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AJvYcCXluHwW4ZwTTSwr3EFsMp/j+F8L9CEpH9rsQDwD1mZ6fyu1TpLqVWyuABDx3MM1qGBy8Krgk9PuLxw4u1UDatCmn6g= X-Gm-Message-State: AOJu0Yw4tKNI1MrA9uFPywBM+bAKT0jQLxUTXTcLZTSnYJB0j6GDJhwO SA8bwbrkJgt0dZhGAkgVMhno5b0hYSt1L4ppgzcjknwPpiqaoZMtbJDMKcszdbQlUP5goQcM4xo Xb5sz1VKh7FRX8HVIfhkEaITwvjLLoEiVqyjjwLa0Gp7SezVsrDt2UF0= X-Google-Smtp-Source: AGHT+IGxmu8tZ2auf5n6LXuKbVU3mYUSyn5t8jODyhg2T6wGz22bjhxnnh9DN4PNnVS8ldrkThRfjtmeyWF/vffwUfg= X-Received: by 2002:a05:6512:2823:b0:512:fe1f:da5b with SMTP id cf35-20020a056512282300b00512fe1fda5bmr2306933lfb.57.1710020266087; Sat, 09 Mar 2024 13:37:46 -0800 (PST) MIME-Version: 1.0 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> In-Reply-To: From: Yosry Ahmed Date: Sat, 9 Mar 2024 13:37:06 -0800 Message-ID: Subject: Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during context switching To: "Kirill A. Shutemov" 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 2795E180012 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: wzcxetuzpq9188tbiduyb93d7q4ka3zz X-HE-Tag: 1710020267-267791 X-HE-Meta: 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 wY4jn6mR d3loqsiVzybBn0+8KUJPqUGQVyvOswEyFX7xCBSpW6Pr9FYI= 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 9, 2024 at 8:34=E2=80=AFAM 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 thi= s > > 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?