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Tue, 04 Jun 2024 10:35:15 -0700 (PDT) MIME-Version: 1.0 References: <0edfcfed-e8c4-4c46-bbce-528c07084792@redhat.com> <8da12503-839d-459f-a2fa-4abd6d21935d@redhat.com> <9d9a5730-161b-4a9d-a696-1cf6d0c5123c@redhat.com> <54ce029f-b16f-4607-bdf1-a1efe904029a@redhat.com> <821cf1d6-92b9-4ac4-bacc-d8f2364ac14f@paulmck-laptop> In-Reply-To: <821cf1d6-92b9-4ac4-bacc-d8f2364ac14f@paulmck-laptop> From: Yang Shi Date: Tue, 4 Jun 2024 10:35:04 -0700 Message-ID: Subject: Re: [linus:master] [mm] efa7df3e3b: kernel_BUG_at_include/linux/page_ref.h To: paulmck@kernel.org Cc: David Hildenbrand , Peter Xu , kernel test robot , Jason Gunthorpe , Vivek Kasireddy , Rik van Riel , oe-lkp@lists.linux.dev, lkp@intel.com, linux-kernel@vger.kernel.org, Andrew Morton , Matthew Wilcox , Christopher Lameter , linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E7A34A0024 X-Stat-Signature: 157dheow4hsogodmqdyhrk1eq568qw9g X-Rspam-User: X-HE-Tag: 1717522517-158863 X-HE-Meta: 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 8Q4STc1N axmfNN1AJjYJVfZTknFAj/64zslK+i4ze/fhGLvIuV4CVpaJie9+jdZBF62eKb2Hc3NAJ96UEk95BgrT6A01afynEFLZJD9odBvPhjgfo0LcRHinEz/JUti69QbP90q8KoRlqnQQSvl8FZjr/NJp/YRnQud2XYltvGb6TEpf7pbKWX3/1xxM+1z9lh5C02cXnw7YGK/3hiOFZaFX+Wsu/B7DowQqbyiUMsMghsBLrH5PBxIoXY7Z1PcEkX04jmWuPLsa05+Z6AaTjyUANh7d4o7HclziRDnQhq2OhyEbJmh5ZiNKOGHWwuhf74Bjgw/hHu2wonGZOhKIM//J5xFmARBkHtow5P2atrm7fLNcXQhU/rZlxeF/7YfXwh5nXS0nlGozBdtwM9zEMCI8n/zuOxiUJH0c6u0X8U2D07uwR768XOAW4LG6QizbL8RFi3RCoLpnNDZrGSOfulwU= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > > > > I chased it further to: > > > > commit 8375ad98cc1defc36adf4a77d9ea1e71db51a371 > > Author: Paul E. McKenney > > Date: Mon Apr 29 15:06:13 2013 -0700 > > > > vm: adjust ifdef for TINY_RCU > > There is an ifdef in page_cache_get_speculative() that checks for !SMP > > and TREE_RCU, which has been an impossible combination since the advent > > of TINY_RCU. The ifdef enables a fastpath that is valid when preemption > > is disabled by rcu_read_lock() in UP systems, which is the case when > > TINY_RCU is enabled. This commit therefore adjusts the ifdef to > > generate the fastpath when TINY_RCU is enabled. > > > > > > Where Paul explicitly restored that fastpath for TINY_RCU instead of removing that code. > > > > So maybe Paul can comment if that is still worth having. CCing him. > > It is currently an atomic operation either way, though the folio_ref_add() > avoids full ordering, but that is immaterial on x86. Some say that it is > in the noise on server-class ARMv8 as well, though they have also said > a great many things in the past. But if that is true, the big benefit > of the TINY_RCU check is that folio_ref_try_add_rcu() is guaranted not > to fail in that case (single CPU with preemption disabled). Except that > everyone has to check the return value anyway, right? > > So the usual advice, unsatisfying though it might be, is to remove that > #ifdef and see if anyone notices. > > After all, both 2013 and 2008 were quite some time ago. ;-) Thanks, Paul. I will submit a patch to remove the #ifdef as the fix for the bug report. And do the clean up in a separate patch which is preferred by David. > > Thanx, Paul