* Mike Rapoport <rppt@xxxxxxxxxx> [250707 07:32]:
> On Mon, Jul 07, 2025 at 01:11:02PM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 04, 2025 at 04:49:38PM +0300, Mike Rapoport wrote:
> > > static bool execmem_cache_free(void *ptr)
> > > {
> > > struct maple_tree *busy_areas = &execmem_cache.busy_areas;
> > > unsigned long addr = (unsigned long)ptr;
> > > MA_STATE(mas, busy_areas, addr, addr);
> > > void *area;
> > > + int err;
> > > +
> > > + guard(mutex)(&execmem_cache.mutex);
> > >
> > > area = mas_walk(&mas);
> > > + if (!area)
> > > return false;
> > >
> > > + err = __execmem_cache_free(&mas, ptr, GFP_KERNEL | __GFP_NORETRY);
> > > + if (err)
> > > + goto err_slowpath;
> > >
> > > schedule_work(&execmem_cache_clean_work);
> > >
> > > return true;
> > > +
> > > +err_slowpath:
> > > + mas_store_gfp(&mas, pending_free_set(ptr), GFP_KERNEL);
> > > + execmem_cache.pending_free_cnt++;
> > > + schedule_delayed_work(&execmem_cache_free_work, FREE_DELAY);
> > > + return true;
> > > }
> >
> > This is a bit if an anti-pattern, using guard() and error goto. Since
>
> Good to know :)
>
> > there is only the one site, its best to write it like so:
> >
> > static bool execmem_cache_free(void *ptr)
> > {
> > struct maple_tree *busy_areas = &execmem_cache.busy_areas;
> > unsigned long addr = (unsigned long)ptr;
> > MA_STATE(mas, busy_areas, addr, addr);
> > void *area;
> > int err;
> >
> > guard(mutex)(&execmem_cache.mutex);
> >
> > area = mas_walk(&mas);
> > if (!area)
> > return false;
> >
> > err = __execmem_cache_free(&mas, ptr, GFP_KERNEL | __GFP_NORETRY);
> > if (err) {
> > mas_store_gfp(&mas, pending_free_set(ptr), GFP_KERNEL);
> > execmem_cache.pending_free_cnt++;
> > schedule_delayed_work(&execmem_cache_free_work, FREE_DELAY);
> > return true;
> > }
> >
> > schedule_work(&execmem_cache_clean_work);
> > return true;
> > }
> >
> > And now I have to ask what happens if mas_store_gfp() returns an error?
>
> AFAIU it won't. mas points to exact slot we've got the area from, nothing else
> can modify the tree because of the mutex, so that mas_store_gfp()
> essentially updates the value at an existing entry.
>
> I'll add a comment about it.
>
> Added @Liam to make sure I'm not saying nonsense :)
>
Yes, if there is already a node with a value with the same range, there
will be no allocations that will happen, so it'll just change the
pointer for you. This is a slot store operation.
But, if it's possible to have no entries (an empty tree, or a single
value at 0), you will most likely allocate a node to store it, which is
256B.
I don't think this is a concern in this particular case though as you
are searching for an entry and storing, so it needs to exist. So
really, the only scenario here is if you store 1 - ULONG_MAX (without
having expanded a root node) or 0 - ULONG_MAX, and that seems invalid.
Thanks,
Liam
