Re: bio_copy_from_iter

[Christoph Hellwig <hch@lst.de>]

On Mon, Jun 01, 2026 at 06:10:18PM +0100, Matthew Wilcox wrote:
> I'd like to remove copy_page_to_iter() (and only have
> copy_folio_to_iter()).  That led me to looking at bio_copy_to_iter().
> At first glance, switching it to bio_for_each_folio_all() makes a lot of
> sense -- if there are large folios involved, then we can copy an entire
> folio at a time instead of a page.
> 
> But what I can't prove to my satisfaction is that every bio passed to
> bio_copy_to_iter() necessarily contains folios.  That's not currently
> necessary, but will become necessary in the future. [1].

bio_copy_to_iter is only called from blk_rq_unmap_user, which is only
used for bios created blk_rq_map_user_iov and only used when they had to
be bounce buffer.  There's two sources of pages for the bounce buffering:
The allocation in bio_copy_user_iov using alloca_page, and whatever sg
and st pass in through struct rq_map_data.  A good step to be able
to validate this would be to kill the mess around struct rq_map_data,
as in removing that structure.  There's no good reason why these
drivers should do their own allocations, this has mostly been
grandfathered in.

> [1] I believe all these bvecs are constructed using
> blk_rq_map_user_iov() which can end up calling bio_add_vmalloc(),
> and vmalloc pages will not be folios.

blk_rq_map_user_iov can't call bio_add_vmalloc.  And if you want to make
the vmalloc backing not folios you will be in a huge world of pain
anyway, as we expect to back folios using vmap/vm_map_ram and treating
vmalloc different from this will be extremely messy and invasive.