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Re: [REVIEW] Prevent direct I/O from mapping extents beyond eof

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [REVIEW] Prevent direct I/O from mapping extents beyond eof
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Fri, 15 Aug 2008 15:27:56 -0700
Cc: lachlan@xxxxxxx, xfs-dev@xxxxxxx, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx
In-reply-to: <20080815220958.GB13770@xxxxxxxxxxxxx>
References: <48A50152.8020104@xxxxxxx> <20080815220958.GB13770@xxxxxxxxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
On Fri, 15 Aug 2008 18:09:58 -0400
Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:

> On Fri, Aug 15, 2008 at 02:08:50PM +1000, Lachlan McIlroy wrote:
> > With the help from some tracing I found that we try to map extents beyond
> > eof when doing a direct I/O read.  It appears that the way to inform the
> > generic direct I/O path (ie do_direct_IO()) that we have breached eof is
> > to return an unmapped buffer from xfs_get_blocks_direct().  This will cause
> > do_direct_IO() to jump to the hole handling code where is will check for
> > eof and then abort.
> >
> > This problem was found because a direct I/O read was trying to map beyond
> > eof and was encountering delayed allocations.  The delayed allocations 
> > beyond
> > eof are speculative allocations and they didn't get converted when the 
> > direct
> > I/O flushed the file because there was only enough space in the current AG
> > to convert and write out the dirty pages within eof.  Note that
> > xfs_iomap_write_allocate() wont necessarily convert all the delayed 
> > allocation
> > passed to it - it will return after allocating the first extent - so if the
> > delayed allocation extends beyond eof then it will stay that way.
> >
> > This change will detect a direct I/O read beyond eof:
> The change looks good to me, but I really think the direct I/O could
> should never send down requests like this down to the filesystems.  akpm
> and -fsdevel Cc'ed.

Oh gee, I forget, and so many people have done drivebys on that code...

We _could_ add additional i_size checking into direct-io.c but bear in
mind that it would be best-effort unreliable stuff.  The code will
still be tripped up by concurrent extends and concurrent truncates.

So we'll still end up calling the fs for blocks outside i_size, only
less commonly.  I think.

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