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 DAAECC433F5 for ; Fri, 25 Mar 2022 00:48:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1357058AbiCYAuS (ORCPT ); Thu, 24 Mar 2022 20:50:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38926 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239525AbiCYAuN (ORCPT ); Thu, 24 Mar 2022 20:50:13 -0400 Received: from mail-pf1-x42d.google.com (mail-pf1-x42d.google.com [IPv6:2607:f8b0:4864:20::42d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 36D0A32078 for ; Thu, 24 Mar 2022 17:48:40 -0700 (PDT) Received: by mail-pf1-x42d.google.com with SMTP id b15so5236075pfm.5 for ; Thu, 24 Mar 2022 17:48:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=zXg1tMAdUsVaG4xJrsonGJrizcZB7/gpq0IZD0yXp2k=; b=fLf/em7TAEh+cdSfAJWuyiN8zD51JBfJx1WtWctYXyqvD4bcoqm1/PX4PIz6HrCgsp Jp/DTCtyY1FTMrxoo34hAUWT/dgLcHUnotyhsaw1CjJX3u9v2EbPNHNdZvtqTh+W5f5P p9iMyD7u9jA2BaY1gPZsu+r5m49+XaCaM92QwG3k94H0bZTZ1We0z1erLFFuyEL67BKm RMV6H06VPmnhOa0Uu496iT0rtERL+MddmvacYMDr/8vRu+EBWEoznZaZuUKesDVIshcO HNCQfRer40lrqDFDScfXz2IC6IBMwAUw/15T6t4gQGLxSCF7vMa4ynN0qq9Yd85nJCiD gx/Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to; bh=zXg1tMAdUsVaG4xJrsonGJrizcZB7/gpq0IZD0yXp2k=; b=wrKK5lP+oE8DpuYpd/NQFYDITlIDoCHRnVli2Dv9fjXUKchiS2YyOuLi+bYcm03Kke R54MRfHM9OeKflcb1eyX8NsISZObJtR2GQOtGvIfSjkum7z1ldb+pACbn3DlQZoEDzTP L0DHfSGksi6zdHbRzE4tROSfB+RbCjffP8XOrBA1mP8QhloZCZz3xzchfMyUiw8Nedc+ mNWqW0R2uLLdCCMj3Fiw1qi4XHL1YUNRcs94wtWF8N7Nf6UVLvpiFLgIL7bw8Tbsg2vn dlEAUEDVkU3NWrTFa3KSwvmzly02Fggs2tbfBQJEaGrrUo15/+597xExXF5ZFWIhxnlx JFog== X-Gm-Message-State: AOAM531Ex7EX3PFK8+1ygmXEBVx7SooxWla+Bjvhq5NOcGayio/qbFUv sVyu5cdFCW6F0NC+ZkjqBy4= X-Google-Smtp-Source: ABdhPJwY8Z8ZqFk2AKMFsNrz3gPdRDcQyu8GHVLdtB+flm261sH6gxGd7Ptp4FyTxdI4YajjDiNkFQ== X-Received: by 2002:a05:6a00:3486:b0:4fa:bb7e:b4c7 with SMTP id cp6-20020a056a00348600b004fabb7eb4c7mr7559822pfb.4.1648169319673; Thu, 24 Mar 2022 17:48:39 -0700 (PDT) Received: from google.com ([2620:15c:211:201:a488:edc:4d69:56d3]) by smtp.gmail.com with ESMTPSA id b14-20020a056a000cce00b004fabc39519esm4987158pfv.5.2022.03.24.17.48.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Mar 2022 17:48:39 -0700 (PDT) Sender: Minchan Kim Date: Thu, 24 Mar 2022 17:48:37 -0700 From: Minchan Kim To: Charan Teja Kalla Cc: Michal Hocko , akpm@linux-foundation.org, surenb@google.com, vbabka@suse.cz, rientjes@google.com, nadav.amit@gmail.com, edgararriaga@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Johannes Weiner Subject: Re: [PATCH 2/2] mm: madvise: return exact bytes advised with process_madvise under error Message-ID: References: <0fa1bdb5009e898189f339610b90ecca16f243f4.1648046642.git.quic_charante@quicinc.com> <602dcc82-519b-bafd-19e6-b373abe572d4@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <602dcc82-519b-bafd-19e6-b373abe572d4@quicinc.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 24, 2022 at 09:15:57PM +0530, Charan Teja Kalla wrote: > Thanks Michal for the inputs. > > On 3/24/2022 6:44 PM, Michal Hocko wrote: > > On Wed 23-03-22 20:54:10, Charan Teja Kalla wrote: > >> From: Charan Teja Reddy > >> > >> The commit 5bd009c7c9a9 ("mm: madvise: return correct bytes advised with > >> process_madvise") fixes the issue to return number of bytes that are > >> successfully advised before hitting error with iovec elements > >> processing. But, when the user passed unmapped ranges in iovec, the > >> syscall ignores these holes and continues processing and returns ENOMEM > >> in the end, which is same as madvise semantic. This is a problem for > >> vector processing where user may want to know how many bytes were > >> exactly processed in a iovec element to make better decissions in the > >> user space. As in ENOMEM case, we processed all bytes in a iovec element > >> but still returned error which will confuse the