On 2/10/19 2:36 AM, Hannes Reinecke wrote:
> On 2/9/19 10:13 PM, Jens Axboe wrote:
>> Add hint on whether a read was served out of the page cache, or if it
>> hit media. This is useful for buffered async IO, O_DIRECT reads would
>> never have this set (for obvious reasons).
>>
>> If the read hit page cache, cqe->flags will have IOCQE_FLAG_CACHEHIT
>> set.
>>
>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>> ---
>> fs/io_uring.c | 7 ++++++-
>> include/uapi/linux/io_uring.h | 5 +++++
>> 2 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 11a549b5dcbf..d7a10484d748 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -587,11 +587,16 @@ static void io_fput(struct io_kiocb *req)
>> static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
>> {
>> struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw);
>> + unsigned ev_flags = 0;
>>
>> kiocb_end_write(kiocb);
>>
>> io_fput(req);
>> - io_cqring_add_event(req->ctx, req->user_data, res, 0);
>> +
>> + if (res > 0 && (req->flags & REQ_F_FORCE_NONBLOCK))
>> + ev_flags = IOCQE_FLAG_CACHEHIT;
>> +
>> + io_cqring_add_event(req->ctx, req->user_data, res, ev_flags);
>> io_free_req(req);
>> }
>>
>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>> index e23408692118..24906e99fdc7 100644
>> --- a/include/uapi/linux/io_uring.h
>> +++ b/include/uapi/linux/io_uring.h
>> @@ -69,6 +69,11 @@ struct io_uring_cqe {
>> __u32 flags;
>> };
>>
>> +/*
>> + * io_uring_event->flags
>> + */
>> +#define IOCQE_FLAG_CACHEHIT (1U << 0) /* IO did not hit media */
>> +
>> /*
>> * Magic offsets for the application to mmap the data it needs
>> */
>>
>
> Hmm. The point of this patch being ... what?
> Just setting a newly introduced flag seems to be a bit pointless.
> Unless it has some magic interaction with io_cqring_add_event().
> But then that function would have had to have knowledge of that flag
> already, which would be ... odd.
Not sure I follow your concern here. The kernel doesn't use the flag,
we just set it. It's used to inform the application of whether or
not the given read was a cachehit, or if it had to be served by
media.
--
Jens Axboe
