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s=arc-20220608; t=1740055260; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=E7B6H9AVIkT1FMFJfXdA/Lfv+V36svDo3jwy4sJTt9I=; b=PrF4ruNpewvoUEtsyU/uUq+upxl7Sbl1okwQSXSeBR+WZAgzwftfO4g3mdkAG6Cizfbock YHsnM96xfkjDO1+lqkFR2gOV7Pt9B+Terf8PR0AEEmHc+b7ahANcPC6xxMIkmwJwaUuP4Q gd3Zmpv0Usk6Uf06RmsQHTfpOhrnH6I= X-AuditID: a67dfc5b-3e1ff7000001d7ae-dc-67b722d7adf2 Date: Thu, 20 Feb 2025 21:40:50 +0900 From: Byungchul Park To: Hillf Danton , torvalds@linux-foundation.org Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com Subject: Re: [RFC PATCH v12 00/26] LUF(Lazy Unmap Flush) reducing tlb numbers over 90% Message-ID: <20250220124050.GB8305@system.software.com> References: <20250220052027.58847-1-byungchul@sk.com> <20250220103223.2360-1-hdanton@sina.com> <20250220114920.2383-1-hdanton@sina.com> MIME-Version: 1.0 Content-Type: text/plain; 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However, I tagged RFC again because there are > > > > still issues that should be resolved to merge to mainline: > > > > > > > > 1. Even though system wide total cpu time for TLB shootdown is > > > > reduced over 95%, page allocation paths should take additional cpu > > > > time shifted from page reclaim to perform TLB shootdown. > > > > > > > > 2. We need luf debug feature to detect when luf goes wrong by any > > > > chance. I implemented just a draft version that checks the sanity > > > > on mkwrite(), kmap(), and so on. I need to gather better ideas > > > > to improve the debug feature. > > > > > > > > --- > > > > > > > > Hi everyone, > > > > > > > > While I'm working with a tiered memory system e.g. CXL memory, I have > > > > been facing migration overhead esp. tlb shootdown on promotion or > > > > demotion between different tiers. Yeah.. most tlb shootdowns on > > > > migration through hinting fault can be avoided thanks to Huang Ying's > > > > work, commit 4d4b6d66db ("mm,unmap: avoid flushing tlb in batch if PTE > > > > is inaccessible"). > > > > > > > > However, it's only for migration through hinting fault. I thought it'd > > > > be much better if we have a general mechanism to reduce all the tlb > > > > numbers that we can apply to any unmap code, that we normally believe > > > > tlb flush should be followed. > > > > > > > > I'm suggesting a new mechanism, LUF(Lazy Unmap Flush), that defers tlb > > > > flush until folios that have been unmapped and freed, eventually get > > > > allocated again. It's safe for folios that had been mapped read-only > > > > and were unmapped, as long as the contents of the folios don't change > > > > while staying in pcp or buddy so we can still read the data through the > > > > stale tlb entries. > > > > > > > Given pcp or buddy, you are opening window for use after free which makes > > > no sense in 99% cases. > > > > Just in case that I don't understand what you meant and for better > > understanding, can you provide a simple and problematic example from > > the u-a-f? > > > Tell us if it is illegal to commit rape without pregnancy in your home town? +to Torvalds Logical blame is welcome but I don't want to see potty-mouthed busters like him in Linux community any more. Please, ban him for better community. Byungchul > PS defering flushing tlb [1,2] is no go. > > Subject: Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs > [1] https://lore.kernel.org/lkml/20250127155146.GB25757@willie-the-truck/ > [2] https://lore.kernel.org/lkml/xhsmhwmdwihte.mognet@vschneid-thinkpadt14sgen2i.remote.csb/