* Re: [PATCH] iommu/iova: Improve 32-bit free space estimate
[not found] <033815732d83ca73b13c11485ac39336f15c3b40.1646318408.git.robin.murphy@arm.com>
@ 2022-03-03 23:36 ` Miles Chen
2022-03-04 9:41 ` Joerg Roedel
0 siblings, 1 reply; 4+ messages in thread
From: Miles Chen @ 2022-03-03 23:36 UTC (permalink / raw)
To: robin.murphy
Cc: iommu, joro, linux-kernel, miles.chen, will, wsd_upstream,
yf.wang, stable
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@mediatek.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Would you mind adding:
Cc: <stable@vger.kernel.org>
to this path? I checked and I think the patch can be applied to
5.4 and later.
thanks,
Miles
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] iommu/iova: Improve 32-bit free space estimate
2022-03-03 23:36 ` [PATCH] iommu/iova: Improve 32-bit free space estimate Miles Chen
@ 2022-03-04 9:41 ` Joerg Roedel
2022-03-04 11:32 ` Robin Murphy
0 siblings, 1 reply; 4+ messages in thread
From: Joerg Roedel @ 2022-03-04 9:41 UTC (permalink / raw)
To: Miles Chen
Cc: robin.murphy, iommu, linux-kernel, will, wsd_upstream, yf.wang, stable
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@mediatek.com>
> > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>
> Would you mind adding:
>
> Cc: <stable@vger.kernel.org>
Applied without stable tag for now. If needed, please consider
re-sending it for stable when this patch is merged upstream.
Regards,
Joerg
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] iommu/iova: Improve 32-bit free space estimate
2022-03-04 9:41 ` Joerg Roedel
@ 2022-03-04 11:32 ` Robin Murphy
2022-03-05 0:03 ` Miles Chen
0 siblings, 1 reply; 4+ messages in thread
From: Robin Murphy @ 2022-03-04 11:32 UTC (permalink / raw)
To: Joerg Roedel, Miles Chen
Cc: iommu, linux-kernel, will, wsd_upstream, yf.wang, stable
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@mediatek.com>
>>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>>
>> Would you mind adding:
>>
>> Cc: <stable@vger.kernel.org>
>
> 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.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] iommu/iova: Improve 32-bit free space estimate
2022-03-04 11:32 ` Robin Murphy
@ 2022-03-05 0:03 ` Miles Chen
0 siblings, 0 replies; 4+ messages in thread
From: Miles Chen @ 2022-03-05 0:03 UTC (permalink / raw)
To: robin.murphy
Cc: iommu, joro, linux-kernel, miles.chen, stable, will,
wsd_upstream, yf.wang
Hi Joerg, Robin,
> 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.
Got it, thank you.
We will try to push to the android LTS trees we need.
Thanks,
Miles
>
> Thanks all!
>
> Robin.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2022-03-03 23:36 ` [PATCH] iommu/iova: Improve 32-bit free space estimate Miles Chen
2022-03-04 9:41 ` Joerg Roedel
2022-03-04 11:32 ` Robin Murphy
2022-03-05 0:03 ` Miles Chen
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