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

From: Robin Murphy <robin.murphy@arm.com>
To: Joerg Roedel <joro@8bytes.org>, Miles Chen <miles.chen@mediatek.com>
Cc: wsd_upstream@mediatek.com, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org, yf.wang@mediatek.com,
	iommu@lists.linux-foundation.org, will@kernel.org
Subject: Re: [PATCH] iommu/iova: Improve 32-bit free space estimate
Date: Fri, 4 Mar 2022 11:32:45 +0000	[thread overview]
Message-ID: <77b0c978-7caa-c333-6015-1d784b5daf3f@arm.com> (raw)
In-Reply-To: <YiHey3lGHAMUp+oC@8bytes.org>

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.
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