Re: [PATCH 1/5] alloc_tag: load module tags into separate continuous memory

[Kent Overstreet <kent.overstreet@linux.dev>]

On Tue, Aug 20, 2024 at 10:13:07AM GMT, Mike Rapoport wrote:
> On Mon, Aug 19, 2024 at 08:15:07AM -0700, Suren Baghdasaryan wrote:
> > When a module gets unloaded there is a possibility that some of the
> > allocations it made are still used and therefore the allocation tags
> > corresponding to these allocations are still referenced. As such, the
> > memory for these tags can't be freed. This is currently handled as an
> > abnormal situation and module's data section is not being unloaded.
> > To handle this situation without keeping module's data in memory,
> > allow codetags with longer lifespan than the module to be loaded into
> > their own separate memory. The in-use memory areas and gaps after
> > module unloading in this separate memory are tracked using maple trees.
> > Allocation tags arrange their separate memory so that it is virtually
> > contiguous and that will allow simple allocation tag indexing later on
> > in this patchset. The size of this virtually contiguous memory is set
> > to store up to 100000 allocation tags and max_module_alloc_tags kernel
> > parameter is introduced to change this size.
> > 
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  .../admin-guide/kernel-parameters.txt         |   4 +
> >  include/asm-generic/codetag.lds.h             |  19 ++
> >  include/linux/alloc_tag.h                     |  13 +-
> >  include/linux/codetag.h                       |  35 ++-
> >  kernel/module/main.c                          |  67 +++--
> >  lib/alloc_tag.c                               | 245 ++++++++++++++++--
> >  lib/codetag.c                                 | 101 +++++++-
> >  scripts/module.lds.S                          |   5 +-
> >  8 files changed, 429 insertions(+), 60 deletions(-)
>  
> ...
> 
> > diff --git a/include/linux/codetag.h b/include/linux/codetag.h
> > index c2a579ccd455..c4a3dd60205e 100644
> > --- a/include/linux/codetag.h
> > +++ b/include/linux/codetag.h
> > @@ -35,8 +35,13 @@ struct codetag_type_desc {
> >  	size_t tag_size;
> >  	void (*module_load)(struct codetag_type *cttype,
> >  			    struct codetag_module *cmod);
> > -	bool (*module_unload)(struct codetag_type *cttype,
> > +	void (*module_unload)(struct codetag_type *cttype,
> >  			      struct codetag_module *cmod);
> > +	void (*module_replaced)(struct module *mod, struct module *new_mod);
> > +	bool (*needs_section_mem)(struct module *mod, unsigned long size);
> > +	void *(*alloc_section_mem)(struct module *mod, unsigned long size,
> > +				   unsigned int prepend, unsigned long align);
> > +	void (*free_section_mem)(struct module *mod, bool unused);
> >  };
> >  
> >  struct codetag_iterator {
> > @@ -71,11 +76,31 @@ struct codetag_type *
> >  codetag_register_type(const struct codetag_type_desc *desc);
> >  
> >  #if defined(CONFIG_CODE_TAGGING) && defined(CONFIG_MODULES)
> > +
> > +bool codetag_needs_module_section(struct module *mod, const char *name,
> > +				  unsigned long size);
> > +void *codetag_alloc_module_section(struct module *mod, const char *name,
> > +				   unsigned long size, unsigned int prepend,
> > +				   unsigned long align);
> > +void codetag_free_module_sections(struct module *mod);
> > +void codetag_module_replaced(struct module *mod, struct module *new_mod);
> >  void codetag_load_module(struct module *mod);
> > -bool codetag_unload_module(struct module *mod);
> > -#else
> > +void codetag_unload_module(struct module *mod);
> > +
> > +#else /* defined(CONFIG_CODE_TAGGING) && defined(CONFIG_MODULES) */
> > +
> > +static inline bool
> > +codetag_needs_module_section(struct module *mod, const char *name,
> > +			     unsigned long size) { return false; }
> > +static inline void *
> > +codetag_alloc_module_section(struct module *mod, const char *name,
> > +			     unsigned long size, unsigned int prepend,
> > +			     unsigned long align) { return NULL; }
> > +static inline void codetag_free_module_sections(struct module *mod) {}
> > +static inline void codetag_module_replaced(struct module *mod, struct module *new_mod) {}
> >  static inline void codetag_load_module(struct module *mod) {}
> > -static inline bool codetag_unload_module(struct module *mod) { return true; }
> > -#endif
> > +static inline void codetag_unload_module(struct module *mod) {}
> > +
> > +#endif /* defined(CONFIG_CODE_TAGGING) && defined(CONFIG_MODULES) */
> 
> Maybe I'm missing something, but can't alloc_tag::module_unload() just copy
> the tags that cannot be freed somewhere outside of module sections and then
> free the module?
> 
> The heavy lifting would be localized to alloc_tags rather than spread all
> over.

The reason they can't be freed is because they're referenced by
slab/page allocatons - can't move them either.