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b=Tkvjsm6P5dTNxmB2HtLkhw2yZ0Sbdm1seN6XgKwQDENk+NheqBT2lAwh7xiZUBfp7xGn2J z1hIW8REyhDku9359Ox9H6l0BwsjYK5aqqZoQGjga3SRlcOlt7NUh06Zy/EWgs27LzU35b Xf7gdb7mKRDYlQnA53LNdqmhb5Wuq9o= X-Rspamd-Queue-Id: 8FA292005E X-Rspam-User: Authentication-Results: imf31.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=Iykvdeq1; spf=pass (imf31.hostedemail.com: domain of jhubbard@nvidia.com designates 40.107.93.75 as permitted sender) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com; arc=pass ("microsoft.com:s=arcselector9901:i=1") X-Rspamd-Server: rspam03 X-Stat-Signature: tjz71g4ny78opkkheybqfrrjtr95uzik X-HE-Tag: 1661979582-774881 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: On 8/31/22 12:43, Yang Shi wrote: >>> It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC. >>> >>> So maybe we should implement pmdp_collapse_flush() for those arches to >>> issue IPI. >> >> ... or find another way to detect and handle this in GUP-fast? >> >> Not sure if, for handling PMDs, it could be sufficient to propagate the >> pmdp pointer + value and double check that the values didn't change. > > Should work too, right before pinning the page. > > pmdp_collapse_flush() is actually just called by khugepaged, so arch > specific implementation should not be a problem and we avoid making > gup fast more complicated. > And just to pile on, about that gup fast complexity: depending upon IPIs added a lot of complexity, not just because of the IPI dependency, but more importantly because only some arches even *have* IPIs. So an entirely different set of reasoning has to be used *in addition* to working through the IPI story. And sure enough, we can see the fallout: you are uncovering lots of half-correct comments in that area. So getting rid of the dependency on IPIs in gup fast would go a long way to simplifying it, and maybe even improving overall CPU load (insert some hand-wavy notes here about IPIs being worse than things like RCU). But the real win is in the complexity reduction in gup fast. thanks, -- John Hubbard NVIDIA