From: Dmitry Monakhov <dmonlist@gmail.com>
To: Dave Chinner <david@fromorbit.com>, Jens Axboe <axboe@fb.com>
Cc: axboe@kernel.dk, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, ming.l@ssi.samsung.com
Subject: Re: [PATCH 1/6] block: add support for carrying a stream ID in a bio
In-Reply-To: <20150325023014.GC31342@dastard>
References: <1427210823-5283-1-git-send-email-axboe@fb.com> <1427210823-5283-2-git-send-email-axboe@fb.com> <20150325023014.GC31342@dastard>
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Date: Sun, 12 Apr 2015 14:42:16 +0400
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Dave Chinner <david@fromorbit.com> writes:

> On Tue, Mar 24, 2015 at 09:26:58AM -0600, Jens Axboe wrote:
>> The top bits of bio->bi_flags are reserved for keeping the
>> allocation pool, set aside the next four bits for carrying
>> a stream ID. That leaves us with support for 15 streams,
>> 0 is reserved as a "stream not set" value.
>> 
>> Add helpers for setting/getting stream ID of a bio.
> ....
>> +/*
>> + * after the pool bits, next 4 bits are for the stream id
>> + */
>> +#define BIO_STREAM_BITS		(4)
>> +#define BIO_STREAM_OFFSET	(BITS_PER_LONG - 8)
>> +#define BIO_STREAM_MASK		((1 << BIO_STREAM_BITS) - 1)
>> +
>> +static inline unsigned long streamid_to_flags(unsigned int id)
>> +{
>> +	return (unsigned long) (id & BIO_STREAM_MASK) << BIO_STREAM_OFFSET;
>> +}
>> +
>> +static inline void bio_set_streamid(struct bio *bio, unsigned int id)
>> +{
>> +	bio->bi_flags |= streamid_to_flags(id);
>> +}
>> +
>> +static inline unsigned int bio_get_streamid(struct bio *bio)
>> +{
>> +	return (bio->bi_flags >> BIO_STREAM_OFFSET) & BIO_STREAM_MASK;
>> +}
>> +
>> +static inline bool bio_streamid_valid(struct bio *bio)
>> +{
>> +	return bio_get_streamid(bio) != 0;
>> +}
>
> Need to reserve at least one stream for filesystem private use (e.g.
> metadata writeback). Potentially 2 streams - one for the journal
> which is frequently overwritten, the other for all other long lived
> persistent metadata.
Definitely. User may set it only if it has CAP_RESOURCES.
This is fun, but we act as a soviet nomenclature who try to monopolize
food distribution system :)
>
> Cheers,
>
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
> --
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