Re: How best to bypass the page cache from within a kernel module?

[Alan Stern <stern@rowland.harvard.edu>]

On Wed, 17 Sep 2003, William Lee Irwin III wrote:

> On Wed, Sep 17, 2003 at 04:33:08PM -0400, Alan Stern wrote:
> > 1. What's the proper way for a kernel thread running in a module to get
> > hold of an mm_struct or to keep the one it had before calling daemonize()?
> 
> Well, you can get one from the slab allocator, though I expect there will
> be a followup question here...

Yes :-)  The slab allocator will give me a nice piece of memory, but I 
will still need to turn that into a valid mm_struct.  I can't call 
alloc_mm() and friends because they're not EXPORTed.

Would this work: atomically increment current->mm->users and save the 
value of current->mm before calling daemonize(), then re-assign the old 
value back to current->mm afterwards?

> On Wed, Sep 17, 2003 at 04:33:08PM -0400, Alan Stern wrote:
> > 2. What's the proper way for a kernel thread to allocate a region of 
> > userspace memory?
> 
> Hmm. Sounds like you want to grab a user address space and do userspace
> stuff inside there. Maybe avoid do_execve() etc. and call sys_*() for
> everything else outright? The question itself probably wants sys_mmap()
> or some such, or handle_mm_fault() depending on what you have in mind
> for allocation.

sys_mmap() or something along those lines would be good.  But I can't call
it directly because 2.6 doesn't EXPORT the sys_xxx functions.  Also, I'm
not clear on whether mmap() lets you create an anonymous mapping -- one
backed by swap space rather than a file -- that's what I would want to do.

> On Wed, Sep 17, 2003 at 04:33:08PM -0400, Alan Stern wrote:
> > 3. What's the proper way to invalidate all entries in the page cache
> > that refer to a particular file?
> 
> invalidate_inode_pages().

Great!  I'll search through the kernel code for it.

Alan Stern


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