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

[Hao Ge]

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>
---
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()) {
+		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);
 		}
 
 		grow_res = vm_module_tags_populate();
-- 
2.25.1