap going on in your reproduction
> case?   That seems the most likely cause to me, if I had to guess....
> 

-aneesh