Re: [PATCH v2 1/2] iova: Remove some magazine pointer NULL checks

[Robin Murphy]

On 2022-09-06 11:50, John Garry wrote:
> On 06/09/2022 10:28, Ethan Zhao wrote:
>
> Hi Ethan,
>
> > > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > > Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>
> > > ---
> > >   drivers/iommu/iova.c | 7 ++-----
> > >   1 file changed, 2 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> > > index 47d1983dfa2a..580fdf669922 100644
> > > --- a/drivers/iommu/iova.c
> > > +++ b/drivers/iommu/iova.c
> > > @@ -661,9 +661,6 @@ iova_magazine_free_pfns(struct iova_magazine 
> > > *mag, struct iova_domain *iovad)
> > >       unsigned long flags;
> > >       int i;
> > > -    if (!mag)
> > > -        return;
> > > -
> >
> > iommu_probe_device
> >    ops->probe_finalize(dev);
> >      intel_iommu_probe_finalize
> >         iommu_setup_dma_ops
> >           iommu_dma_init_domain(domain, dma_base, dma_limit, dev)
> >             iova_domain_init_rcaches
> >               {
> >               ...
> >               cpu_rcache->loaded = iova_magazine_alloc(GFP_KERNEL);
> >               cpu_rcache->prev = iova_magazine_alloc(GFP_KERNEL);
> >             if (!cpu_rcache->loaded || !cpu_rcache->prev) {
> >                  ret = -ENOMEM;
> >                        goto out_err;
> >
> > Do you mean iova_magazine_alloc() is impossible to fail ?
>
> No, iova_magazine_alloc() may fail and return NULL. But if it does then 
> we set iovad rcache pointer = NULL in the error path and don't use the 
> rcache.
>
> However we have a !iovad->rcache check on the "fast" alloc but not 
> "insert". I need to check why that is again.

Right, if you find a good reason to respin the patch then perhaps also 
tweaking the commit message to clarify that it's impossible to have a 
NULL rcache *at any point where those checks are made* might avoid all 
possible doubt, however I'd hope that it's clear enough that the 
transient case while iova_domain_init_rcaches() is in the process of 
failing really doesn't need consideration in its own right.

I guess the check in iova_rcache_get() was maybe with the intent of 
allowing alloc_iova_fast() to seamlessly fall back to standard 
allocation, so an API user can treat iova_domain_init_rcaches() failure 
as non-fatal? That makes a fair amount of sense, but does mean that 
we're missing the equivalent in iova_rcache_insert() for it to actually 
work. Or we just remove it and tighten up the documentation to say 
that's not valid - I would like a way to make rcaches optional in 
iommu-dma for systems where they're a pointless waste of memory, but we 
can always revisit this when we get there.

Cheers,
Robin.