Re: How can I force a read to hit the disk?

["Richard B. Johnson" <root@chaos.analogic.com>]

On Mon, 8 Sep 2003, Alan Stern wrote:

> On Fri, 5 Sep 2003, Andreas Dilger wrote:
>
> > On Sep 05, 2003  13:46 -0400, Alan Stern wrote:
> > > My kernel module for Linux-2.6 needs to be able to verify that the media
> > > on which a file resides actually is readable.  How can I do that?
> > >
> > > It would certainly suffice to use the normal VFS read routines, if there
> > > was some way to force the system to actually read from the device rather
> > > than just returning data already in the cache.  So I guess it would be
> > > enough to perform an fdatasync for the file and then invalidate the file's
> > > cache entries.  How does one invalidate a file's cache entries?  Does
> > > filemap_flush() perform both these operations for you?
> >
> > If you open the file with O_DIRECT, it should read/write directly on the
> > disk, and it will also invalidate any existing cache for the read/written
> > area.
>
> 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.

Cheers,
Dick Johnson
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