Re: bio segment constraints

[Sean Anderson <seanga2@gmail.com>]

On 4/7/25 03:07, Christoph Hellwig wrote:
> On Sun, Apr 06, 2025 at 03:40:04PM -0400, 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,
> 
> First you need to define what segment you mean.  We have at least two and
> a half historical uses of the name.  One is for each bio_vec attached to
> the bio, either directly as submitted into ->submit_bio for bio based
> drivers (case 1a), or generated by bio_split_to_limits (case 1b), which
> is called for every blk-mq driver before calling into ->queue_rq(s) or
> explicitly called by a few bio based driver.
> 
> The other is the bio-vec synthesized by bio_for_each_segment (case 2).

I'm referring to the bio_vecs you get from queue_mq. Which I think is the
latter.

>> - Is bv_len aligned to SECTOR_SIZE?
> 
> Yes.
> 
>> - To logical_sector_size?
> 
> Yes.

OK, but...

>> - What if logical_sector_size > PAGE_SIZE?
> 
> Still always aligned to logical_sector_size.
> 
>> - What about bv_offset?
> 
> bv_offset is a memory offset and must only be aligned to the
> dma_alignment limit.
> 
>> - Is it possible to have a bio where the total length is a multiple of
>>    logical_sector_size, but the data is split across several segments
>>    where each segment is a multiple of SECTOR_SIZE?
> 
> Yes.

...if this is the case, then for some of those segments wouldn't bv_len
not be a multiple of logical_sector_size?

>> - Is is possible to have segments not even aligned to SECTOR_SIZE?
> 
> No.
> 
>> - Can I somehow request to only get segments with bv_len aligned to
>>    logical_sector_size?
> 
> For drivers that use bio_split_to_limits implicitly or explicitly you can
> do that by setting the right seg_boundary_mask.

Is that the right knob? It operates on the physical address, so it looked
more like something for broken DMA engines. For example (if I recall correctly)
MMC SDMA can't cross a page boundary, so you could use seg_boundary_mask to
enforce that.

>> make some big assumptions (which might be bugs?) For example, in
>> drivers/mtd/mtd_blkdevs.c, do_blktrans_request looks like:
> 
>> - There is only one bio in a request. This one is a bit of a soft
>>    assumption since we should only flush the pages in the bio and not the
>>    whole request otherwise.
> 
> It always operates on the first bio in the request and then uses
> blk_update_request to move the context past that.  It is an old
> and somewhat arkane way to write drivers, but should work.  The
> rq_for_each_segment looks do call flush_dcache_page look horribly
> wrong for this model, though.
> 
>> - The data is in lowmem OR bv_offset + bv_len <= PAGE_SIZE. kmap() only
>>    maps a single page, so if we go past one page we end up in adjacent
>>    kmapped pages.
> 
> Yes, this looks broken.
> 
>> Am I missing something here? Handling highmem seems like a persistent
>> issue. E.g. drivers/mtd/ubi/block.c doesn't even bother doing a kmap.
>> Should both of these have BLK_FEAT_BOUNCE_HIGH?
> 
> BLK_FEAT_BOUNCE_HIGH needs to go away rather sooner than later.
> 
> in the short run the best fix would be to synthesized a
> bio_for_each_segment like bio_vec that stays inside a single page
> using bio_iter_iovec) at the top of do_blktrans_request and use
> that for all references to the data.
> 

OK, but if you have to stay inside a single page couldn't you end up
with a sector spanning a page boundary due to only being aligned to
dma_alignment? Or maybe we set seg_boundary_mask to PAGE_MASK to enforce that?

--Sean

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