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[93.72.151.96]) by smtp.googlemail.com with ESMTPSA id o14sm1636656lfn.56.2020.05.14.04.16.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 May 2020 04:16:17 -0700 (PDT) Subject: Re: zfs deadlock on r360452 relating to busy vm page To: Mark Johnston Cc: Bryan Drewery , freebsd-current@freebsd.org References: <2bdc8563-283b-32cc-8a1a-85ff52aca99e@FreeBSD.org> <0e9cceba-84d0-ec4f-8784-36703452201d@FreeBSD.org> <889cb93b-85c7-3ec4-4ccf-5fb56ec38fa5@FreeBSD.org> <20200513144257.GA24239@raichu> From: Andriy Gapon Openpgp: preference=signencrypt Autocrypt: addr=avg@FreeBSD.org; prefer-encrypt=mutual; keydata= mQINBFm4LIgBEADNB/3lT7f15UKeQ52xCFQx/GqHkSxEdVyLFZTmY3KyNPQGBtyvVyBfprJ7 mAeXZWfhat6cKNRAGZcL5EmewdQuUfQfBdYmKjbw3a9GFDsDNuhDA2QwFt8BmkiVMRYyvI7l N0eVzszWCUgdc3qqM6qqcgBaqsVmJluwpvwp4ZBXmch5BgDDDb1MPO8AZ2QZfIQmplkj8Y6Z AiNMknkmgaekIINSJX8IzRzKD5WwMsin70psE8dpL/iBsA2cpJGzWMObVTtCxeDKlBCNqM1i gTXta1ukdUT7JgLEFZk9ceYQQMJJtUwzWu1UHfZn0Fs29HTqawfWPSZVbulbrnu5q55R4PlQ /xURkWQUTyDpqUvb4JK371zhepXiXDwrrpnyyZABm3SFLkk2bHlheeKU6Yql4pcmSVym1AS4 dV8y0oHAfdlSCF6tpOPf2+K9nW1CFA8b/tw4oJBTtfZ1kxXOMdyZU5fiG7xb1qDgpQKgHUX8 7Rd2T1UVLVeuhYlXNw2F+a2ucY+cMoqz3LtpksUiBppJhw099gEXehcN2JbUZ2TueJdt1FdS ztnZmsHUXLxrRBtGwqnFL7GSd6snpGIKuuL305iaOGODbb9c7ne1JqBbkw1wh8ci6vvwGlzx rexzimRaBzJxlkjNfMx8WpCvYebGMydNoeEtkWldtjTNVsUAtQARAQABtB5BbmRyaXkgR2Fw b24gPGF2Z0BGcmVlQlNELm9yZz6JAlQEEwEIAD4WIQS+LEO7ngQnXA4Bjr538m7TUc1yjwUC WbgsiAIbIwUJBaOagAULCQgHAgYVCAkKCwIEFgIDAQIeAQIXgAAKCRB38m7TUc1yj+JAEACV l9AK/nOWAt/9cufV2fRj0hdOqB1aCshtSrwHk/exXsDa4/FkmegxXQGY+3GWX3deIyesbVRL rYdtdK0dqJyT1SBqXK1h3/at9rxr9GQA6KWOxTjUFURsU7ok/6SIlm8uLRPNKO+yq0GDjgaO LzN+xykuBA0FlhQAXJnpZLcVfPJdWv7sSHGedL5ln8P8rxR+XnmsA5TUaaPcbhTB+mG+iKFj GghASDSfGqLWFPBlX/fpXikBDZ1gvOr8nyMY9nXhgfXpq3B6QCRYKPy58ChrZ5weeJZ29b7/ QdEO8NFNWHjSD9meiLdWQaqo9Y7uUxN3wySc/YUZxtS0bhAd8zJdNPsJYG8sXgKjeBQMVGuT eCAJFEYJqbwWvIXMfVWop4+O4xB+z2YE3jAbG/9tB/GSnQdVSj3G8MS80iLS58frnt+RSEw/ psahrfh0dh6SFHttE049xYiC+cM8J27Aaf0i9RflyITq57NuJm+AHJoU9SQUkIF0nc6lfA+o JRiyRlHZHKoRQkIg4aiKaZSWjQYRl5Txl0IZUP1dSWMX4s3XTMurC/pnja45dge/4ESOtJ9R 8XuIWg45Oq6MeIWdjKddGhRj3OohsltKgkEU3eLKYtB6qRTQypHHUawCXz88uYt5e3w4V16H lCpSTZV/EVHnNe45FVBlvK7k7HFfDDkryLkCDQRZuCyIARAAlq0slcsVboY/+IUJdcbEiJRW be9HKVz4SUchq0z9MZPX/0dcnvz/gkyYA+OuM78dNS7Mbby5dTvOqfpLJfCuhaNYOhlE0wY+ 1T6Tf1f4c/uA3U/YiadukQ3+6TJuYGAdRZD5EqYFIkreARTVWg87N9g0fT9BEqLw9lJtEGDY EWUE7L++B8o4uu3LQFEYxcrb4K/WKmgtmFcm77s0IKDrfcX4doV92QTIpLiRxcOmCC/OCYuO