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block: improve handling of the magic discard payload

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Instead of allocating a single unused biovec for discard requests, send
them down without any payload.  Instead we allow the driver to add a
"special" payload using a biovec embedded into struct request (unioned
over other fields never used while in the driver), and overloading
the number of segments for this case.

This has a couple of advantages:

 - we don't have to allocate the bio_vec
 - the amount of special casing for discard requests in the block
   layer is significantly reduced
 - using this same scheme for other request types is trivial,
   which will be important for implementing the new WRITE_ZEROES
   op on devices where it actually requires a payload (e.g. SCSI)
 - we can get rid of playing games with the request length, as
   we'll never touch it and completions will work just fine
 - it will allow us to support ranged discard operations in the
   future by merging non-contiguous discard bios into a single
   request
 - last but not least it removes a lot of code

This patch is the common base for my WIP series for ranges discards and to
remove discard_zeroes_data in favor of always using REQ_OP_WRITE_ZEROES,
so it would be good to get it in quickly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Christoph Hellwig
2016-12-08 15:20:32 -07:00
committed by Jens Axboe
parent be07e14f96
commit f9d03f96b9
13 changed files with 76 additions and 138 deletions
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34
block/blk-core.c
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@@ -1475,38 +1475,6 @@ void blk_put_request(struct request *req)
}
EXPORT_SYMBOL(blk_put_request);
/**
* blk_add_request_payload - add a payload to a request
* @rq: request to update
* @page: page backing the payload
* @offset: offset in page
* @len: length of the payload.
*
* This allows to later add a payload to an already submitted request by
* a block driver. The driver needs to take care of freeing the payload
* itself.
*
* Note that this is a quite horrible hack and nothing but handling of
* discard requests should ever use it.
*/
void blk_add_request_payload(struct request *rq, struct page *page,
int offset, unsigned int len)
{
struct bio *bio = rq->bio;
bio->bi_io_vec->bv_page = page;
bio->bi_io_vec->bv_offset = offset;
bio->bi_io_vec->bv_len = len;
bio->bi_iter.bi_size = len;
bio->bi_vcnt = 1;
bio->bi_phys_segments = 1;
rq->__data_len = rq->resid_len = len;
rq->nr_phys_segments = 1;
}
EXPORT_SYMBOL_GPL(blk_add_request_payload);
bool bio_attempt_back_merge(struct request_queue *q, struct request *req,
struct bio *bio)
{
@@ -2642,6 +2610,8 @@ bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
return false;
}
WARN_ON_ONCE(req->rq_flags & RQF_SPECIAL_PAYLOAD);
req->__data_len -= total_bytes;
/* update sector only for requests with clear definition of sector */