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a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1629383380; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=+hha0WK4MGSbErqnpiDyGubQ7lCqoDx3pKTyzY9GuGA=; b=h8KkCnflI5JHXAeG5mykE+w5uQK6FCeVJoA8k6DG8MBvUTDsv0tX+94mWY1KREXw/aOCHq Nlvv9IyPj8T2n7SYYnGzBjR1E6qzI8GcDzb2d56sUTFYwLMop6lfqBZuYgINLpavCaFCON t9M7NLyDVcPI7IQjMbz25NIWJVm7Xqc= Subject: Re: [PATCH v2 03/13] libxenguest: deal with log-dirty op stats overflow To: Jan Beulich Cc: Andrew Cooper , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , George Dunlap , Ian Jackson , "xen-devel@lists.xenproject.org" References: <0bebfe8c-6897-dc8b-7fe0-9127d4996eb8@suse.com> <668ef720-389f-4cf1-608e-64aca4f7c73d@suse.com> <347edc1f-a104-1bc3-0cb2-2cfffcb6214a@suse.com> From: Juergen Gross Message-ID: <4441fb9f-d3eb-d443-b04a-534545671ff4@suse.com> Date: Thu, 19 Aug 2021 16:29:39 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Y7HHd6IA1lFKkx72IQJmwiPikEen7LfMB" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --Y7HHd6IA1lFKkx72IQJmwiPikEen7LfMB Content-Type: multipart/mixed; boundary="Lnr00Y0f6XIijPBfqCSzNvjbvAqjc9RW2"; protected-headers="v1" From: Juergen Gross To: Jan Beulich Cc: Andrew Cooper , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , George Dunlap , Ian Jackson , "xen-devel@lists.xenproject.org" Message-ID: <4441fb9f-d3eb-d443-b04a-534545671ff4@suse.com> Subject: Re: [PATCH v2 03/13] libxenguest: deal with log-dirty op stats overflow References: <0bebfe8c-6897-dc8b-7fe0-9127d4996eb8@suse.com> <668ef720-389f-4cf1-608e-64aca4f7c73d@suse.com> <347edc1f-a104-1bc3-0cb2-2cfffcb6214a@suse.com> In-Reply-To: --Lnr00Y0f6XIijPBfqCSzNvjbvAqjc9RW2 Content-Type: multipart/mixed; boundary="------------742C01FE52AAAED5C3D73417" Content-Language: en-US This is a multi-part message in MIME format. --------------742C01FE52AAAED5C3D73417 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 19.08.21 13:51, Jan Beulich wrote: > On 19.08.2021 13:25, Juergen Gross wrote: >> On 19.08.21 13:06, Jan Beulich wrote: >>> On 19.08.2021 12:20, Juergen Gross wrote: >>>> On 05.07.21 17:13, Jan Beulich wrote: >>>>> In send_memory_live() the precise value the dirty_count struct fiel= d >>>>> gets initialized to doesn't matter much (apart from the triggering = of >>>>> the log message in send_dirty_pages(), see below), but it is import= ant >>>>> that it not be zero on the first iteration (or else send_dirty_page= s() >>>>> won't get called at all). Saturate the initializer value at the max= imum >>>>> value the field can hold. >>>>> >>>>> While there also initialize struct precopy_stats' respective field = to a >>>>> more sane value: We don't really know how many dirty pages there ar= e at >>>>> that point. >>>>> >>>>> In suspend_and_send_dirty() and verify_frames() the local variables= >>>>> don't need initializing at all, as they're only an output from the >>>>> hypercall which gets invoked first thing. >>>>> >>>>> In send_checkpoint_dirty_pfn_list() the local variable can be dropp= ed >>>>> altogether: It's optional to xc_logdirty_control() and not used any= where >>>>> else. >>>>> >>>>> Note that in case the clipping actually takes effect, the "Bitmap >>>>> contained more entries than expected..." log message will trigger. = This >>>>> being just an informational message, I don't think this is overly >>>>> concerning. >>>> >>>> Is there any real reason why the width of the stats fields can't be >>>> expanded to avoid clipping? This could avoid the need to set the >>>> initial value to -1, which seems one of the more controversial chang= es. >>> >>> While not impossible, it comes with a price tag, as we'd either need >>> to decouple xc_shadow_op_stats_t from struct xen_domctl_shadow_op_sta= ts >>> or alter the underlying domctl. Neither of which looked either >> >> I was thinking about the domctl. >> >> Apart of struct xen_sysctl_page_offline_op this seems to be the only >> left domctl/sysctl structure limiting guest or host size to values >> being relevant. Changing those would be a sensible thing to do IMO. >=20 > Yet in the context of v1 of this series, which included "x86/paging: > deal with log-dirty stats overflow" (now commit 17e91570c5a4) we > settled on these fields not needing widening. This doesn't prevent > us doing what you suggest, but it would look pretty odd to me at > least. Sorry I was too busy at that time to have a detailed look at the patches. TBH I'd rather undo the stats overflow handling and widen the fields. This is a rather simple patch and a much cleaner solution in the end. I'm not insisting on that, but in case I had to decide this would be my preferred way to handle it. 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