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client-ip=162.221.158.21; receiver=esa2.hc3370-68.iphmx.com; envelope-from="Andrew.Cooper3@citrix.com"; x-sender="postmaster@mail.citrix.com"; x-conformance=sidf_compatible IronPort-SDR: o03CIUk9V5gNn97oaUq886kh1dm/X2WXBiYBG+zZo1OmST283lpTnvczdfbOLNPYyujYxlv/PS aFq72/lOwUyahjn8/wHC0BsCM9iokLoXZ4f/EQioOY7Jn7jqoWxesklie+xp0A40EPokOZz0Pg UPww/FF/oTaqsoNW9yU0o3ZZBmoxuXVgaNf/rvLVabJQbsDscf5xH7smWqk8qa/87TvWEf2z4K f30DkaQYLuiaXUSmSWJnPsttXkyIJexLBGprMMjqjhIyYAsBDSSZKqJUhTGx3T16F83WcYyznR CWs= X-SBRS: 2.7 X-MesageID: 3138803 X-Ironport-Server: esa2.hc3370-68.iphmx.com X-Remote-IP: 162.221.158.21 X-Policy: $RELAYED X-IronPort-AV: E=Sophos;i="5.64,279,1559534400"; d="scan'208";a="3138803" Subject: Re: [Xen-devel] [PATCH 1/2] xen/gntdev: replace global limit of mapped pages by limit per call To: Juergen Gross , , CC: Boris Ostrovsky , Stefano Stabellini References: <20190718065222.31310-1-jgross@suse.com> <20190718065222.31310-2-jgross@suse.com> From: Andrew Cooper Openpgp: preference=signencrypt Autocrypt: addr=andrew.cooper3@citrix.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190718065222.31310-2-jgross@suse.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Content-Language: en-GB X-ClientProxiedBy: AMSPEX02CAS02.citrite.net (10.69.22.113) To AMSPEX02CL02.citrite.net (10.69.22.126) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/07/2019 07:52, Juergen Gross wrote: > Today there is a global limit of pages mapped via /dev/xen/gntdev set > to 1 million pages per default. The Xen default limit even for dom0 is 1024 pages * 16 entries per page, which is far lower than this limit. > There is no reason why that limit is > existing, as total number of foreign mappings is limited by the s/foreign/grant/ ? > hypervisor anyway and preferring kernel mappings over userspace ones > doesn't make sense. Its probably also worth stating that this a root-only device, which further brings in to question the user/kernel split. > > Additionally checking of that limit is fragile, as the number of pages > to map via one call is specified in a 32-bit unsigned variable which > isn't tested to stay within reasonable limits (the only test is the > value to be <= zero, which basically excludes only calls without any > mapping requested). So trying to map e.g. 0xffff0000 pages while > already nearly 1000000 pages are mapped will effectively lower the > global number of mapped pages such that a parallel call mapping a > reasonable amount of pages can succeed in spite of the global limit > being violated. > > So drop the global limit and introduce per call limit instead. Its probably worth talking about this new limit.  What is it trying to protect? ~Andrew