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.xenproject.org (lists.xenproject.org [192.237.175.120]) (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 022E7C021B8 for ; Wed, 26 Feb 2025 14:36:58 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.896753.1305507 (Exim 4.92) (envelope-from ) id 1tnIWb-0006fm-2Y; Wed, 26 Feb 2025 14:36:37 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 896753.1305507; Wed, 26 Feb 2025 14:36:37 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1tnIWa-0006ff-Tz; Wed, 26 Feb 2025 14:36:36 +0000 Received: by outflank-mailman (input) for mailman id 896753; Wed, 26 Feb 2025 14:36:36 +0000 Received: from se1-gles-sth1-in.inumbo.com ([159.253.27.254] helo=se1-gles-sth1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1tnIWa-0006fZ-2e for xen-devel@lists.xenproject.org; Wed, 26 Feb 2025 14:36:36 +0000 Received: from mail-wm1-x331.google.com (mail-wm1-x331.google.com [2a00:1450:4864:20::331]) by se1-gles-sth1.inumbo.com (Halon) with ESMTPS id 1419100f-f44f-11ef-9aae-95dc52dad729; Wed, 26 Feb 2025 15:36:35 +0100 (CET) Received: by mail-wm1-x331.google.com with SMTP id 5b1f17b1804b1-43994ef3872so43029145e9.2 for ; Wed, 26 Feb 2025 06:36:35 -0800 (PST) Received: from [10.156.60.236] (ip-037-024-206-209.um08.pools.vodafone-ip.de. [37.24.206.209]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43aba52b591sm24475015e9.6.2025.02.26.06.36.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Feb 2025 06:36:34 -0800 (PST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 1419100f-f44f-11ef-9aae-95dc52dad729 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=google; t=1740580594; x=1741185394; darn=lists.xenproject.org; h=content-transfer-encoding:in-reply-to:autocrypt:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:from:to:cc:subject:date:message-id:reply-to; bh=IjKqxD6G3KD6g5YrviTD+8jL/Tf+30e9/VjAQuiPHG8=; b=dwP1SUQzrHPL6UxwEqAwaMNEETPYDwSaFw+bCFgf1t8KO9X1AIV2d9JU//YojMrI0W ZHW2XReI2vFJmwX1B4lQIZRIzkoeBOfKuRqSMIn3FuWbbBiE1BYcD6LbJBP3IA1xs0Ce tT2sNMzLavpABi09UNycGi8MGZtAf/O+HUe0CUA6ExxWOnwxPRyz0tRwQeiXH2nh3/6E ma4ORVLwmKQn8LP13LHHQGmcYGkLdz7rX3IKzZbRELP705LiY/igOrkaBWekuXe/Zovh sYH92YWxRremMCFanWTv2U/1WngQ6cIfSE3jcWWl/P9Vs9jN5o516hFKTwS1VlLf6y0v V3KA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1740580594; x=1741185394; h=content-transfer-encoding:in-reply-to:autocrypt:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=IjKqxD6G3KD6g5YrviTD+8jL/Tf+30e9/VjAQuiPHG8=; b=Jw152cxHhKpDhZAdWv1P9M5jXLSCS6lCIj6N1egiQji7s5VIvKAAG3i96kL5xxb84E wf3kSTuj1mgc56lVOwwwLdm5Ot93H9gf3NA2Bl3tWpcW+oZEr1oP5X0tIyG0G73ELFRB kKu5F6L7FFRaqHEK5lUM5uBZ4HO3zOBXOPT8SVLKG3kpY7RNzokzSkmSFPaktc/KbbMM z3luY9ZSNIfAhmafsvjP2LG9NCBoZGTPw4PFTI2e5xmkmHc87jEn4X+5k8lSZFg+FRWp JVA/qrx9eZ54fU/UBtegJbxgdJE7NDUvye42MTh6EBr24/huqRqWDjKSugs7Ec+GXhKL iZPw== X-Gm-Message-State: AOJu0Yx4NZkAGqyCf3HwOQC7v892CeK/6s/T3gISmDBs+ASn+3QRuDtx 8uqCY+2KZp761fHSiXcNzJjiBdPP8k+yG2TK9Wf/KBfT17FNgzdzHeqhHMDpyQ== X-Gm-Gg: ASbGncvN3jc5keJtJgw4B/Avil2MVQPZoeKGQiCAyBk4wn2Zi4QWa7oUZMKI27RTDu7 vFG4iAt+9woU1qLJMhvQc8HaXFqsLUmL59h1hIceSaAy38Avh+aVu4/D2WtJk2XLhuPSRPjGIWQ rVUeYuMrS/r4zxJadN1SZQ11Bq/67QB9gxdVC6uBm0mPSWadVcb7+cHi2RT+i8V6mGlC0Yj668Q CXGU6mLkfYeXh1UWU9WZzCLrnMFxot3bQpRFIRZ45pqUkQPjZDBWvH82ZGLd1ELz1p+VTh2awPq 05PeBbXBiGk87ptBmwRhmHit8CtlHnOofAUfmnKn3QvEFZd03HBvL86+fBH1Jx17nHYyoxxrlaL aW6Va33wIsag= X-Google-Smtp-Source: AGHT+IHL55gaBL0DdF2pxtoq64HZs+4zzY67oKFp94IXWc/gtgxLj6Y3NMgbbtTst4bR3I+Hx6V+Tg== X-Received: by 2002:a05:600c:5487:b0:439:9863:e876 with SMTP id 5b1f17b1804b1-43ab75011bbmr38650155e9.24.1740580594348; Wed, 26 Feb 2025 06:36:34 -0800 (PST) Message-ID: <292f8373-705a-4405-bbdb-af750eb5f0ac@suse.com> Date: Wed, 26 Feb 2025 15:36:33 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: xen-devel@lists.xenproject.org, Andrew Cooper , Anthony PERARD , Michal Orzel , Julien Grall , Stefano Stabellini , Alejandro Vallejo References: <20250224132724.9074-1-alejandro.vallejo@cloud.