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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id m8-20020adffe48000000b002c55521903bsm1942945wrs.51.2023.04.06.07.42.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Apr 2023 07:42:04 -0700 (PDT) Message-ID: <1654e2d5-5a32-a253-e335-0ee42f69f5ef@redhat.com> Date: Thu, 6 Apr 2023 16:42:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.1 Content-Language: en-US To: Peter Zijlstra , Marcelo Tosatti Cc: Frederic Weisbecker , Yair Podemsky , linux@armlinux.org.uk, mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, will@kernel.org, aneesh.kumar@linux.ibm.com, akpm@linux-foundation.org, arnd@arndb.de, keescook@chromium.org, paulmck@kernel.org, jpoimboe@kernel.org, samitolvanen@google.com, ardb@kernel.org, juerg.haefliger@canonical.com, rmk+kernel@armlinux.org.uk, geert+renesas@glider.be, tony@atomide.com, linus.walleij@linaro.org, sebastian.reichel@collabora.com, nick.hawkins@hpe.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, vschneid@redhat.com, dhildenb@redhat.com, alougovs@redhat.com, jannh@google.com References: <20230404134224.137038-1-ypodemsk@redhat.com> <20230404134224.137038-4-ypodemsk@redhat.com> <20230405195226.GB365912@hirez.programming.kicks-ass.net> <20230406132928.GM386572@hirez.programming.kicks-ass.net> <20230406140423.GA386634@hirez.programming.kicks-ass.net> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI only to CPUs in kernel mode In-Reply-To: <20230406140423.GA386634@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On 06.04.23 16:04, Peter Zijlstra wrote: > On Thu, Apr 06, 2023 at 03:29:28PM +0200, Peter Zijlstra wrote: >> On Thu, Apr 06, 2023 at 09:38:50AM -0300, Marcelo Tosatti wrote: >> >>>> To actually hit this path you're doing something really dodgy. >>> >>> Apparently khugepaged is using the same infrastructure: >>> >>> $ grep tlb_remove_table khugepaged.c >>> tlb_remove_table_sync_one(); >>> tlb_remove_table_sync_one(); >>> >>> So just enabling khugepaged will hit that path. >> >> Urgh, WTF.. >> >> Let me go read that stuff :/ > > At the very least the one on collapse_and_free_pmd() could easily become > a call_rcu() based free. > > I'm not sure I'm following what collapse_huge_page() does just yet. It wants to replace a leaf page table by a THP (Transparent Huge Page mapped by a PMD). So we want to rip out a leaf page table while other code (GUP-fast) might still be walking it. In contrast to freeing the page table, we put it into a list where it can be reuse when having to PTE-map a THP again. Now, similar to after freeing the page table, someone else could reuse that page table and modify it. If we have GUP-fast walking the page table while that is happening, we're in trouble. So we have to make sure GUP-fast is done before enqueuing the now-free page table. That's why the tlb_remove_table_sync_one() was recently added (by Jann IIRC). -- Thanks, David / dhildenb