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Date: Wed, 25 Mar 2015 13:30:14 +1100
From: Dave Chinner <david@fromorbit.com>
To: 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
Message-ID: <20150325023014.GC31342@dastard>
References: <1427210823-5283-1-git-send-email-axboe@fb.com>
 <1427210823-5283-2-git-send-email-axboe@fb.com>
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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.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com