Re: [PATCH v5 3/6] iommu/iova: Extend rbtree node caching

[tn]

On 22.09.2017 18:21, Tomasz Nowicki wrote:
> Hi Robin,
>
> On 21.09.2017 17:52, Robin Murphy wrote:
> > The cached node mechanism provides a significant performance benefit for
> > allocations using a 32-bit DMA mask, but in the case of non-PCI devices
> > or where the 32-bit space is full, the loss of this benefit can be
> > significant - on large systems there can be many thousands of entries in
> > the tree, such that walking all the way down to find free space every
> > time becomes increasingly awful.
> >
> > Maintain a similar cached node for the whole IOVA space as a superset of
> > the 32-bit space so that performance can remain much more consistent.
> >
> > Inspired by work by Zhen Lei <thunder.leizhen@xxxxxxxxxx>.
> >
> > Tested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > Tested-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> > Tested-by: Nate Watterson <nwatters@xxxxxxxxxxxxxx>
> > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> > ---
> >
> > v5: Fixed __cached_rbnode_delete_update() logic to update both nodes
> > ÂÂÂÂ when necessary
> >
> > Â drivers/iommu/iova.c | 60 
> > ++++++++++++++++++++++++----------------------------
> > Â include/linux/iova.h |Â 3 ++-
> > Â 2 files changed, 30 insertions(+), 33 deletions(-)
> >
> > diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> > index 20be9a8b3188..c6f5a22f8d20 100644
> > --- a/drivers/iommu/iova.c
> > +++ b/drivers/iommu/iova.c
> > @@ -48,6 +48,7 @@ init_iova_domain(struct iova_domain *iovad, unsigned 
> > long granule,
> > ÂÂÂÂÂ spin_lock_init(&iovad->iova_rbtree_lock);
> > ÂÂÂÂÂ iovad->rbroot = RB_ROOT;
> > +ÂÂÂ iovad->cached_node = NULL;
> > ÂÂÂÂÂ iovad->cached32_node = NULL;
> > ÂÂÂÂÂ iovad->granule = granule;
> > ÂÂÂÂÂ iovad->start_pfn = start_pfn;
> > @@ -110,48 +111,44 @@ EXPORT_SYMBOL_GPL(init_iova_flush_queue);
> > Â static struct rb_node *
> > Â __get_cached_rbnode(struct iova_domain *iovad, unsigned long 
> > *limit_pfn)
> > Â {
> > -ÂÂÂ if ((*limit_pfn > iovad->dma_32bit_pfn) ||
> > -ÂÂÂÂÂÂÂ (iovad->cached32_node == NULL))
> > +ÂÂÂ struct rb_node *cached_node = NULL;
> > +ÂÂÂ struct iova *curr_iova;
> > +
> > +ÂÂÂ if (*limit_pfn <= iovad->dma_32bit_pfn)
> > +ÂÂÂÂÂÂÂ cached_node = iovad->cached32_node;
> > +ÂÂÂ if (!cached_node)
> > +ÂÂÂÂÂÂÂ cached_node = iovad->cached_node;
> > +ÂÂÂ if (!cached_node)
> > ÂÂÂÂÂÂÂÂÂ return rb_last(&iovad->rbroot);
> > -ÂÂÂ else {
> > -ÂÂÂÂÂÂÂ struct rb_node *prev_node = rb_prev(iovad->cached32_node);
> > -ÂÂÂÂÂÂÂ struct iova *curr_iova =
> > -ÂÂÂÂÂÂÂÂÂÂÂ rb_entry(iovad->cached32_node, struct iova, node);
> > -ÂÂÂÂÂÂÂ *limit_pfn = curr_iova->pfn_lo;
> > -ÂÂÂÂÂÂÂ return prev_node;
> > -ÂÂÂ }
> > +
> > +ÂÂÂ curr_iova = rb_entry(cached_node, struct iova, node);
> > +ÂÂÂ *limit_pfn = min(*limit_pfn, curr_iova->pfn_lo);
>
> I guess this it the fix for stale pointer in iovad->cached32_node from 
> previous series but I think this is something more.
>
> With this min() here we have real control over the limit_pfn we would 
> like to allocate at most. In other works, without your series two 
> subsequent calls can give us:
> iova (ffff) = alloc_iova_fast(iovad, 1, DMA_BIT_MASK(32) >> shift);
>
> iova (fffe)= alloc_iova_fast(iovad, 1, DMA_BIT_MASK(16) >> shift);
>
> We do not see this since nobody uses limit_pfn below DMA_BIT_MASK(32) 
> now. That might be done intentionally so I ask for your opinion.
>
> Also, with your patch and two the same alloc_iova_fast() calls in 
> iteration may get 32-bit space full much faster. Please correct me if I 
> messed things up.

After few more thoughts, I think this is unrealistic case.

In any case, please consider below fix:
 static void
-__cached_rbnode_insert_update(struct iova_domain *iovad,
-	unsigned long limit_pfn, struct iova *new)
+__cached_rbnode_insert_update(struct iova_domain *iovad, struct iova *new)
 {
-	if (limit_pfn != iovad->dma_32bit_pfn)
-		return;
-	iovad->cached32_node = &new->node;
+	if (new->pfn_hi == iovad->dma_32bit_pfn)
+		iovad->cached32_node = &new->node;
+	else
+		iovad->cached_node = &new->node;
 }

Thanks,
Tomasz