Re: [PATCH 3/6] direct-io: add support for write stream IDs

[Jens Axboe <axboe@fb.com>]

On 04/10/2015 05:50 PM, Ming Lin wrote:
> On Wed, Mar 25, 2015 at 7:26 AM, Jens Axboe <axboe@kernel.dk> wrote:
>> On 03/24/2015 08:43 PM, Dave Chinner wrote:
>>>
>>> On Tue, Mar 24, 2015 at 09:27:00AM -0600, Jens Axboe wrote:
>>>>
>>>> Get the streamid from the file, if any, and set it on the bio.
>>>>
>>>> Signed-off-by: Jens Axboe <axboe@fb.com>
>>>> ---
>>>>    fs/direct-io.c | 4 ++++
>>>>    1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/fs/direct-io.c b/fs/direct-io.c
>>>> index e181b6b2e297..5d2750346451 100644
>>>> --- a/fs/direct-io.c
>>>> +++ b/fs/direct-io.c
>>>> @@ -77,6 +77,7 @@ struct dio_submit {
>>>>          int reap_counter;               /* rate limit reaping */
>>>>          sector_t final_block_in_request;/* doesn't change */
>>>>          int boundary;                   /* prev block is at a boundary */
>>>> +       int streamid;                   /* Write stream ID */
>>>>          get_block_t *get_block;         /* block mapping function */
>>>>          dio_submit_t *submit_io;        /* IO submition function */
>>>>
>>>> @@ -372,6 +373,8 @@ dio_bio_alloc(struct dio *dio, struct dio_submit
>>>> *sdio,
>>>>
>>>>          sdio->bio = bio;
>>>>          sdio->logical_offset_in_bio = sdio->cur_page_fs_offset;
>>>> +
>>>> +       bio_set_streamid(bio, sdio->streamid);
>>>>    }
>>>>
>>>>    /*
>>>> @@ -1205,6 +1208,7 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb,
>>>> struct inode *inode,
>>>>          sdio.blkbits = blkbits;
>>>>          sdio.blkfactor = i_blkbits - blkbits;
>>>>          sdio.block_in_file = offset >> blkbits;
>>>> +       sdio.streamid = iocb->ki_filp->f_streamid;
>>>
>>>
>>> If iocb->ki_filp->f_streamid is not set, then it should fall back to
>>> whatever is set on the inode->i_streamid.
>
> Why should do the fall back?

Because the assumption is that, in general, the specific file is a good 
indication of the data lifetime, if someone has already set that. It's a 
better guess than writing without any stream attached.

> That change causes problem for direct IO, for example
>
> process 1:
> fd = open("/dev/nvme0n1", O_DIRECT...);
> //set stream_id 1
> fadvise(fd, 1, 0, POSIX_FADV_STREAMID);
> pwrite(fd, ....);
>
> process 2:
> fd = open("/dev/nvme0n1", O_DIRECT...);
> //should be legacy stream_id 0
> pwrite(fd, ....);
>
> But now process 2 also see stream_id 1, which is wrong.

I guess for that case, it is a problem. Basically the fallback breaks 
down for full block devices, or huge files that are used as a general 
backing store (like a vm image, for instance). Hmm, not sure what the 
right solution would be here, or if there really is one. It's probably 
best NOT to do the fallback after all.

-- 
Jens Axboe