jB1oaaqXQzZrCutXRK0L5XN1Y1PYjIrEzHMIXmCDlLYnpFkK+itlXwlE2ZQxkfMruCWdQXye syl2fynAe8hvp7Mms9qU2r2K9EcJiR5N1t1C2/kTKNUhcRv7Yd/vwusK7BqJbhlng5ZgRx0m WxdntU/JLEntz3QBsBsWM9Y9wf2V4tLv6/DuDBta781RsCB/UrU2zNuOEkSixlUiHxw1dccI 6CVlaWkkJBxmHX22GdDFrcjvwMNIbbyfQLuBq6IOh8nvu9vuItup7qemDG3Ms6TVwA7BD3j+ 3fGprtyW8Fd/RR2bW2+LWkMrqHffAr6Y6V3h5kd2G9Q8ZWpEJk+LG6Mk3fhZhmCnHhDu6CwN MeUvxXDVO+fqc3JjFm5OxhmfVeJKrbCEUJyM8ESWLoNHLqjywdZga4Q7P12g8DUQ1mRxYg/L HgZY3zfKOqcAEQEAAYkCPAQYAQgAJhYhBL4sQ7ueBCdcDgGOvnfybtNRzXKPBQJZuCyIAhsM BQkFo5qAAAoJEHfybtNRzXKPBVwQAKfFy9P7N3OsLDMB56A4Kf+ZT+d5cIx0Yiaf4n6w7m3i ImHHHk9FIetI4Xe54a2IXh4Bq5UkAGY0667eIs+Z1Ea6I2i27Sdo7DxGwq09Qnm/Y65ADvXs 3aBvokCcm7FsM1wky395m8xUos1681oV5oxgqeRI8/76qy0hD9WR65UW+HQgZRIcIjSel9vR XDaD2HLGPTTGr7u4v00UeTMs6qvPsa2PJagogrKY8RXdFtXvweQFz78NbXhluwix2Tb9ETPk LIpDrtzV73CaE2aqBG/KrboXT2C67BgFtnk7T7Y7iKq4/XvEdDWscz2wws91BOXuMMd4c/c4 OmGW9m3RBLufFrOag1q5yUS9QbFfyqL6dftJP3Zq/xe+mr7sbWbhPVCQFrH3r26mpmy841ym dwQnNcsbIGiBASBSKksOvIDYKa2Wy8htPmWFTEOPRpFXdGQ27awcjjnB42nngyCK5ukZDHi6 w0qK5DNQQCkiweevCIC6wc3p67jl1EMFY5+z+zdTPb3h7LeVnGqW0qBQl99vVFgzLxchKcl0 R/paSFgwqXCZhAKMuUHncJuynDOP7z5LirUeFI8qsBAJi1rXpQoLJTVcW72swZ42IdPiboqx NbTMiNOiE36GqMcTPfKylCbF45JNX4nF9ElM0E+Y8gi4cizJYBRr2FBJgay0b9Cp Message-ID: Date: Thu, 14 May 2020 14:16:15 +0300 User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:60.0) Gecko/20100101 Firefox/60.0 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20200513144257.GA24239@raichu> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 49N89w50n8z4BwL X-Spamd-Bar: -- Authentication-Results: mx1.freebsd.org; dkim=none; dmarc=none; spf=pass (mx1.freebsd.org: domain of agapon@gmail.com designates 209.85.208.195 as permitted sender) smtp.mailfrom=agapon@gmail.com X-Spamd-Result: default: False [-2.06 / 15.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCVD_TLS_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[3]; R_SPF_ALLOW(-0.20)[+ip4:209.85.128.0/17:c]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_GOOD(-0.10)[text/plain]; MIME_TRACE(0.00)[0:+]; DMARC_NA(0.00)[FreeBSD.org]; TO_DN_SOME(0.00)[]; NEURAL_HAM_LONG(-1.00)[-0.998,0]; RCVD_COUNT_THREE(0.00)[3]; IP_SCORE(-0.09)[ip: (0.40), ipnet: 209.85.128.0/17(-0.39), asn: 15169(-0.42), country: US(-0.05)]; RCVD_IN_DNSWL_NONE(0.00)[195.208.85.209.list.dnswl.org : 127.0.5.0]; NEURAL_HAM_MEDIUM(-0.97)[-0.971,0]; FORGED_SENDER(0.30)[avg@FreeBSD.org,agapon@gmail.com]; RWL_MAILSPIKE_POSSIBLE(0.00)[195.208.85.209.rep.mailspike.net : 