Re: [PATCH] IB/isert: fix unaligned immediate-data handling

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 31, 2020 at 09:48:12AM -0700, Sagi Grimberg wrote:
> 
> > > Currently we allocate rx buffers in a single contiguous buffers
> > > for headers (iser and iscsi) and data trailer. This means
> > > that most likely the data starting offset is aligned to 76
> > > bytes (size of both headers).
> > > 
> > > This worked fine for years, but at some point this broke.
> > > To fix this, we should avoid passing unaligned buffers for
> > > I/O.
> > 
> > That is a bit vauge - what suddenly broke it?
> 
> Somewhere around the multipage bvec work that Ming did. The issue was
> that brd assumed a 512 aligned page vector. IIRC the discussion settled
> that the block layer expects a 512B aligned buffer(s).

I don't think the limit is from multipage bvec, and the limit is
actually from driver since block layer just sets up the vectors which
is passed to driver, see the following discussion:

https://lore.kernel.org/linux-block/20191204230225.GA26189@ming.t460p/

So what is the exact setting for reproducing the issue? Some underlying
driver does require aligned buffer, such as loop/dio, brd, or when kasan
is involved for buffer allocated via kmalloc().


Thanks, 
Ming