Re: [PATCH] iommu/iova: Improve 32-bit free space estimate

[Robin Murphy]

On 2022-03-04 09:41, Joerg Roedel wrote:
> On Fri, Mar 04, 2022 at 07:36:46AM +0800, Miles Chen wrote:
> > Hi Robin,
> >
> > > For various reasons based on the allocator behaviour and typical
> > > use-cases at the time, when the max32_alloc_size optimisation was
> > > introduced it seemed reasonable to couple the reset of the tracked
> > > size to the update of cached32_node upon freeing a relevant IOVA.
> > > However, since subsequent optimisations focused on helping genuine
> > > 32-bit devices make best use of even more limited address spaces, it
> > > is now a lot more likely for cached32_node to be anywhere in a "full"
> > > 32-bit address space, and as such more likely for space to become
> > > available from IOVAs below that node being freed.
> > >
> > > At this point, the short-cut in __cached_rbnode_delete_update() really
> > > doesn't hold up any more, and we need to fix the logic to reliably
> > > provide the expected behaviour. We still want cached32_node to only move
> > > upwards, but we should reset the allocation size if *any* 32-bit space
> > > has become available.
> > >
> > > Reported-by: Yunfei Wang <yf.wang@xxxxxxxxxxxx>
> > > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> >
> > Would you mind adding:
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
>
> Applied without stable tag for now. If needed, please consider
> re-sending it for stable when this patch is merged upstream.

Yeah, having figured out the history, I ended up with the opinion that 
it was a missed corner-case optimisation opportunity, rather than an 
actual error with respect to intent or implementation, so I 
intentionally left that out. Plus figuring out an exact Fixes tag might 
be tricky - as above I reckon it probably only started to become 
significant somwehere around 5.11 or so.

All of these various levels of retry mechanisms are only a best-effort 
thing, and ultimately if you're making large allocations from a small 
space there are always going to be *some* circumstances that still 
manage to defeat them. Over time, we've made them try harder, but that 
fact that we haven't yet made them try hard enough to work well for a 
particular use-case does not constitute a bug. However as Joerg says, 
anyone's welcome to make a case to Greg to backport a mainline commit if 
it's a low-risk change with significant benefit to real-world stable 
kernel users.

Thanks all!

Robin.