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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 05, 2020 at 12:04:27PM +0100, Jürgen Groß wrote:
> 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.

Can you undef it afterwards then? I don't think it's supposed to be
used by the rest of the file.

> 
> > 
> > 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.

Dereferences of info and the increase of idx should have parentheses
IMO.

You could rename the rinfo parameter to entry or some such.

> > 
> > > +
> > > +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.

This function has two lines, so I think it's suitable to be inlined:
"A reasonable rule of thumb is to not put inline at functions that
have more than 3 lines of code in them"

I bet the compiler would do this already, but I think adding inline
here is fine according to coding style.

Thanks, Roger.

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