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Fri, 29 Oct 2021 23:44:38 +0000 Message-ID: <2365e65f-7431-4bf5-4ced-5e146776b9ac@oracle.com> Date: Fri, 29 Oct 2021 19:44:31 -0400 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.2.1 Subject: Re: [PATCH v3] xen/balloon: add late_initcall_sync() for initial ballooning done Content-Language: en-US To: =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= Cc: Juergen Gross , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Stefano Stabellini , stable@vger.kernel.org References: <20211029142049.25198-1-jgross@suse.com> <11956c14-f1f7-70f0-40a6-aad31a264af6@oracle.com> From: Boris Ostrovsky In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SN7PR04CA0194.namprd04.prod.outlook.com (2603:10b6:806:126::19) To BLAPR10MB5009.namprd10.prod.outlook.com (2603:10b6:208:321::10) MIME-Version: 1.0 Received: from [10.74.104.68] (138.3.200.4) by SN7PR04CA0194.namprd04.prod.outlook.com (2603:10b6:806:126::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4649.15 via Frontend Transport; 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Can be set with xen_scrub_pages cmdline >>> parameter. Default value controlled with CONFIG_XEN_SCRUB_PAGES_DEFAULT. >>> + >>> +What: /sys/devices/system/xen_memory/xen_memory0/boot_timeout >>> +Date: November 2021 >>> +KernelVersion: 5.16 >>> +Contact: xen-devel@lists.xenproject.org >>> +Description: >>> + The time (in seconds) to wait before giving up to boot in case >>> + initial ballooning fails to free enough memory. Applies only >>> + when running as HVM or PVH guest and started with less memory >>> + configured than allowed at max. >> >> How is this going to be used? We only need this during boot. >> >> >>> - state = update_schedule(state); >>> + balloon_state = update_schedule(balloon_state); >> >> Now that balloon_state has whole file scope it can probably be updated inside update_schedule(). >> >> >>> + while ((credit = current_credit()) < 0) { >>> + if (credit != last_credit) { >>> + last_changed = jiffies; >>> + last_credit = credit; >>> + } >>> + if (balloon_state == BP_ECANCELED) { >> >> What about other states? We are really waiting for BP_DONE, aren't we? > BP_DONE is set also as an intermediate step: > > balloon_state = decrease_reservation(n_pages, > GFP_BALLOON); > if (balloon_state == BP_DONE && n_pages != -credit && > n_pages < totalreserve_pages) > balloon_state = BP_EAGAIN; > > It would be bad to finish waiting in this case. RIght, but if we were to say 'if (balloon_state != BP_DONE)' the worst that can happen is that we will continue on to the next iteration without warning and/or panicing. Of course, there is a chance thaton the next iteration the same thing will happen but I think chances of hitting this race every time are infinitely low. We can also check for current_credit() again. The question is whether we do want to continue waiting if we are in BP_AGAIN. I don't think BP_WAIT is possible in this case although this may change in the future and we will forget to update this code. -boris