Re: [PATCH] nfs4: Ensure that ACL pages sent over NFS were not allocated from the slab

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Mar 04, 2011 at 01:45:06PM -0500, Neil Horman wrote:
> On Fri, Mar 04, 2011 at 12:13:21PM -0500, J. Bruce Fields wrote:
> > On Fri, Mar 04, 2011 at 11:44:13AM -0500, Neil Horman wrote:
> > > We can't create a buffer with kmalloc and free it later in the tcp
> > > ack path with put_page, so we need to either:
> > 
> > But out of curiosity: why is there this rule?
> > 
> Its an artifact the results from needing to free memory using a method
> in keeping with the way in which it was allocated.  To use this bug as
> an example, the acl data was allocated by the VFS using kmalloc, which
> gets data from the slab.  Even though this data is a size that is a
> multiple of a page, slab objects can be less than a page, and multiple
> objects can be stored in a single page.  As such, anything allocated
> from the slab allocator needs to be freed by the slab allocator, so
> that object reference counts internally maintained by the slab can be
> kept accurate.  

OK.  I guess my naive mental model was that the slab allocator was
layered on top of the page allocator--so it got pages with alloc_pages()
or equivalent, then handed out pieces of them to people using the slab
allocator.

So I assumed the slab allocator would hold a reference to the page like
any other user would, in which case the tcp code could take a second
reference of its own.

--b.