Re: [RFC PATCH v6 2/2] module: allocate codetag sections before the regular module layout

Hi Suren

Thanks for your review.

On 2026/9/2 02:36, Suren Baghdasaryan wrote:

On Tue, Sep 1, 2026 at 8:04 AM Petr Pavlu [off-list ref] wrote:

quoted

On 8/31/26 9:21 AM, Hao Ge wrote:

quoted

Whether a codetag section goes to the codetag region is decided by
layout_sections() and asked again in move_module(). A concurrent
load can shut profiling down in between, and move_module() then
copies the section to offset 0 of its regular destination,
overwriting whatever is there.

Decide and allocate in one pass, before the layout. Allocation
errors fail the load. On a tag area overflow profiling is already
disabled, so -EAGAIN makes the section fall back to regular module
data and the module still loads.

The overflow and populate failure paths of reserve_module_tags() now
release their reservation instead of leaking the maple tree entry.
When profiling was toggled off, the overflow check did not run, a
module could load with more tags than the page flags can address,
and re-enabling profiling then silently corrupted /proc/allocinfo.
The check no longer depends on mem_alloc_profiling_enabled().

Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Based-on-a-patch-by: Petr Pavlu [off-list ref]
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
Changes against Petr's prototype:
- allocate_codetag_sections() returns an error instead of void, and
  only -EAGAIN falls back to a regular section. Any other error now
  fails the load. The prototype fell back on everything, which can
  leave live tags in module memory.
- reserve_module_tags() releases its reservation when populate fails
  too, that path used to leak the maple tree entry.
- codetag_free_module_sections() on the move_module() error path uses
  info->mod, the local mod is assigned only after a successful move.
- The percpu section is marked only when index.pcpu != 0, otherwise
  sechdrs[0] gets marked.
- Dropped the SHF_ALLOC check, .codetag.* sections always have it.
---
 include/linux/module.h   |   2 +
 kernel/module/internal.h |   4 ++
 kernel/module/main.c     | 120 ++++++++++++++++++++-------------------
 mm/alloc_tag.c           |   9 ++-
 4 files changed, 75 insertions(+), 60 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 7566815fabbe..33548daa31a3 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h

@@ -325,6 +325,8 @@ enum mod_mem_type {
      MOD_INIT_RODATA,

      MOD_MEM_NUM_TYPES,
+
+     MOD_STANDALONE = -2,
      MOD_INVALID = -1,
 };

It might be better to split this patch into two: the first to introduce
MOD_STANDALONE and use it only for the percpu section, and the second
with all the codetag-related changes.

The introduction of SH_ENTSIZE_STANDALONE should also allow us to clean
up the current resetting of SHF_ALLOC for the percpu section, and now
also for codetag sections. The problem is that find_sec(".data..percpu")
can currently return different results depending on whether it is called
before layout_and_allocate() or later. In addition, apply_relocations()
needs a special case for SH_ENTSIZE_STANDALONE, where it could otherwise
just test SHF_ALLOC.

Instead of resetting SHF_ALLOC for percpu/codetag sections, both
__layout_sections() and move_module() can check for
SH_ENTSIZE_STANDALONE to determine whether a section is handled
specially and should be skipped.

I think it would be useful to include this change in the first patch
introducing MOD_STANDALONE, but I'm also ok with the current version.
I can send a separate patch later to make more use of
SH_ENTSIZE_STANDALONE in this way.

I ran some tests on my side and nothing blew up.

Thanks.

Petr's suggestion to split the patch sounds good to me and
release_module_tags() change in alloc_tag.c could also be done in a
separate patch. It's the cleanup after we do shutdown_mem_profiling(),
so I think it would be correct on its own.

OK, if I understand you correctly, you'd like the release_module_tags() call
on vm_module_tags_populate() failure to go into its own separate patch.
That's because our -EAGAIN fallback depends on the reserve_module_tags() change.
Without it the overflow entry stays in the maple tree, codetag_module_replaced()
retargets it at the live module, and rmmod then walks the never-populated tag area
and faults.                                                                                                                  

Thanks
Best Regards
Hao

Thanks,
Suren.

quoted

quoted

@@ -2966,18 +2967,23 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
       */
      module_mark_ro_after_init(info->hdr, info->sechdrs, info->secstrings);

-     /*
-      * Determine total sizes, and put offsets in sh_entsize.  For now
-      * this is done generically; there doesn't appear to be any
-      * special cases for the architectures.
-      */
+     /* Allow codetag sections to be allocated separately first. */
+     err = allocate_codetag_sections(info);
+     if (err) {
+             codetag_free_module_sections(info->mod);

The usual convention is that functions clean up after themselves on
error. That means this codetag_free_module_sections() call should be
done ideally by allocate_codetag_sections().

--
Thanks,
Petr