Re: [PATCH v2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Hao Ge]

On 2026/8/5 10:26, Hao Ge wrote:
> Hi Suren
>
>
> Thanks a lot for your review.
>
>
> On 2026/8/5 04:08, Suren Baghdasaryan wrote:
> > On Tue, Aug 4, 2026 at 5:21 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
> > > In reserve_module_tags(), the tag overflow check is gated on
> > > mem_alloc_profiling_enabled():
> > >
> > >      if (mem_alloc_profiling_enabled() && !tags_addressable())
> > >
> > > If profiling is toggled off at runtime and a module is loaded whose
> > > tags exceed the compressed-mode limit, shutdown_mem_profiling() is
> > > skipped. vm_module_tags_populate() still maps memory for the tags and
> > > the module loads successfully, but the total tag count now exceeds what
> > > NR_UNUSED_PAGEFLAG_BITS can address.
> > >
> > > Once profiling is re-enabled, ref_to_idx() computes each tag's index
> > > as its position in the alloc_tag array. update_page_tag_ref() masks
> > > it to alloc_tag_ref_mask before storing in page->flags. Indices
> > > beyond the mask are truncated and idx_to_ref() resolves them to wrong
> > > tags.
> > >
> > > mem_alloc_profiling_enabled() and mem_profiling_compressed are
> > > independent. Once compressed mode is established at boot, it stays
> > > active regardless of runtime toggles of mem_profiling.
> > >
> > > Remove the mem_alloc_profiling_enabled() guard. Also return an error
> > > after shutdown_mem_profiling() to skip vm_module_tags_populate(), as
> > > the mapped pages would never be reused - shutdown_mem_profiling() sets
> > > mem_profiling_support to false, so no future module load enters the
> > > codetag path.
> > >
> > > Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag 
> > > compression")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>
> > Thanks for the fix, Hao!
> >
> > > ---
> > > Changes in v2:
> > > - Return error after shutdown_mem_profiling() to skip unnecessary
> > >    vm_module_tags_populate()
> > >    v1 link: 
> > > https://lore.kernel.org/all/20260804064408.105033-1-hao.ge@xxxxxxxxx/
> > > ---
> > >   mm/alloc_tag.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> > > index 52aece27b00e..d8c36430f1c0 100644
> > > --- a/mm/alloc_tag.c
> > > +++ b/mm/alloc_tag.c
> > > @@ -904,10 +904,11 @@ static void *reserve_module_tags(struct module 
> > > *mod, unsigned long size,
> > >                  int grow_res;
> > >
> > >                  module_tags.size = offset + size;
> > > -               if (mem_alloc_profiling_enabled() && 
> > > !tags_addressable()) {
> > Makes sense but how about replacing mem_alloc_profiling_enabled() with
> > mem_profiling_support? Otherwise this warning will be issued multiple
> > times if we are loading multiple modules.
>
>
> That's a really good point, this duplicate warning issue will indeed 
> happen.
>
> When I thought over your point, I realized using mem_profiling_support 
> creates a small race window.
>
>
> Thread A（insmod A）                      Thread B (insmod B)
>
> --------- ---------
>
> needs_section_mem() -> true
>
>     needs_section_mem() -> true
>
>     (profiling still supported)
>
> reserve_module_tags()
>
>     overflow -> shutdown
>
>     mem_profiling_support=false
>
>     return -ENOSPC
>
>     reserve_module_tags()
>
>         mem_profiling_support==false
>
>         -> overflow check skipped
>
>         -> vm_module_tags_populate()
>
>             maps unused tag pages


Sorry, my mail client messed up the race diagram, resend it.

Thread A (insmod A)                   Thread B (insmod B)
---------------------                 ---------------------
needs_section_mem() -> true
                                      needs_section_mem() -> true
                                      (profiling still supported)
reserve_module_tags()
  overflow -> shutdown_mem_profiling()
  mem_profiling_support = false
  return -ENOMEM
                                      reserve_module_tags()
                                        mem_profiling_support == false
                                        -> overflow check skipped
                                        -> vm_module_tags_populate()
                                           maps unused tag pages

> So I'd rather go with pr_warn_once here.
>
>
> > > +               if (!tags_addressable()) {
> > >                          shutdown_mem_profiling(true);
> > >                          pr_warn("With module %s there are too many 
> > > tags to fit in %d page flag bits. Memory allocation profiling is 
> > > disabled!\n",
> > >                                  mod->name, NR_UNUSED_PAGEFLAG_BITS);
> > > +                       return ERR_PTR(-ENOSPC);
> > This ENOSPC error will be propagated all the way up to the init_module
> > syscall and it's not among the error codes currently expected (see:
> > https://man7.org/linux/man-pages/man2/init_module.2.html). I suggest
> > returning ENOMEM instead.
>
>
> Agree, will change
>
>
> Thanks
>
> Best Regards
>
> Hao
>
> > >                  }
> > >
> > >                  grow_res = vm_module_tags_populate();
> > > --
> > > 2.25.1