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Shutemov" To: Yosry Ahmed Cc: Dave Hansen , Andy Lutomirski , Andrew Morton , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "Peter Zijlstra (Intel)" , the arch/x86 maintainers , linux-mm@kvack.org, Linux Kernel Mailing List Subject: Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during context switching Message-ID: References: <20240307133916.3782068-1-yosryahmed@google.com> <20240307133916.3782068-3-yosryahmed@google.com> <420fcb06-c3c3-4e8f-a82d-be2fb2ef444d@app.fastmail.com> <35b670e2-9ef5-4d3a-b6ea-f8016dfa088d@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Stat-Signature: 5bimr1a4jn1xpd4himi7b8x7qxemuaon X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 0BBD880021 X-Rspam-User: X-HE-Tag: 1710160942-109196 X-HE-Meta: U2FsdGVkX1/om6DxEtg3UgAWlTy5FE3Eoy2yfmY7GuQ2IartLLFJGwK5tAo/is1SoxKwz7Bmg93nCG82kSM7wz1RJYFMaR4aCcvj/Zg9ll3OD415CUceUzkPejMITwlw5rkFNYz/YQ++SWMwjQkM735yy+lDSaHyP794V+6ylGAZhBSqGz8uWAph4uFOro6OkUAsq5SqxXggDuJNbiJ5geXK1aKEOHjH94tSXPyeJpmw5JakaQjNpcm/tOSaQMb6/iUJ7DyWl520prGdJQNXJwFq1R8j9OpVDd5AoAqzT073GWTR+1zSyZpEFdmQOQwBqjkRh7534EkbdRe51FfiKtKJeJIpzmK4hY7MgpETrOxIq6k8XIB2HeyHcHqmsBEBQmIcsL2roIoD60vsZM9hziPnef5FVyhqdrD+aU949xic025Pe5ThIDg5pd1q5u1BZKeve44S/1jkYcfJoJTqGhF1vciURiFsXQiLUswGRbdA+AfNBcP1q0xS/wGf66xwG6AV4an9HGrmdqBL1sysgW+QSTB0mUBNJpPs+qK0vQjCCFuh0wN1O0jBbQtvN/YOkkT7sDykiCIplqMxUnNjPquiw+bqC1mbPWdMPFzFsK/2EmdJydG4Y2/StUBnOPTNstUv77q3FsDjM6anDtHPyZLD0aYYW5APTIiMlJoVP6PBi2G3dpllQ6wW2HK725GpxVrwldNPiS7toc+sheQAtp/DURRXVn8yNWgFlxxgbzg1BXKhYFEScMuuh9iXnXAUnva86gQy0LgNgHM/yhf3om3HexqdkCG5EmZ5mloOyXc63AT1KtgGxXVTDU8FO/Fy3WlYu9/6DvttxFdES+bR0Vx57etxh2nueedVbXZkOJFmpPFRIum6HGYnambYqVCGhoDmJpDGtn05Kj11zaUsrIb28iCQeZbj28jl6SaIHTFjNh5zdIZyHcPIvoXKvdzAwkkiMNzUBWy6e9MgrLA YD7/iUTl 57uOqcrgxftmPAxoSFml1mAM7rv7v9QHwypMT3PKwvI9UaqjHZqUbicyjn7QoPJf2gL5DV4yp9wDaj4bOYaYEkIq1tA8wFqyHrQci3EO6mT+MpmksY9qsnzRFr+VwYGSER7AMgbd+DzTAED1JC9tsV3U5sg== 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: List-Subscribe: List-Unsubscribe: On Sat, Mar 09, 2024 at 01:37:06PM -0800, Yosry Ahmed wrote: > On Sat, Mar 9, 2024 at 8:34 AM Kirill A. Shutemov > wrote: > > > > On Sat, Mar 09, 2024 at 02:19:19AM +0000, Yosry Ahmed wrote: > > > I don't see how skipping set_tlbstate_lam_mode() for kthreads fixes this > > > problem. Do you mind elaborating? > > > > Define what problem is. > > > > Yes, in this scenario kthread gets more permissive LAM mode than it needs. > > But nothing breaks. > > > The problem here is not how the kthread runs at all. It is the fact > that if that kthread context switches into the user process that has > enabled LAM, it may not update CR3 because the mm doesn't change. > switch_mm_irqs_off() will only update CR3 in this case if there is a > pending TLB flush. Otherwise, we just return, even if the LAM for this > mm has changed. > > This can cause the process that has enabled LAM to run with LAM > disabled and fault on tagged addresses, right? Did I miss something? You are right. I think IPI is the way to go. Will you prepare a patch? -- Kiryl Shutsemau / Kirill A. Shutemov