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 26.02.2025 15:28, Roger Pau Monné wrote: > On Wed, Feb 26, 2025 at 03:08:33PM +0100, Jan Beulich wrote: >> On 26.02.2025 14:56, Roger Pau Monné wrote: >>> On Mon, Feb 24, 2025 at 01:27:24PM +0000, Alejandro Vallejo wrote: >>>> --- a/xen/common/page_alloc.c >>>> +++ b/xen/common/page_alloc.c >>>> @@ -490,13 +490,11 @@ static long outstanding_claims; /* total outstanding claims by all domains */ >>>> >>>> unsigned long domain_adjust_tot_pages(struct domain *d, long pages) >>>> { >>>> - long dom_before, dom_after, dom_claimed, sys_before, sys_after; >>>> - >>>> ASSERT(rspin_is_locked(&d->page_alloc_lock)); >>>> d->tot_pages += pages; >>>> >>>> /* >>>> - * can test d->claimed_pages race-free because it can only change >>>> + * can test d->outstanding_pages race-free because it can only change >>>> * if d->page_alloc_lock and heap_lock are both held, see also >>>> * domain_set_outstanding_pages below >>>> */ >>>> @@ -504,17 +502,16 @@ unsigned long domain_adjust_tot_pages(struct domain *d, long pages) >>>> goto out; >>> >>> I think you can probably short-circuit the logic below if pages == 0? >>> (and avoid taking the heap_lock) >> >> Are there callers passing in 0? > > Not sure, but if there are no callers expected we might add an ASSERT > to that effect then. > >>>> spin_lock(&heap_lock); >>>> - /* adjust domain outstanding pages; may not go negative */ >>>> - dom_before = d->outstanding_pages; >>>> - dom_after = dom_before - pages; >>>> - BUG_ON(dom_before < 0); >>>> - dom_claimed = dom_after < 0 ? 0 : dom_after; >>>> - d->outstanding_pages = dom_claimed; >>>> - /* flag accounting bug if system outstanding_claims would go negative */ >>>> - sys_before = outstanding_claims; >>>> - sys_after = sys_before - (dom_before - dom_claimed); >>>> - BUG_ON(sys_after < 0); >>>> - outstanding_claims = sys_after; >>>> + BUG_ON(outstanding_claims < d->outstanding_pages); >>>> + if ( pages > 0 && d->outstanding_pages < pages ) >>>> + { >>>> + /* `pages` exceeds the domain's outstanding count. Zero it out. */ >>>> + outstanding_claims -= d->outstanding_pages; >>>> + d->outstanding_pages = 0; >>>> + } else { >>>> + outstanding_claims -= pages; >>>> + d->outstanding_pages -= pages; >>> >>> I wonder if it's intentional for a pages < 0 value to modify >>> outstanding_claims and d->outstanding_pages, I think those values >>> should only be set from domain_set_outstanding_pages(). >>> domain_adjust_tot_pages() should only decrease the value, but never >>> increase either outstanding_claims or d->outstanding_pages. >>> >>> At best the behavior is inconsistent, because once >>> d->outstanding_pages reaches 0 there will be no further modification >>> from domain_adjust_tot_pages(). >> >> Right, at that point the claim has run out. While freeing pages with an >> active claim means that the claim gets bigger (which naturally needs >> reflecting in the global). > > domain_adjust_tot_pages() is not exclusively called when freeing > pages, see steal_page() for example. Or also when pages were allocated. steal_page() ... > When called from steal_page() it's wrong to increase the claim, as > it assumes that the page removed from d->tot_pages is freed, but > that's not the case. The domain might end up in a situation where > the claim is bigger than the available amount of memory. ... is a case that likely wasn't considered when the feature was added. I never really liked this; I'd be quite happy to see it ripped out, as long as we'd be reasonably certain it isn't in active use by people. Jan