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Date: Tue, 9 Sep 2003 08:13:10 -0400 (EDT)
From: "Richard B. Johnson" <root@chaos.analogic.com>
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To: Alan Stern <stern@rowland.harvard.edu>
cc: Andreas Dilger <adilger@clusterfs.com>, linux-kernel@vger.kernel.org
Subject: Re: How can I force a read to hit the disk?
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On Mon, 8 Sep 2003, Alan Stern wrote:

> On Mon, 8 Sep 2003, Richard B. Johnson wrote:
>
> > On Mon, 8 Sep 2003, Alan Stern wrote:
> >
> > > I tried doing that, but it caused a segmentation violation.  The trouble
> > > was this line near the start of fs/direct_io.c:dio_refill_pages()
> > >
> > > 	down_read(&current->mm->mmap_sem);
> > >
> > > Unfortunately, in a kernel thread (which is where my code runs)
> > > current->mm is NULL.
> > >
> > > Can anybody offer additional advice?  How about a way to invalidate all
> > > the page cache entries that contain a page from the file?
> > >
> > > Alan Stern
> >
> > When your thread code starts up, execute
> >                 init_rwsem(&current->mm->mmap_sem);
> >
> > ... This is in the thread's code, not the module init code.
>
> That doesn't work either; it also causes a segmentation violation.  As I
> said before, current->mm is NULL.  It gets set that way by exit_mm() which
> is called from daemonize().
>
> Alan Stern
>

Gawd. I assumed you knew how to initialize a pointer. I just
located the procedure used to initialize the semaphore.


Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (794.73 BogoMips).
            Note 96.31% of all statistics are fiction.