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

[Alan Stern]

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

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