Re: [PATCH v3 08/12] io_uring: overflow processing for CQE32

[Jens Axboe <axboe@kernel.dk>]

On 4/26/22 12:28 AM, Kanchan Joshi wrote:
> On Mon, Apr 25, 2022 at 11:25:26AM -0700, Stefan Roesch wrote:
>> This adds the overflow processing for large CQE's.
>>
>> This adds two parameters to the io_cqring_event_overflow function and
>> uses these fields to initialize the large CQE fields.
>>
>> Allocate enough space for large CQE's in the overflow structue. If no
>> large CQE's are used, the size of the allocation is unchanged.
>>
>> The cqe field can have a different size depending if its a large
>> CQE or not. To be able to allocate different sizes, the two fields
>> in the structure are re-ordered.
>>
>> Co-developed-by: Jens Axboe <axboe@kernel.dk>
>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>> fs/io_uring.c | 31 ++++++++++++++++++++++---------
>> 1 file changed, 22 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 68b61d2b356d..3630671325ea 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -220,8 +220,8 @@ struct io_mapped_ubuf {
>> struct io_ring_ctx;
>>
>> struct io_overflow_cqe {
>> -    struct io_uring_cqe cqe;
>>     struct list_head list;
>> +    struct io_uring_cqe cqe;
>> };
>>
>> struct io_fixed_file {
>> @@ -2017,10 +2017,14 @@ static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
>> static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
>> {
>>     bool all_flushed, posted;
>> +    size_t cqe_size = sizeof(struct io_uring_cqe);
>>
>>     if (!force && __io_cqring_events(ctx) == ctx->cq_entries)
>>         return false;
>>
>> +    if (ctx->flags & IORING_SETUP_CQE32)
>> +        cqe_size <<= 1;
>> +
>>     posted = false;
>>     spin_lock(&ctx->completion_lock);
>>     while (!list_empty(&ctx->cq_overflow_list)) {
>> @@ -2032,7 +2036,7 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
>>         ocqe = list_first_entry(&ctx->cq_overflow_list,
>>                     struct io_overflow_cqe, list);
>>         if (cqe)
>> -            memcpy(cqe, &ocqe->cqe, sizeof(*cqe));
>> +            memcpy(cqe, &ocqe->cqe, cqe_size);
> 
> Maybe a nit, but if we do it this way -
> memcpy(cqe, &ocqe->cqe,     sizeof(*cqe) << (ctx->flags & IORING_SETUP_CQE32));

Unless you make that:

memcpy(cqe, &ocqe->cqe, sizeof(*cqe) << !!(ctx->flags & IORING_SETUP_CQE32));

that will end in tears, and that just makes it less readable. So I don't
think that's a good idea at all.

-- 
Jens Axboe