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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id F1386C433F5 for ; Tue, 22 Mar 2022 16:46:30 +0000 (UTC) Received: from localhost ([::1]:40446 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nWheQ-0006AP-2f for qemu-devel@archiver.kernel.org; Tue, 22 Mar 2022 12:46:30 -0400 Received: from eggs.gnu.org ([209.51.188.92]:55882) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nWhbp-0003Rr-Pc for qemu-devel@nongnu.org; Tue, 22 Mar 2022 12:43:49 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:55701) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nWhbn-0001Xm-V6 for qemu-devel@nongnu.org; Tue, 22 Mar 2022 12:43:49 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1647967427; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=6xkt2QT5/9L7XnUNnuE3MDXANt+r20rg5iJKFn8wdMI=; b=d4RX3/kuwkieFtfJJQ+wo4yYuJb4li2iG/gIU0zw3HG5Aqs0xA/9BXRGfLy4Kvv4stpOsx Qr+TcXo4tEiPWLUVO0FJCSaPGfFSOihST41Bmqdr0h8ORXOk2RHGbKmLxl0QAFJOFEv2vR 5/EcMo3XZcfw6EZFgGFFZ9dVsWx8gnY= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-610--4DBV6vjNgi92Zaomy7uGQ-1; Tue, 22 Mar 2022 12:43:44 -0400 X-MC-Unique: -4DBV6vjNgi92Zaomy7uGQ-1 Received: by mail-wr1-f69.google.com with SMTP id j67-20020adf9149000000b00203e6b7d151so3191498wrj.13 for ; Tue, 22 Mar 2022 09:43:43 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:organization:in-reply-to :content-transfer-encoding; bh=6xkt2QT5/9L7XnUNnuE3MDXANt+r20rg5iJKFn8wdMI=; b=caOp9qq4GQ6qKvmlIWNqaJr9ek6OdxeFXdpPYkeCrIFEncR5h6cC99zyXFpq5Q36mG TlOuki48hjAm6LNO4IAneFamOAE9cmobHsawufYEyxRhVYe8U41YETXgOWSrEUo5l1ux tXSD4ulaXED8PH+vIvvwQ59XF/EWnwdOWpvxQ/bSo3jIZwL1iZKMFy1VKMmVSM8cpsIH DlVfK6WERDcND4AdU4wkdKttf83nak5IYqK4MsVNQZzfz5PyZrP/wkJtLt1aZdwikdN6 pNaHrD4ZTfteoR3MXq0/rw697RUsSL664BfwbyjaAqn0lQnF/cjKDTrOTyxkCwHvlMAK t/0A== X-Gm-Message-State: AOAM531N8QCquPe0jiOoeSqCALszZmKoCAGPjr+cYZ0NrfUWjRuKSTtI 9Gx5E19nV0382xD4QiYjfOKGqkKSyhSxi5N/Y1X8+wnQlOq1bMB7ZtBRra0+4PIvLeggaPM2Qx0 Y8lxQo1bg/PXaVhA= X-Received: by 2002:a05:600c:4fcb:b0:389:a82b:8f8a with SMTP id o11-20020a05600c4fcb00b00389a82b8f8amr4660696wmq.167.1647967422673; Tue, 22 Mar 2022 09:43:42 -0700 (PDT) X-Google-Smtp-Source: ABdhPJykyCyZXlnZYp3vNEVQDGDs8cXrmugk+whmhUyzcXXyPY+W6Ej3h+DKomTWqZp08Wcvv3vBTQ== X-Received: by 2002:a05:600c:4fcb:b0:389:a82b:8f8a with SMTP id o11-20020a05600c4fcb00b00389a82b8f8amr4660672wmq.167.1647967422396; Tue, 22 Mar 2022 09:43:42 -0700 (PDT) Received: from ?IPV6:2003:cb:c708:de00:549e:e4e4:98df:ff72? (p200300cbc708de00549ee4e498dfff72.dip0.t-ipconnect.de. [2003:cb:c708:de00:549e:e4e4:98df:ff72]) by smtp.gmail.com with ESMTPSA id l13-20020adfbd8d000000b002040daf5dffsm9982350wrh.18.2022.03.22.09.43.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Mar 2022 09:43:42 -0700 (PDT) Message-ID: <4cf0654d-8e2f-8f5d-73f9-d0592c0eaa2f@redhat.com> Date: Tue, 22 Mar 2022 17:43:40 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH] softmmu/physmem: Use qemu_madvise To: Andrew Deason References: <20220316040405.4131-1-adeason@sinenomine.net> <9c36fe6b-39e1-0bfc-d2bb-97b106828ee1@redhat.com> <20220322113949.d297e615317fa34894de8ed2@sinenomine.net> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220322113949.d297e615317fa34894de8ed2@sinenomine.net> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , qemu-devel@nongnu.org, =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Peter Xu , "Dr. David Alan Gilbert" , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 22.03.22 17:39, Andrew Deason wrote: > On Wed, 16 Mar 2022 10:41:41 +0100 > David Hildenbrand wrote: > >> On 16.03.22 10:37, Dr. David Alan Gilbert wrote: >>> * Peter Maydell (peter.maydell@linaro.org) wrote: >>>> On Wed, 16 Mar 2022 at 07:53, David Hildenbrand wrote: >>>>> >>>>> On 16.03.22 05:04, Andrew Deason wrote: >>>>>> We have a thin wrapper around madvise, called qemu_madvise, which >>>>>> provides consistent behavior for the !CONFIG_MADVISE case, and works >>>>>> around some platform-specific quirks (some platforms only provide >>>>>> posix_madvise, and some don't offer all 'advise' types). This specific >>>>>> caller of madvise has never used it, tracing back to its original >>>>>> introduction in commit e0b266f01dd2 ("migration_completion: Take >>>>>> current state"). >>>>>> >>>>>> Call qemu_madvise here, to follow the same logic as all of our other >>>>>> madvise callers. This slightly changes the behavior for >>>>>> !CONFIG_MADVISE (EINVAL instead of ENOSYS, and a slightly different >>>>>> error message), but this is now more consistent with other callers >>>>>> that use qemu_madvise. >>>>>> >>>>>> Signed-off-by: Andrew Deason >>>>>> --- >>>>>> Looking at the history of commits that touch this madvise() call, it >>>>>> doesn't _look_ like there's any reason to be directly calling madvise vs >>>>>> qemu_advise (I don't see anything mentioned), but I'm not sure. >>>>>> >>>>>> softmmu/physmem.c | 12 ++---------- >>>>>> 1 file changed, 2 insertions(+), 10 deletions(-) >>>>>> >>>>>> diff --git a/softmmu/physmem.c b/softmmu/physmem.c >>>>>> index 43ae70fbe2..900c692b5e 100644 >>>>>> --- a/softmmu/physmem.c >>>>>> +++ b/softmmu/physmem.c >>>>>> @@ -3584,40 +3584,32 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length) >>>>>> rb->idstr, start, length, ret); >>>>>> goto err; >>>>>> #endif >>>>>> } >>>>>> if (need_madvise) { >>>>>> /* For normal RAM this causes it to be unmapped, >>>>>> * for shared memory it causes the local mapping to disappear >>>>>> * and to fall back on the file contents (which we just >>>>>> * fallocate'd away). >>>>>> */ >>>>>> -#if defined(CONFIG_MADVISE) >>>>>> if (qemu_ram_is_shared(rb) && rb->fd < 0) { >>>>>> - ret = madvise(host_startaddr, length, QEMU_MADV_REMOVE); >>>>>> + ret = qemu_madvise(host_startaddr, length, QEMU_MADV_REMOVE); >>>>>> } else { >>>>>> - ret = madvise(host_startaddr, length, QEMU_MADV_DONTNEED); >>>>>> + ret = qemu_madvise(host_startaddr, length, QEMU_MADV_DONTNEED); >>>>> >>>>> posix_madvise(QEMU_MADV_DONTNEED) has completely different semantics >>>>> then madvise() -- it's not a discard that we need here. >>>>> >>>>> So ram_block_discard_range() would now succeed in environments (BSD?) >>>>> where it's supposed to fail. >>>>> >>>>> So AFAIKs this isn't sane. >>>> >>>> But CONFIG_MADVISE just means "host has madvise()"; it doesn't imply >>>> "this is a Linux madvise() with MADV_DONTNEED". Solaris madvise() >>>> doesn't seem to have MADV_DONTNEED at all; a quick look at the >>>> FreeBSD manpage suggests its madvise MADV_DONTNEED is identical >>>> to its posix_madvise MADV_DONTNEED. >>>> >>>> If we need "specifically Linux MADV_DONTNEED semantics" maybe we >>>> should define a QEMU_MADV_LINUX_DONTNEED which either (a) does the >>>> right thing or (b) fails, and use qemu_madvise() regardless. >>>> >>>> Certainly the current code is pretty fragile to being changed by >>>> people who don't understand the undocumented subtlety behind >>>> the use of a direct madvise() call here. >>> >>> Yeh and I'm not sure I can remembe rall the subtleties; there's a big >>> hairy set of ifdef's in include/qemu/madvise.h that makes >>> sure we always have the definition of QEMU_MADV_REMOVE/DONTNEED >>> even on platforms that might not define it themselves. >>> >>> But I think this code is used for things with different degrees >>> of care about the semantics; e.g. 'balloon' just cares that >>> it frees memory up and doesn't care about the detailed semantics >>> that much; so it's probably fine with that. >>> Postcopy is much more touchy, but then it's only going to be >>> calling this on Linux anyway (because of the userfault dependency). >> >> MADV_DONTNEED/MADV_REMOVE only provides discard semantics on Linux IIRC >> -- and that's what we want to achieve: ram_block_discard_range() >> >> So I agree with Peter that we might want to make this more explicit. > > I was looking at the comments/history around this code to try to make > this more explicit/clear, and it seems like the whole function is very > Linux-specific. All we ever do is: > > - fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE) > - madvise(MADV_REMOVE) > - madvise(MADV_DONTNEED) with Linux semantics > > All of those operations are Linux-only, so trying to figure out the > cross-platform way to model this seems kind of pointless. Is it fine to > just #ifdef the whole thing to be just for Linux? > Fine with me, as long as it compiles on other OSs :) -- Thanks, David / dhildenb