Re: [Xen-devel] [PATCH] xen/blkfront: fix ring info addressing

[Jürgen Groß <jgross@xxxxxxxx>]

On 05.03.20 11:49, Roger Pau Monné wrote:
> On Thu, Mar 05, 2020 at 11:03:31AM +0100, Juergen Gross wrote:
> > Commit 0265d6e8ddb890 ("xen/blkfront: limit allocated memory size to
> > actual use case") made struct blkfront_ring_info size dynamic. This is
> > fine when running with only one queue, but with multiple queues the
> > addressing of the single queues has to be adapted as the structs are
> > allocated in an array.
> Thanks, and sorry for not catching this during review.
>
> > Fixes: 0265d6e8ddb890 ("xen/blkfront: limit allocated memory size to actual use 
> > case")
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >   drivers/block/xen-blkfront.c | 82 ++++++++++++++++++++++++--------------------
> >   1 file changed, 45 insertions(+), 37 deletions(-)
> >
> > diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> > index e2ad6bba2281..a8d4a3838e5d 100644
> > --- a/drivers/block/xen-blkfront.c
> > +++ b/drivers/block/xen-blkfront.c
> > @@ -213,6 +213,7 @@ struct blkfront_info
> >         struct blk_mq_tag_set tag_set;
> >         struct blkfront_ring_info *rinfo;
> >         unsigned int nr_rings;
> > +       unsigned int rinfo_size;
> >         /* Save uncomplete reqs and bios for migration. */
> >         struct list_head requests;
> >         struct bio_list bio_list;
> > @@ -259,6 +260,21 @@ static int blkfront_setup_indirect(struct 
> > blkfront_ring_info *rinfo);
> >   static void blkfront_gather_backend_features(struct blkfront_info *info);
> >   static int negotiate_mq(struct blkfront_info *info);
> >   
> > +#define rinfo_ptr(rinfo, off) \
> > +       (struct blkfront_ring_info *)((unsigned long)(rinfo) + (off))
>                                        ^ void * would seem more natural IMO.
>
> Also if you use void * you don't need the extra (struct
> blkfront_ring_info *) cast I think?
Yes, can change that.

> I however think this macro is kind of weird, since it's just doing an
> addition. I would rather have that calculation in get_rinfo and code
> for_each_rinfo on top of that.
I wanted to avoid the multiplication in the rather common
for_each_rinfo() usage.

> I agree this might be a question of taste, so I'm not going to insist
> but that would reduce the number of helpers from 3 to 2.
>
> > +
> > +#define for_each_rinfo(info, rinfo, idx)                               \
> > +       for (rinfo = info->rinfo, idx = 0;                           \
> > +            idx < info->nr_rings;                                        \
> > +            idx++, rinfo = rinfo_ptr(rinfo, info->rinfo_size))
> I think the above is missing proper parentheses around macro
> parameters.
rinfo and idx are simple variables, so I don't think they need
parentheses. info maybe. But just seeing it now: naming the
parameter "rinfo" and trying to access info->rinfo isn't a good
idea. It is working only as I always use "rinfo" as the pointer.

> > +
> > +static struct blkfront_ring_info *get_rinfo(struct blkfront_info *info,
> > +                                           unsigned int i)
> inline attribute might be appropriate here.
See "the inline disease" in the kernel's coding style.


Juergen


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