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

[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@vger.kernel.org
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
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@linux.dev/
---
 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

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

[Andrew Morton]

On Tue,  4 Aug 2026 20:20:38 +0800 Hao Ge <hao.ge@linux.dev> 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.

Thanks.  Sashiko pointed at a few things, one pre-existing.
	https://sashiko.dev/#/patchset/20260804122038.190270-1-hao.ge@linux.dev

Is there any sanity in there?

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

[Hao Ge]

On 2026/8/5 03:43, Andrew Morton wrote:
> On Tue,  4 Aug 2026 20:20:38 +0800 Hao Ge <hao.ge@linux.dev> 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.
> 
> Thanks.  Sashiko pointed at a few things, one pre-existing.
> 	https://sashiko.dev/#/patchset/20260804122038.190270-1-hao.ge@linux.dev
> 
> Is there any sanity in there?
Hi Andrew

Thanks for pointing these out.
I went through all the Sashiko feedback from v1 and v2.
Most of what it flagged is pre-existing and I'll fix it separately.
The only point that actually touches this patch is the -ENOMEM failing 
the first load. Between wasting a few KB of tag memory to let the load 
succeed and returning -ENOMEM so the user just reloads the module once,
I prefer the latter: a re-insmod works because the codetag section is 
skipped once profiling is off, whereas those mapped tag pages would 
never be used or reclaimed.

Thanks
Best Regards
Hao

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

[Suren Baghdasaryan]

On Tue, Aug 4, 2026 at 5:21 AM Hao Ge <hao.ge@linux.dev> 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@vger.kernel.org
> Signed-off-by: Hao Ge <hao.ge@linux.dev>

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@linux.dev/
> ---
>  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.

> +               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.

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

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@linux.dev> 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@vger.kernel.org
>> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> 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@linux.dev/
>> ---
>>   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
>>

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@linux.dev> 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@vger.kernel.org
>>> Signed-off-by: Hao Ge <hao.ge@linux.dev>
>> 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@linux.dev/
>>> ---
>>>   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
>>>

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

[Suren Baghdasaryan]

On Tue, Aug 4, 2026 at 7:26 PM Hao Ge <hao.ge@linux.dev> 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@linux.dev> 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@vger.kernel.org
> >> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> > 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@linux.dev/
> >> ---
> >>   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.

Yeah, you are right.

>
>
> 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.

pr_warn_once() works. Thanks!

>
>
> >> +               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
> >>