On 4/8/25 02:10, Hannes Reinecke wrote:
On 4/7/25 16:14, Sean Anderson wrote:
On 4/7/25 03:10, Hannes Reinecke wrote:
On 4/6/25 21:40, Sean Anderson wrote:
Hi all,
I'm not really sure what guarantees the block layer makes regarding the
segments in a bio as part of a request submitted to a block driver. As
far as I can tell this is not documented anywhere. In particular,
- Is bv_len aligned to SECTOR_SIZE?
The block layer always uses a 512 byte sector size, so yes.
- To logical_sector_size?
Not necessarily. Bvecs are a consecutive list of byte ranges which
make up the data portion of a bio.
The logical sector size is a property of the request queue, which is
applied when a request is formed from one or several bios.
For the request the overall length need to be a multiple of the logical
sector size, but not necessarily the individual bios.
Oh, so this is worse than I thought. So if you care about e.g. only submitting I/O in units of logical_block_size, you have to combine
> segments across the entire request.>
Well, yes, and no.
You might be seeing a request with several bios, each having small
bvecs. But in the driver you will want to use an iov iterator or map
it into a sg list via blk_rq_map_sg(), and then iterate over that
for submission.
[ .. ]
- Can I somehow request to only get segments with bv_len aligned to
logical_sector_size? Or do I need to do my own coalescing and bounce
buffering for that?
The driver surely can. You should be able to set 'max_segment_size' to
the logical block size, and that should give you what you want.
But couldn't I get segments smaller than that? max_segment_size seems like
it would only restrict the maximum size, leaving the possibility open for
smaller segments.
As mentioned: individual segments might. The overall request still will adhere to the logical block size setting (ie it will never be smaller than the logical block size).
Maybe have a look at drivers/mtd/ubi/block.c. There the driver will
map the request onto a scatterlist, and then iterate over the sg entries
to read in the data.
Note: mapping onto a scatterlist will coalesce adjacent bvecs, so on the
scatterlist you will find only contiguous segments which adhere to the
logical block size linmitations.
How can this happen? From above, you could have a bvec where it contains only
one sector. So it will always be discontiguous in terms of the logical block size.
I've looked at ubiblock, and that only works for that driver because it's read-only.
The MTD API allows you to read unaligned buffers (although it's inefficient since
you may re-read the same page multiple times), but for writes the data must be
aligned to the page size.
I think the only way to solve this is to create a (mtd-)page-sized buffer and bounce
the incoming data in if the bvec is shorter (or if it's in high memory).
--Sean
