Re: [PATCH v4 0/2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

On Mon, Aug 10, 2026 at 8:52 PM Andrew Morton <[email protected]> wrote:
>
> On Mon, 10 Aug 2026 17:39:53 +0800 Hao Ge <[email protected]> wrote:
>
> > v3 was a single patch. After discussion with Suren and Andrew we went
> > for a more graceful approach: rather than failing the module load on
> > overflow, let it load without profiling. Once profiling is disabled,
> > codetag_needs_module_section() returns false, so on retry the codetag
> > section is placed as regular module data.
> >
> > A new patch (1/2) is added to move release_module_tags() above
> > reserve_module_tags(), since the overflow path now has to call it and
> > the helper sits below it.
>
> Thing is, [2/2] has cc:stable but it requires [1/2] to be able to be
> compiled.  [1/2] doesn't have cc:stable so we're asking -stable folks
> to backport a patch which doesn't compile.
>
> Resolve this by using the same Fixes: and cc:stable in both patches.
>
> > release_module_tags() is what module unload calls to drop a module's
> > reservation from the maple tree. By the time reserve_module_tags()
> > detects the overflow it has already stored that reservation, and the
> > -EAGAIN return skips vm_module_tags_populate(), so the backing pages
> > never get mapped. If reserve_module_tags() returns without calling
> > release_module_tags(), the stale entry keeps pointing at that unmapped
> > range; when the module is later unloaded, release_module_tags() walks
> > it and panics.
>
> AI review had a lot to say about this patchset.  Some pre-existing, some
> not:
>         https://sashiko.dev/#/patchset/[email protected]

Yeah, some of them are not related to this change but at least one does.
I need to address the unrelated ones. Will do that as a separate patchset.

>
>
>
> offtopic: alloc_tag isn't getting allmodconfig build coverage at this
> time because:
>
> 1: MEM_ALLOC_PROFILING depends on !DEBUG_FORCE_WEAK_PER_CPU (why?  I
>    can't figure that out)

DEBUG_FORCE_WEAK_PER_CPU forces weak percpu definitions everywhere,
even in the core kernel. This introduces the restriction of [1]:

2. Static percpu variables cannot be defined inside a function.

Memory allocation profiling relies on percpu variables inside a
function in DEFINE_ALLOC_TAG(). For
CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU we comporomise by accounting all
module allocations to a statically defined _shared_alloc_tag (see [2])
but we can't do that for all kernel allocations because profiling
becomes quite meaningless at that point (all allocations being
accounted in the same counter is not useful).

[1] https://elixir.bootlin.com/linux/v7.2-rc6/source/include/linux/percpu-defs.h#L64
[2]  https://elixir.bootlin.com/linux/v7.2-rc6/source/include/linux/alloc_tag.h#L91

>
> 2: x86_64 allmodconfig enables DEBUG_FORCE_WEAK_PER_CPU, despite it
>    being for s390 and alpha.  In fact it might be alpha-only.
>
> Adding
>
>         depends on ALPHA || S390
>
> in there fixes this.