Re: [7.0-rc1] codetag: kernel warning "alloc_tag was not set" during boot

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Feb 24, 2026 at 8:15 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Tue, Feb 24, 2026 at 6:26 AM Harry Yoo <harry.yoo@oracle.com> wrote:
> >
> > On Tue, Feb 24, 2026 at 11:16:12PM +0900, Harry Yoo wrote:
> > > Just sharing updates on this before going to bed...
> > >
> > > I was able to reproduce it on my machine I have a working fix
> > > (nowhere close to upstream quality, though)
> > >
> > > The reason why we're seeing "alloc_tag not was set" is
> > > because SLUB allocates empty sheaves for kmalloc
> > > with __GFP_NO_OBJ_EXT and it doesn't teach memory profiling
> > > how to handle this when such sheaves are freed.
> > >
> > > When __GFP_NO_OBJ_EXT is used in alloc_slab_obj_exts(), it later
> > > avoids this "alloc_tag was not set" warning by marking alloc_tags
> > > empty in free_slab_obj_exts(), just before freeing obj_exts.
> > >
> > > And we don't handle this when allocating freeing sheaves
> > > that were allocated with __GFP_NO_OBJ_EXT.
>
> Ah, thanks for the analyses! So, the fact that __alloc_empty_sheaf()
> sets __GFP_NO_OBJ_EXT when allocating the sheaf leads to this issue
> later on. Makes sense. I only wonder why I could not reproduce this...
> Will try to find out later.

The reason I could not reproduce this is because with my config
free_empty_sheaf() just happen to never been called with SLAB_KMALLOC
caches. Once I remove SLAB_KMALLOC condition in both
__alloc_empty_sheaf() and free_empty_sheaf() I can reproduce the
problem.

>
> > >
> > > Passing __GFP_NO_OBJ_EXT skips 1) allocation of obj_exts,
> > > and 2) alloc_tag_add() even when obj_exts is already allocated,
> > > and this confuses memory profiling later.
> > >
> > > I'm adding the fix I have now. (I guess Suren might have some preference
> > > on how to solve it though)
> > >
> > >   1. mark obj_exts allocation failure when slab has no obj_exts and
> > >      gfp flag has __GFP_NO_OBJ_EXT, so that a later obj_exts allocation
> > >      will mark alloc_tags empty.
> > >
> > >   2. Set alloc_tag when obj_exts is allocated available,
> > >      even when __GFP_NO_OBJ_EXT is set.
> > >
> > >      Because it's already allocated, we don't have to worry about
> > >      recursive allocation.
> >
> > Wait, isn't it much simpler to just do mark_objexts_empty(sheaf);
> > just before freeing sheaves, if s->flags has SLAB_KMALLOC?
>
> I think that's the best fix and it makes logical sense.
> alloc_slab_obj_exts() allocates obj_exts with __GFP_NO_OBJ_EXT and
> then free_slab_obj_exts() does mark_objexts_empty() before freeing
> them. Similarly,  __alloc_empty_sheaf() allocates sheaves with
> __GFP_NO_OBJ_EXT (for SLAB_KMALLOC objects), therefore
> free_empty_sheaf() should also call mark_objexts_empty(sheaf) before
> freeing them. mark_objexts_empty() currently accepts only slabobj_ext
> but I think we can change that. I'll take a stab at it.

I posted a fix at
https://lore.kernel.org/all/20260224165250.1322946-1-surenb@google.com/.
David, please give it a try.
Thanks,
Suren.



>
> >
> > /me goes to bed anyway
> >
> > --
> > Cheers,
> > Harry / Hyeonggon