Re: [patch] Re: Interaction between Xen and XFS: stray RW mappings

[Andi Kleen <andi@firstfloor.org>]

On Tue, Oct 23, 2007 at 05:04:14PM +1000, David Chinner wrote:
> On Tue, Oct 23, 2007 at 10:36:41AM +1000, David Chinner wrote:
> > On Tue, Oct 23, 2007 at 01:35:14AM +0200, Andi Kleen wrote:
> > > On Tue, Oct 23, 2007 at 08:32:25AM +1000, David Chinner wrote:
> > > > Could vmap()/vunmap() take references to the pages that are mapped? That
> > > > way delaying the unmap would also delay the freeing of the pages and hence
> > > > we'd have no problems with the pages being reused before the mapping is
> > > > torn down. That'd work for Xen even with XFS's lazy unmapping scheme, and
> > > > would allow Nick's more advanced methods to work as well....
> > > 
> > > You could always just keep around an array of the pages and then drop the
> > > reference count after unmap. Or walk the vmalloc mapping and generate such
> > > an array before freeing, then unmap and then drop the reference counts.
> > 
> > You mean like vmap() could record the pages passed to it in the area->pages
> > array, and we walk and release than in __vunmap() like it already does
> > for vfree()?
> > 
> > If we did this, it would probably be best to pass a page release function
> > into the vmap or vunmap call - we'd need page_cache_release() called on
> > the page rather than __free_page()....
> > 
> > The solution belongs behind the vmap/vunmap interface, not in XFS....
> 
> Lightly tested(*) patch that does this with lazy unmapping
> below for comment.

Thanks

> 
> (*) a) kernel boots, b) made an XFS filesystem with 64k directory
> blocks, created ~100,000 files in a directory to get a wide btree
> (~1700 blocks, still only a single level) and run repeated finds
> across it dropping caches in between.  Each traversal maps and
> unmaps every btree block.

Hmm, the __free_page -> page_cache_release() change in vfree() would
have been simpler wouldn't it? 

But if it works it is fine.

-Andi