Re: [PATCH RFC] mm, slab: warn when increasing refcount on large kmalloc page

[David Rientjes <rientjes@google.com>]

On Fri, 18 Apr 2025, Harry Yoo wrote:

> > Since slab pages are now frozen, increasing refcount on a page
> > containing a kmalloc() allocation is not possible anymore. Large kmalloc
> > pages should ideally behave the same, because the decision for which
> > allocation size to use them is the slab allocator's implementation
> > detail, and sizes passed to kmalloc() might depend on e.g. user input.
> 
> Agreed.
> 
> > Because of some unexpected fallout in the slab pages case (see commit
> > b9c0e49abfca ("mm: decline to manipulate the refcount on a slab page"),
> > let's take a more cautious approach and before making large kmalloc
> > pages actually frozen, start warning about code that would try to
> > increase refcount on them.
> >
> > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> > ---
> > I'd like to expose this via slab-next and see if there are any reports.
> > If not for few weeks, maybe proceed immediately to freezing refcount and
> > handling it in get_page/put_page exactly like folio_test_slab. Thoughts?
> 
> +1
> 
> Let's give testing a try.
> 
> ...at least it does not hit anything on my box.
> 

Same, I ran this through a number of benchmarks on x86 and couldn't find 
any occurrences of the warning.