Re: [PATCH V9 3/7] io_uring: shrink io_mapped_buf

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

On Wed, Nov 06, 2024 at 08:09:38AM -0700, Jens Axboe wrote:
> On 11/6/24 5:26 AM, Ming Lei wrote:
> > `struct io_mapped_buf` will be extended to cover kernel buffer which
> > may be in fast IO path, and `struct io_mapped_buf` needs to be per-IO.
> > 
> > So shrink sizeof(struct io_mapped_buf) by the following ways:
> > 
> > - folio_shift is < 64, so 6bits are enough to hold it, the remained bits
> >   can be used for the coming kernel buffer
> > 
> > - define `acct_pages` as 'unsigned int', which is big enough for
> >   accounting pages in the buffer
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  io_uring/rsrc.c | 2 ++
> >  io_uring/rsrc.h | 6 +++---
> >  2 files changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
> > index 9b8827c72230..16f5abe03d10 100644
> > --- a/io_uring/rsrc.c
> > +++ b/io_uring/rsrc.c
> > @@ -685,6 +685,8 @@ static bool io_try_coalesce_buffer(struct page ***pages, int *nr_pages,
> >  		return false;
> >  
> >  	data->folio_shift = folio_shift(folio);
> > +	WARN_ON_ONCE(data->folio_shift >= 64);
> 
> Since folio_shift is 6 bits, how can that be try?
> 
> I think you'd want:
> 
> 	WARN_ON_ONCE(folio_shift(folio) >= 64);
> 
> instead.

imu->folio_shift is 6 bits, and it is only copied from data->folio_shift(char),
that is why the warning is added for data->folio_shift.

Thanks,
Ming