user whether it is > >> failed or succeeded to advise. > > Do you have any specific example where the initial semantic is really > > problematic or is this mostly a theoretical problem you have found when > > reading the code? > > > > > >> As an example, consider below ranges were passed by the user in struct > >> iovec: iovec1(ranges: vma1), iovec2(ranges: vma2 -- vma3 -- hole) and > >> iovec3(ranges: vma4). In the current implementation, it fully advise > >> iovec1 and iovec2 but just returns number of processed bytes as iovec1 > >> range. Then user may repeat the processing of iovec2, which is already > >> processed, which then returns with ENOMEM. Then user may want to skip > >> iovec2 and starts processing from iovec3. Here because of wrong return > >> processed bytes, iovec2 is processed twice. > > I think you should be much more specific why this is actually a problem. > > This would surely be less optimal but is this a correctness issue? > > > > Yes, this is a problem found when reading the code, but IMO we can > easily expect an invalid vma/hole in the passed range because we are > operating on other process VMA. More than solving the problem of being > less optimal, this can be looked in the direction of helping the user to > take better policy decisions with this syscall. And, not better policy > decisions from user is just being sub optimal(i.e. issuing the syscall > again on the processed range) with this syscall. > > Having said that, at present I don't have any reports/unit test showing > the existing semantic is really a problematic. > > > [...] > >> + vma = find_vma_prev(mm, start, &prev); > >> + if (vma && start > vma->vm_start) > >> + prev = vma; > >> + > >> + blk_start_plug(&plug); > >> + for (;;) { > >> + /* > >> + * It it hits a unmapped address range in the [start, end), > >> + * stop processing and return ENOMEM. > >> + */ > >> + if (!vma || start < vma->vm_start) { > >> + error = -ENOMEM; > >> + goto out; > >> + } > >> + > >> + tmp = vma->vm_end; > >> + if (end < tmp) > >> + tmp = end; > >> + > >> + error = madvise_vma_behavior(vma, &prev, start, tmp, behavior); > >> + if (error) > >> + goto out; > >> + tmp_bytes_advised += tmp - start; > >> + start = tmp; > >> + if (prev && start < prev->vm_end) > >> + start = prev->vm_end; > >> + if (start >= end) > >> + goto out; > >> + if (prev) > >> + vma = prev->vm_next; > >> + else > >> + vma = find_vma(mm, start); > >> + } > >> +out: > >> + /* > >> + * partial_bytes_advised may contain non-zero bytes indicating > >> + * the number of bytes advised before failure. Holds zero incase > >> + * of success. > >> + */ > >> + *partial_bytes_advised = error ? tmp_bytes_advised : 0; > > Although this looks like a fix I am not sure it is future proof. > > madvise_vma_behavior doesn't report which part of the range has been > > really processed. I do not think that currently supported madvise modes > > for process_madvise support an early break out with return to the > > userspace (madvise_cold_or_pageout_pte_range bails on fatal signals for > > example) but this can change in the future and then you are back to > > "imprecise" return value problem. Yes, this is a theoretical problem > > Agree here with the "imprecise" return value problem with processing a > VMA range. Yes when it is decided to return proper processed value from > madvise_vma_behavior(), this code too may need the maintenance. > > > but so it sounds the problem you are trying to fix IMHO. I think it > > would be better to live with imprecise return values reporting rather > > than aiming for perfection which would be fragile and add a future > > maintenance burden. > > > Hmm. Should atleast this imprecise return values be documented in man > page or in madvise.c file? I don't think we need to document it in man page. madvice.c would be enough, IMHO.