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 00210C6379F for ; Wed, 22 Feb 2023 10:11:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232327AbjBVKLq (ORCPT ); Wed, 22 Feb 2023 05:11:46 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50452 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231135AbjBVKLj (ORCPT ); Wed, 22 Feb 2023 05:11:39 -0500 Received: from madras.collabora.co.uk (madras.collabora.co.uk [46.235.227.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4EB38311C1; Wed, 22 Feb 2023 02:11:38 -0800 (PST) Received: from [192.168.10.12] (unknown [39.45.217.110]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: usama.anjum) by madras.collabora.co.uk (Postfix) with ESMTPSA id A5DDC660215E; Wed, 22 Feb 2023 10:11:13 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1677060696; bh=NXcADlIg87ZcIH/rmsyo4aL3MQ7pMgv/uza3wicJzPE=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=THO6C2nljd7jwF7SJMnlnE7I332qeIT0Y2k8E29LrPGIExRXzjvMgw2wW1j8gtrfj 7HIfAe/s+pErJgN8cPqOVUFTraUY0QBLiW7zuNwVh2QSz2yv7BI1HRD+MVET5GEVvO Pba592XrXyPsLQHCWUkk9rL1Yxodfefnz9GqUDJdle0ki/yLi2TxYibwN5M3e1gdtZ k0vOAPGfooR81D9X2QnPI1IMAAYSMNkDI9vH0iyVrxnIiuqQZj5sUtAjLUbMB2IgPK szoZfOQEwiY4wu8Loq7NoTH+jIoW8Rq4kmmH+GrhvSjYbbDo7d2Gb3r7OX+BlQxBWv NxJtjfUNiZ0Ow== Message-ID: <6d2b40c6-bed9-69a6-e198-537b50953acd@collabora.com> Date: Wed, 22 Feb 2023 15:11:06 +0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Cc: Muhammad Usama Anjum , Andrei Vagin , Mike Rapoport , Nadav Amit , David Hildenbrand , Andrew Morton , Paul Gofman , Cyrill Gorcunov , Alexander Viro , Shuah Khan , Christian Brauner , Yang Shi , Vlastimil Babka , "Liam R . Howlett" , Yun Zhou , Suren Baghdasaryan , Alex Sierra , Peter Xu , Matthew Wilcox , Pasha Tatashin , Axel Rasmussen , "Gustavo A . R . Silva" , Dan Williams , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Greg KH , kernel@collabora.com, Danylo Mocherniuk Subject: Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs To: =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= References: <20230202112915.867409-1-usama.anjum@collabora.com> <20230202112915.867409-4-usama.anjum@collabora.com> <36ddfd75-5c58-197b-16c9-9f819099ea6d@collabora.com> Content-Language: en-US From: Muhammad Usama Anjum In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/21/23 5:42 PM, Michał Mirosław wrote: > On Tue, 21 Feb 2023 at 11:28, Muhammad Usama Anjum > wrote: >> >> Hi Michał, >> >> Thank you so much for comment! >> >> On 2/17/23 8:18 PM, Michał Mirosław wrote: > [...] >>> For the page-selection mechanism, currently required_mask and >>> excluded_mask have conflicting >> They are opposite of each other: >> All the set bits in required_mask must be set for the page to be selected. >> All the set bits in excluded_mask must _not_ be set for the page to be >> selected. >> >>> responsibilities. I suggest to rework that to: >>> 1. negated_flags: page flags which are to be negated before applying >>> the page selection using following masks; >> Sorry I'm unable to understand the negation (which is XOR?). Lets look at >> the truth table: >> Page Flag negated_flags >> 0 0 0 >> 0 1 1 >> 1 0 1 >> 1 1 0 >> >> If a page flag is 0 and negated_flag is 1, the result would be 1 which has >> changed the page flag. It isn't making sense to me. Why the page flag bit >> is being fliped? >> >> When Anrdei had proposed these masks, they seemed like a fancy way of >> filtering inside kernel and it was straight forward to understand. These >> masks would help his use cases for CRIU. So I'd included it. Please can you >> elaborate what is the purpose of negation? > > The XOR is a way to invert the tested value of a flag (from positive > to negative and the other way) without having the API with invalid > values (with required_flags and excluded_flags you need to define a > rule about what happens if a flag is present in both of the masks - > either prioritise one mask over the other or reject the call). At minimum, one mask (required, any or excluded) must be specified. For a page to get selected, the page flags must fulfill the criterion of all the specified masks. If a flag is present in both required_mask and excluded_mask, the required_mask would select a page. But exculded_mask would drop the page. So page page would be dropped. It is responsibility of the user to correctly specify the flags. matched = true; if (p->required_mask) matched = ((p->required_mask & bitmap) == p->required_mask); if (matched && p->anyof_mask) matched = (p->anyof_mask & bitmap); if (matched && p->excluded_mask) matched = !(p->excluded_mask & bitmap); if (matched && bitmap) { // page selected } Do you accept/like this behavior of masks after explaintation? > (Note: the XOR is applied only to the value of the flags for the > purpose of testing page-selection criteria.) > > So: > 1. if a flag is not set in negated_flags, but set in required_flags, > then it means "this flag must be one" - equivalent to it being set in > required_flag (in your current version of the API). > 2. if a flag is set in negated_flags and also in required_flags, then > it means "this flag must be zero" - equivalent to it being set in > excluded_flags. Lets translate words into table: pageflags required_flags negated_flags matched 1 1 0 yes 0 1 1 yes > > The same thing goes for anyof_flags: if a flag is set in anyof_flags, > then for it to be considered matched: > 1. it must have a value of 1 if it is not set in negated_flags > 2. it must have a value of 0 if it is set in negated_flags pageflags anyof_flags negated_flags matched 1 1 0 yes 0 1 1 yes > > BTW, I think I assumed that both conditions (all flags in > required_flags and at least one in anyof_flags is present) need to be > true for the page to be selected - is this your intention? All the masks are optional. If all or any of the 3 masks are specified, the page flags must pass these masks to get selected. > The example > code has a bug though, in that if anyof_flags is zero it will never > match. Let me fix the selection part: > > // calc. a mask of flags that have expected ("active") values > tested_flags = page_flags ^ negated_flags; > // are all required flags in "active" state? [== all zero when negated] > if (~tested_flags & required_mask) > skip page; > // is any extra flag "active"? > if (anyof_flags && !(tested_flags & anyof_flags)) > skip page; > After taking a while to understand this and compare with already present flag system, `negated flags` is comparatively difficult to understand while already present flags seem easier. > > Best Regards > Michał Mirosław -- BR, Muhammad Usama Anjum