From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6B70747D926; Tue, 1 Sep 2026 15:29:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788276555; cv=none; b=m+yfmdIGb80zUyLGtAZPgaoARKQyHtnmwQ7wycK2s1d40kATIOW7DscauxtBaGIbsNC8VkybM1K0OJICiXessFjKcbeUOD/ev7xIhX1MvgUqGjFbDbsz9ltZNdQ8htT9lORAISdF6n6VFKcAW6IlI8+R5g2aaIrOFdlXei2oloc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788276555; c=relaxed/simple; bh=gAuzGRimVjj78jhrlRXNHnfbradPAO79wFfbtCzp4s8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=bMJgZiWDJj6lqGlgWWZgZ5vvt77fByjfr7rOnCVVVkCgZVRankO11GcHOIFJl4TCmHAHn9fGK7oYBsaZ1TAJ8lGUBdfECjropaHK5YLHSW1vdWEQ0Vr83JUSPGFityMDgyASn+flxLJ0Fs8meb3gD1+EjnMam1WWkEOE1Q+5h14= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LfcivOSb; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LfcivOSb" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9BDC81F00A3E; Tue, 1 Sep 2026 15:29:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788276554; bh=76k3ebeTiGHZdNKuTSQi/NWwAeUMZ+iwU02Jwt/dttM=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=LfcivOSbSNA9RWVTIQCS5VwJvAly6t4EmMGccL6I3BGCEmGObVhvyZieZT4g4qACu aF51VLZtUbROnSOzeEvxDMjG+n+AzoLaZbPVdTLdwrIBkE+OWwB9HDvjwc8z31Wqks QMM+8lXtjv5Zy4+t01MHdoR+BxHu/MXx89XoVrRr23n77vHGxSgtOyoYOCSJwtq0Ik eiH/kjr7kQ5u1gsg4hTaYnTmyBJSsG/LqsXooivMk5Apd9mKIEO/Jb/6DBBs9c7EiB exzbv2gAB9y1+8NDMAJrFR/UPThN3lS5azkUwjKYYsp4zBdbvM0ujbS66XryEQ/87l Pv43kYheGE4aw== Received: from phl-compute-05.internal (phl-compute-05.internal [10.202.2.45]) by mailfauth.ams.internal (Postfix) with ESMTP id 058231980054; Tue, 1 Sep 2026 11:28:51 -0400 (EDT) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-05.internal (MEProxy); Tue, 01 Sep 2026 11:29:07 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: dmFkZTFktg/hTQ+O6MbEiEwc2myH07+rMXf0DZqVtTUmRXg4RwvnbNppg3BbYl6I9K2dJp zj2kvOaUXcu2ztpPSJQZ8Ogheq7n6crddipwfoV80/nKOVQCCUOZnwgAEYU3Av2yZmZYHP 7I24oVqb6S67vLQnUUVvqgo9Ogx60q3Zj3bOCLUAsSUEIPZoTe4V2V2wqRJqBwtgC7FsoJ 6rYeXjf2LD4UuiBTZ1hgiB6DybGuursalkVC5p+rE3Z61vyySwM3+mUWXx3KxMVsF55pfW Y63Mg9mEwLnf5UI83C+MMBITxZq8Ug1aRj6MAutPXRzdrg4OYxGJGhFXVYpjOziQBAb9RI 1M1hA5OH+Ae4cWmjEpmcov0cbF7J1JTYooBZB1U/4PkwrHTcu5roVcjACm7Wf3tkWztBte OY+5bwnhGsoj0FTHeT0/cau4njSfVbXlJZ1jSVb2nMKJnYqlsW3moVE3n8V2D8es2M3EPy 5xaMi7C2kz6Mg9KuRoNC4hwC7/Re1QxT08Hsh9sEILCxwqWQNx2QWNFYFS3vApiRt/qeEh QZMs8OhwcUjv+4Gjxqp6zUOfl3bCas78PtJvCW7P93kuisSD3Qf06s4bxmIr/Ytjmx/fZ1 Qv+oLYQOOBA+Bv+X1KZMEAcNK1ia85teinZBzPxGV9H8TQ/W36HCzjE2uc7g X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Tue, 1 Sep 2026 11:28:49 -0400 (EDT) Date: Tue, 1 Sep 2026 16:28:48 +0100 From: Kiryl Shutsemau To: "Lorenzo Stoakes (ARM)" Cc: Jason Gunthorpe , Andrew Morton , David Hildenbrand , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Guo Ren , Brian Cain , Geert Uytterhoeven , Dinh Nguyen , Simon Schuster , Jonas Bonn , Stefan Kristiansson , Stafford Horne , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Russell King , Vineet Gupta , Michal Simek , Chris Zankel , Max Filippov , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , "David S. Miller" , Andreas Larsson , Richard Henderson , Matt Turner , Magnus Lindholm , Catalin Marinas , Mark Rutland , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , "James E.J. Bottomley" , Helge Deller , Madhavan Srinivasan , Michael Ellerman , "Christophe Leroy (CS GROUP)" , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Richard Weinberger , Anton Ivanov , Johannes Berg , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Arnd Bergmann , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , John Hubbard , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org, linux-sh@vger.kernel.org, linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-snps-arc@lists.infradead.org, linux-arch@vger.kernel.org, sparclinux@vger.kernel.org, linux-alpha@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-um@lists.infradead.org, Hugh Dickins , Qi Zheng Subject: Re: [PATCH 01/12] mm/huge_memory: zap deposited page tables after an RCU grace period Message-ID: References: <20260901-rcu-pagetable-freeing-v1-0-5456a81c8212@kernel.org> <20260901-rcu-pagetable-freeing-v1-1-5456a81c8212@kernel.org> <20260901142408.GA56830@ziepe.ca> Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Sep 01, 2026 at 03:41:17PM +0100, Lorenzo Stoakes (ARM) wrote: > On Tue, Sep 01, 2026 at 11:24:08AM -0300, Jason Gunthorpe wrote: > > On Tue, Sep 01, 2026 at 03:12:45PM +0100, Lorenzo Stoakes (ARM) wrote: > > > > > It won't be costly at the time of the calls obviously as its deferred. Maybe > > > increase some time spent in softirq but again is 512x that big of a deal? > > > > > > I'm not sure how you'd both defer the free and somehow utilise mmu_gather here > > > either really, certainly not without it becoming extremely messy. > > > > The less costly version is to thread the page to be freed onto the > > mmu_gather through a linked list in the struct page memory. This is > > super cheap since it is just a singly linked list operation. > > > > Then when the mmu_gather is flushed it does a single call_rcu using > > the rcu head of the struct page of the head of the list. The callback > > clears the entire linked list of pages. > > > > Since you have to tlb flush anyhow, it makes sense to always use the > > mmu_gather. For example the design I ended up with for iommupt > > accumulates all the invalidations and all the free-able memory into a > > gather then invalidates and frees. > > > > This allows maximizing the tlbi efficiency too. You can't do call_srcu > > until you flush the tlb and if you call once per table then you are > > also tlb flushing once per table too. > > > > So if the kernel really does want to clear out 512 leaf tables the > > optimal implementation is one range tlbi for 512 entries followed by > > one call_rcu to free the memory. Hence the gather.. > > I think there's some confusion here. > > This isn't the path in which a page table is being freed, the _deposited_ > table is zapped, in zap_deposited_table(). > > That is, the page table kept in reserve for THP split, that is not > currently mapped. > > It amounts to a __free_pages() call. > > The TLB operations are in e.g. zap_huge_pmd() etc. and nobody has > complained about inefficiencies there. > > So, unless I'm missing something here, TLB flushes play no role in this > whatsoever. > > The issue Kiryl raised was that instead of immediately freeing page tables, > they are now batched up individually by call_rcu(). > > I personally find it difficult to imagine the numbers here would be > problematic or certainly cause anything observable beyond what is > observable now. > > So I'm going to have to say, unless it can be clearly demonstrated this is > problematic, I don't think there's any reason to add additional complexity > here. It would be nice to measure munmap() overhead here. I am worried about hitting DEFAULT_MAX_RCU_BLIMIT and trigger rcu_force_quiescent_state() which can be disruptive to the system. DEFAULT_MAX_RCU_BLIMIT is 10K, so it is ~20G of THP unmapped on x86. munmap() of 64G worth of THP should be enough to demonstrate the problem. -- Kiryl Shutsemau / Kirill A. Shutemov