Re: [PATCH 2/2] io_uring: get rid of alloc cache init_once handling

[Pavel Begunkov <asml.silence@gmail.com>]

On 1/23/25 14:27, Pavel Begunkov wrote:
> On 1/23/25 14:21, Jens Axboe wrote:
>> init_once is called when an object doesn't come from the cache, and
>> hence needs initial clearing of certain members. While the whole
>> struct could get cleared by memset() in that case, a few of the cache
>> members are large enough that this may cause unnecessary overhead if
>> the caches used aren't large enough to satisfy the workload. For those
>> cases, some churn of kmalloc+kfree is to be expected.
>>
>> Ensure that the 3 users that need clearing put the members they need
>> cleared at the start of the struct, and place an empty placeholder
>> 'init' member so that the cache initialization knows how much to
>> clear.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>   include/linux/io_uring/cmd.h   |  3 ++-
>>   include/linux/io_uring_types.h |  3 ++-
>>   io_uring/alloc_cache.h         | 30 +++++++++++++++++++++---------
>>   io_uring/futex.c               |  4 ++--
>>   io_uring/io_uring.c            | 13 ++++++++-----
>>   io_uring/io_uring.h            |  5 ++---
>>   io_uring/net.c                 | 11 +----------
>>   io_uring/net.h                 |  7 +++++--
>>   io_uring/poll.c                |  2 +-
>>   io_uring/rw.c                  | 10 +---------
>>   io_uring/rw.h                  |  5 ++++-
>>   io_uring/uring_cmd.c           | 10 +---------
>>   12 files changed, 50 insertions(+), 53 deletions(-)
>>
>> diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
>> index a3ce553413de..8d7746d9fd23 100644
>> --- a/include/linux/io_uring/cmd.h
>> +++ b/include/linux/io_uring/cmd.h
>> @@ -19,8 +19,9 @@ struct io_uring_cmd {
>>   };
>>   struct io_uring_cmd_data {
>> -    struct io_uring_sqe    sqes[2];
>>       void            *op_data;
>> +    int            init[0];
> 
> What do you think about using struct_group instead?

And why do we care not clearing it all on initial alloc? If that's
because of kasan, we can disable it until ("kasan, mempool: don't
store free stacktrace in io_alloc_cache objects") lands.

-- 
Pavel Begunkov