Re: [PATCH v3 15/16] mm: replace __GFP_NO_CODETAG with ALLOC_NO_CODETAG

[Hao Ge <hao.ge@linux.dev>]

CC Andrew and Suren


On 2026/6/29 23:56, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
>
> New issues:
> - [Low] The fallback `#else` definition of `pgalloc_tag_add` was not updated to accept `unsigned int alloc_flags`.
>
> Pre-existing issues:
> - [Critical] TOCTOU race in `__alloc_tag_add_early_pfn` leading to an out-of-bounds array write.
> --
>
> commit ccee7f6d4bcf9d00da38c878e97dd852c5e649b3
> Author: Brendan Jackman <jackmanb@google.com>
>
> mm: replace __GFP_NO_CODETAG with ALLOC_NO_CODETAG
>
> This commit replaces the mm-internal __GFP_NO_CODETAG flag with a
> new alloc_flags bit, ALLOC_NO_CODETAG. It plumbs alloc_flags through
> several allocator functions to allow passing this flag directly to the
> alloc_tag code.
>
>> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
>> index d9be1cf5187d9..a32a94e759b94 100644
>> --- a/mm/alloc_tag.c
>> +++ b/mm/alloc_tag.c
> [ ... ]
>> @@ -806,7 +795,8 @@ static void __init __alloc_tag_add_early_pfn(unsigned long pfn)
>>   	do {
>>   		pool = READ_ONCE(current_pfn_pool);
>>   		if (!pool || atomic_read(&pool->count) >= PFN_POOL_SIZE) {
>> -			struct page *new_page = alloc_page(__GFP_HIGH | __GFP_NO_CODETAG);
>> +			struct page *new_page = __alloc_pages(__GFP_HIGH, 0, numa_mem_id(),
>> +							      NULL, ALLOC_NO_CODETAG);
>>   			struct pfn_pool *new;
> [Severity: Critical]
> This is a pre-existing issue, but is there a time-of-check to time-of-use
> race condition here?
>
> If multiple CPUs are active during the deferred init window, could they
> concurrently observe the same remaining capacity?
>
> CPU1:
>      if (!pool || atomic_read(&pool->count) >= PFN_POOL_SIZE)
>
> CPU2:
>      if (!pool || atomic_read(&pool->count) >= PFN_POOL_SIZE)
>
> If both see 1 remaining slot, won't they both proceed to call
> atomic_fetch_add() on the pool counter further down in this function?
> This could cause an out-of-bounds array write at pool->pfns[PFN_POOL_SIZE].


It's interesting.

I didn't get this warning from Sashiko when I submitted the patch earlier,

I'm not sure what changed on its end to trigger it now.

No worries though, unless I've overlooked some detail,

I believe this is a false positive


Even if both CPUs see 1 remaining slot and both skip the pool-growing 
branch,

they still have to go through atomic_cmpxchg to actually claim it.

static void __init __alloc_tag_add_early_pfn(unsigned long pfn)
{
     struct pfn_pool *pool;
     int idx;

     do {
         pool = READ_ONCE(current_pfn_pool);
         if (!pool || atomic_read(&pool->count) >= PFN_POOL_SIZE) {
         .........
         }
         idx = atomic_read(&pool->count);
         if (idx >= PFN_POOL_SIZE)
             continue;
         if (atomic_cmpxchg(&pool->count, idx, idx + 1) == idx)
             break;
     } while (1);

     pool->pfns[idx] = pfn;
}


Only one wins, the other gets a CAS failure and loops back.

There's no path where both write to pfns[PFN_POOL_SIZE].


Given that, I think we can ignore this Sashiko report and

move forward with Brendan's patch.


Thanks

Best Regards

Hao


> [ ... ]
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 026f33f217036..803b32e5a5e47 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
> [ ... ]
>> @@ -1263,17 +1263,17 @@ void __pgalloc_tag_add(struct page *page, struct task_struct *task,
>>   		alloc_tag_add_early_pfn(page_to_pfn(page), alloc_flags);
>>   		if (task->alloc_tag)
>>   			alloc_tag_set_inaccurate(task->alloc_tag);
>>   	}
>>   }
>>   
>>   static inline void pgalloc_tag_add(struct page *page, struct task_struct *task,
>> -				   unsigned int nr, gfp_t gfp_flags)
>> +				   unsigned int nr, unsigned int alloc_flags)
>>   {
>>   	if (mem_alloc_profiling_enabled())
>> -		__pgalloc_tag_add(page, task, nr, gfp_flags);
>> +		__pgalloc_tag_add(page, task, nr, alloc_flags);
>>   }
> [Severity: Low]
> Does this change miss updating the fallback definition of pgalloc_tag_add()
> when memory allocation profiling is disabled?
>
> If the kernel is compiled without mem-alloc profiling, the dummy function
> signature appears to still use the old type:
>
> static inline void pgalloc_tag_add(struct page *page, struct task_struct *task,
>                     unsigned int nr, gfp_t gfp_flags) {}
>
> Since callers like post_alloc_hook() are now passing an unsigned int
> for alloc_flags, won't this cause a Sparse type mismatch warning because
> gfp_t is a restricted bitwise type?
>