127.0.0.17]; R_DKIM_NA(0.00)[]; FREEMAIL_ENVFROM(0.00)[gmail.com]; ASN(0.00)[asn:15169, ipnet:209.85.128.0/17, country:US]; FROM_NEQ_ENVFROM(0.00)[avg@FreeBSD.org,agapon@gmail.com]; MID_RHS_MATCH_FROM(0.00)[]; RECEIVED_SPAMHAUS_PBL(0.00)[96.151.72.93.khpj7ygk5idzvmvt5x4ziurxhy.zen.dq.spamhaus.net : 127.0.0.10] X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.33 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 14 May 2020 11:16:21 -0000 On 13/05/2020 17:42, Mark Johnston wrote: > On Wed, May 13, 2020 at 10:45:24AM +0300, Andriy Gapon wrote: >> On 13/05/2020 10:35, Andriy Gapon wrote: >>> In r329363 I re-worked zfs_getpages and introduced range locking to it. >>> At the time I believed that it was safe and maybe it was, please see the commit >>> message. >>> There, indeed, have been many performance / concurrency improvements to the VM >>> system and r358443 is one of them. >> >> Thinking more about it, it could be r352176. >> I think that vm_page_grab_valid (and later vm_page_grab_valid_unlocked) are not >> equivalent to the code that they replaced. >> The original code would check valid field before any locking and it would >> attempt any locking / busing if a page is invalid. The object was required to >> be locked though. >> The new code tries to busy the page in any case. >> >>> I am not sure how to resolve the problem best. Maybe someone who knows the >>> latest VM code better than me can comment on my assumptions stated in the commit >>> message. > > The general trend has been to use the page busy lock as the single point > of synchronization for per-page state. As you noted, updates to the > valid bits were previously interlocked by the object lock, but this is > coarse-grained and hurts concurrency. I think you are right that the > range locking in getpages was ok before the recent change, but it seems > preferable to try and address this in ZFS. > >>> In illumos (and, I think, in OpenZFS/ZoL) they don't have the range locking in >>> this corner of the code because of a similar deadlock a long time ago. > > Do they just not implement readahead? I think so, but not 100% sure. I recall seeing a comment in illumos code that they do not care about read-ahead because there is ZFS prefetch and the data will be cached in ARC. That makes sense from the I/O point of view, but it does not help with page faults. > Can you explain exactly what the > range lock accomplishes here - is it entirely to ensure that znode block > size remains stable? As far as I can recall, this is the reason indeed. -- Andriy Gapon