Re: [PATCH] mm/percpu: avoid IO dependency in alloc_percpu

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/16/26 04:08, Khazhy Kumykov wrote:
> On Mon, Jun 15, 2026 at 6:23 PM Khazhismel Kumykov <khazhy@chromium.org> wrote:
>>
>> percpu_alloc users within noio context may block on other percpu_alloc
>> users doing IO due to the shared pcpu_alloc_mutex. pcpu_balance_workfn
>> already grabs under noio context - update remaining holders to also run
>> under noio.
>>
>> Lockdep reported this as a circular locking dependency, with one caller
>> having entered fs_reclaim holding pcpu_alloc_mutex, and the second
>> caller (elevator init) grabbing pcpu_alloc_mutex in noio context.
>>
>> Fixes: 9a5b183941b5 ("mm, percpu: do not consider sleepable allocations atomic")
>>
>> Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
>> ---
>>  mm/percpu.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/percpu.c b/mm/percpu.c
>> index b0676b8054ed..50057403ae79 100644
>> --- a/mm/percpu.c
>> +++ b/mm/percpu.c
>> @@ -1749,8 +1749,13 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
>>         size_t bits, bit_align;
>>
>>         gfp = current_gfp_context(gfp);
>> -       /* whitelisted flags that can be passed to the backing allocators */
>> -       pcpu_gfp = gfp & (GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN);
>> +       /*
>> +        * whitelisted flags that can be passed to the backing allocators.
>> +        *
>> +        * Allocations must be GFP_NOIO, else users in noio/nofs contexts may
>> +        * form dependency through pcpu_alloc_mutex
>> +        */
>> +       pcpu_gfp = gfp & (GFP_NOIO | __GFP_NORETRY | __GFP_NOWARN);
> ah, this is incomplete since we have subsequent allocations that use
> "GFP_KERNEL", I'll send a version with memalloc_noio instead.
> Apologies...

Please see https://lore.kernel.org/all/20260612022648.13008-1-kaitao.cheng@linux.dev/
 
>>         is_atomic = !gfpflags_allow_blocking(gfp);
>>         do_warn = !(gfp & __GFP_NOWARN);
>>
>> --
>> 2.54.0.1189.g8c84645362-goog
>>