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Subject: Re: O_DIRECT fails in some kernel and FS
From: Steve Lord <lord@sgi.com>
To: Chris Wedgwood <cw@f00f.org>
Cc: Jeff Garzik <garzik@havoc.gtf.org>, Chris Mason <mason@suse.com>,
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Date: 04 Feb 2002 09:15:30 -0600
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On Sun, 2002-02-03 at 16:44, Chris Wedgwood wrote:
> On Sun, Feb 03, 2002 at 09:05:04AM -0600, Stephen Lord wrote:
> 
>     but page faults are not blocked out for the duration of the I/O so
>     the coherency is weak.
> 
> I was thinking this would also be goof, basically invalidate those
> pages and remove them from the VMAs, marking them as unusable pending
> IO completion --- the logic her being if you were to fault on an
> invalidated page during IO you deserve to block indefinitely until the
> IO completes.
> 
>     However, if an application is doing a combination of mmapped and
>     direct I/O to a file at the same time, then it should generally
>     have some form of user space synchronization anyway.
> 
> I hadn't considered that.  I imagined an application doing either but
> not both, and the kernel enforcing this.  However, in the case when
> you want to mmap a large file, you may want to manipulate some pages
> using mmap whilst writing others with O_DIRECT.  Although, in such
> cases arguably you could using multiple mapping's.
> 
> 

If an application is single threaded then it cannot be doing both at
the same time - so all we need to do is flush and invalidate mappings
at the start of I/O. This is really only needed for the range covered by
the direct read/write.

If an application is multithreaded and is doing mmap and direct I/O
from different threads without doing its own synchronization, then it
is broken, there is no ordering guarantee provided by the kernel as
to what happens first.

> 
>    --cw

Steve

-- 

Steve Lord                                      voice: +1-651-683-3511
Principal Engineer, Filesystem Software         email: lord@sgi.com
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