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a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1629372308; 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=K+/Zke3M1fTKkbZu+8B/ox03FCgT0Pi7r2HiZ8xIwFc=; b=lfgto82k1DEG3t4pt5O0SbJyIbQUgdJnu6M406tLqMnyGvXk8skK/n5LYci4Ny0oesltGI zeeEFTBA7cbgBn83yjn87aBWiMHDUqnJ7BEwZo+r2LNuw/1pm/xaeMyQv+rKN9llXP8HLq X9fSPksuKdND6SRA2Dz+wYRx83rKm94= 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> From: Juergen Gross Subject: Re: [PATCH v2 03/13] libxenguest: deal with log-dirty op stats overflow Message-ID: <347edc1f-a104-1bc3-0cb2-2cfffcb6214a@suse.com> Date: Thu, 19 Aug 2021 13:25:07 +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="RY2HSgQLmFtTNkuQrkyaejGqogh9H2iZb" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --RY2HSgQLmFtTNkuQrkyaejGqogh9H2iZb Content-Type: multipart/mixed; boundary="Er0ubBy9SQHg74Ki91yuZRdUetcf6mo7Q"; 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: <347edc1f-a104-1bc3-0cb2-2cfffcb6214a@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> In-Reply-To: --Er0ubBy9SQHg74Ki91yuZRdUetcf6mo7Q Content-Type: multipart/mixed; boundary="------------F5A225BCA0B86820088128BB" Content-Language: en-US This is a multi-part message in MIME format. --------------F5A225BCA0B86820088128BB Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable 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 field >>> gets initialized to doesn't matter much (apart from the triggering of= >>> the log message in send_dirty_pages(), see below), but it is importan= t >>> that it not be zero on the first iteration (or else send_dirty_pages(= ) >>> won't get called at all). Saturate the initializer value at the maxim= um >>> 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 are = 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 dropped= >>> altogether: It's optional to xc_logdirty_control() and not used anywh= ere >>> else. >>> >>> Note that in case the clipping actually takes effect, the "Bitmap >>> contained more entries than expected..." log message will trigger. Th= is >>> 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 changes= =2E >=20 > 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_stats= > 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. > appealing or necessary to me; instead I'm still struggling with > Andrew's comments, yet I didn't receive any clarification of further > explanation. Plus I continue to think that statistics output like this > shouldn't be assumed to be precise anyway, and for practical purposes > I don't think it really matters how large the counts actually are once > they've moved into the billions. In case the underlying problem can easily be avoided be changing the type of the field I don't see why this shouldn't be done. Especially as in this case a misleading message will just be avoided. 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