On Thu, Mar 07, 2024 at 07:22:36PM +0200, Kirill A. Shutemov wrote: > On Thu, Mar 07, 2024 at 01:39:14PM +0000, Yosry Ahmed wrote: > > In switch_mm_irqs_off(), we read the 'mm->context.lam_cr3_mask' into > > 'new_lam', which is later passed to load_new_mm_cr3(). However, there is > > a call to set_tlbstate_lam_mode() in between which will read > > 'mm->context.lam_cr3_mask' again and set 'cpu_tlbstate.lam' accordingly. > > If we race with another thread updating 'mm->context.lam_cr3_mask', the > > value in 'cpu_tlbstate.lam' could end up being different from CR3. > > What other thread? LAM can only be enabled when the process has single > thread. And cannot be disabled. See MM_CONTEXT_LOCK_LAM. Right, but a kthread may run with that single-threaded process's mm IIUC. I think this can happen via kthread_use_mm() or if we context switch directly from the user process to the kthread (context_switch() doesn't seem to update the mm in this case). > > > While we are at it, remove the misguiding comment that states that > > 'new_lam' may not match tlbstate_lam_cr3_mask() if a race occurs. > > The comment is indeed misguiding, but for different reason. It is leftover > from the earlier version of LAM patchset.
