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

[Hao Ge]

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

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