Re: [PATCH 4/5] brd: split I/O at page boundaries

Keith Busch
2025-04-29T21:17:38Z

On Tue, Apr 29, 2025 at 02:15:29PM +0200, Christoph Hellwig wrote:
> On Tue, Apr 29, 2025 at 09:38:28AM +0800, Yu Kuai wrote:
> > Hi,
> >
> > 在 2025/04/29 2:07, Keith Busch 写道:
> >> On Mon, Apr 28, 2025 at 07:09:50AM -0700, Christoph Hellwig wrote:
> >>> A lot of complexity in brd stems from the fact that it tries to handle
> >>> I/O spanning two backing pages. Instead limit the size of a single
> >>> bvec iteration so that it never crosses a page boundary and remove all
> >>> the now unneeded code.
> >>
> >> Doesn't bio_for_each_segment() already limit bvecs on page boundaries?
> >> You'd need to use bio_for_each_bvec() to get multi-page bvecs.
> >
> > I think it only limit bvecs on page boundaries on the issue side, not
> > disk side.
> >
> > For example, if user issue an IO (2k + 4k), will bio_for_each_segment()
> > split this IO into (2k + 2k) and (4k + 2k), I do not test yet, but I
> > think the answer is no.
>
> Exactly. I got this wrong with zram, where it only triggers with larger
> than 4k page sizes, and I got this wrong here on my first attempt as
> well. Fortunately testing found it quickly. I thought the comment and
> commit message document the issue well enough, but I'm open to better
> wording.

Ah, it just clicked for me that you're talking about the pages returned
from brd_lookup_page (the "backing pages", as you said), not the bio's
pages. Sorry about that.