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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> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Sep 01, 2026 at 04:28:48PM +0100, Kiryl Shutsemau wrote: > 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. I mean you're going to hit that from RCU freeing page tables already, which most architectures already do right? So if RCU saturation is a problem, that problem already exists, but I've not heard of that being a problem at all? So you're going to have to demonstrate why this situation is markedly different from that. And it's the same scale. Overall I think freeing 64 GiB of mapped memory all at once will inevitably be a slow operation, freeing them directly will also be a lengthily process. And also it seems to me that RCU mishandling heavy load to the point of causing system instability should a bug filed with RCU no? Also note pte_free_defer() is already used in retract_page_tables() so a large collapse could also hit this problem? I'm not sure I'm convinced there's an issue here. > > -- > Kiryl Shutsemau / Kirill A. Shutemov -- Cheers, Lorenzo