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

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Date:   Fri, 4 Mar 2022 07:08:21 +0800
From:   Miles Chen <miles.chen@...iatek.com>
To:     <robin.murphy@....com>
CC:     <iommu@...ts.linux-foundation.org>, <joro@...tes.org>,
        <linux-kernel@...r.kernel.org>, <miles.chen@...iatek.com>,
        <will@...nel.org>, <wsd_upstream@...iatek.com>,
        <yf.wang@...iatek.com>
Subject: Re: [PATCH] iommu/iova: Improve 32-bit free space estimate

> 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@...iatek.com>
> Signed-off-by: Robin Murphy <robin.murphy@....com>

Reviewed-by: Miles Chen <miles.chen@...iatek.com>