From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B1FABC433FE for ; Fri, 4 Mar 2022 11:32:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239623AbiCDLdl (ORCPT ); Fri, 4 Mar 2022 06:33:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52344 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231348AbiCDLdj (ORCPT ); Fri, 4 Mar 2022 06:33:39 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id EC4FA14866F; Fri, 4 Mar 2022 03:32:51 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id B8797143D; Fri, 4 Mar 2022 03:32:51 -0800 (PST) Received: from [10.57.39.47] (unknown [10.57.39.47]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7723B3F70D; Fri, 4 Mar 2022 03:32:50 -0800 (PST) Message-ID: <77b0c978-7caa-c333-6015-1d784b5daf3f@arm.com> Date: Fri, 4 Mar 2022 11:32:45 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Thunderbird/91.6.1 Subject: Re: [PATCH] iommu/iova: Improve 32-bit free space estimate Content-Language: en-GB To: Joerg Roedel , Miles Chen Cc: iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, will@kernel.org, wsd_upstream@mediatek.com, yf.wang@mediatek.com, stable@vger.kernel.org References: <033815732d83ca73b13c11485ac39336f15c3b40.1646318408.git.robin.murphy@arm.com> <20220303233646.13773-1-miles.chen@mediatek.com> From: Robin Murphy In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.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 >>> Signed-off-by: Robin Murphy >> >> Would you mind adding: >> >> Cc: > > 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.