From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 15E161FC3; Fri, 24 Oct 2025 12:13:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761308029; cv=none; b=tLouTISTxspmwPd6ut101FNEp2FSviy/W2mujysL7GHYmF6y+B7aoIxO9YzJO5jPRisBh4tHBFMa5tAXPnelihji8IrbC0qDFojVO7r7ol4rUbpoJgXgGkLdytxv/cbJzp3akan4dce4QbcdX/3ooHellNV1u//2qVTgqAtNrhM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761308029; c=relaxed/simple; bh=X3sXu4j8MfMq52gqBAavsMTcndSosYtjB52S1c8LUjs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=EN+RDv7kBWDzrEj9xItGj4uae7MN6v81d0BtxwG5xZU5UcId1jtNY5Ge2o2UkOtDc2h+h/9ydgnpOgRl9D2iB+bTNsRQG/jdKYKjE3lHsyBLP3ksjZxGpPetDT2g1exU4a+JONK0cD1gO2DzoH4eP/avGZlxcXVgBFDJLRRdVis= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5EA231515; Fri, 24 Oct 2025 05:13:38 -0700 (PDT) Received: from [10.44.160.74] (e126510-lin.lund.arm.com [10.44.160.74]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6C2833F66E; Fri, 24 Oct 2025 05:13:38 -0700 (PDT) Message-ID: <390e41ae-4b66-40c1-935f-7a1794ba0b71@arm.com> Date: Fri, 24 Oct 2025 14:13:35 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 06/13] mm: introduce generic lazy_mmu helpers To: David Hildenbrand , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Alexander Gordeev , Andreas Larsson , Andrew Morton , Boris Ostrovsky , Borislav Petkov , Catalin Marinas , Christophe Leroy , Dave Hansen , "David S. Miller" , "H. Peter Anvin" , Ingo Molnar , Jann Horn , Juergen Gross , "Liam R. Howlett" , Lorenzo Stoakes , Madhavan Srinivasan , Michael Ellerman , Michal Hocko , Mike Rapoport , Nicholas Piggin , Peter Zijlstra , Ryan Roberts , Suren Baghdasaryan , Thomas Gleixner , Vlastimil Babka , Will Deacon , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org References: <20251015082727.2395128-1-kevin.brodsky@arm.com> <20251015082727.2395128-7-kevin.brodsky@arm.com> <73b274b7-f419-4e2e-8620-d557bac30dc2@redhat.com> Content-Language: en-GB From: Kevin Brodsky In-Reply-To: <73b274b7-f419-4e2e-8620-d557bac30dc2@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 23/10/2025 21:52, David Hildenbrand wrote: > On 15.10.25 10:27, Kevin Brodsky wrote: >> [...] >> >> * madvise_*_pte_range() call arch_leave() in multiple paths, some >>    followed by an immediate exit/rescheduling and some followed by a >>    conditional exit. These functions assume that they are called >>    with lazy MMU disabled and we cannot simply use pause()/resume() >>    to address that. This patch leaves the situation unchanged by >>    calling enable()/disable() in all cases. > > I'm confused, the function simply does > > (a) enables lazy mmu > (b) does something on the page table > (c) disables lazy mmu > (d) does something expensive (split folio -> take sleepable locks, >     flushes tlb) > (e) go to (a) That step is conditional: we exit right away if pte_offset_map_lock() fails. The fundamental issue is that pause() must always be matched with resume(), but as those functions look today there is no situation where a pause() would always be matched with a resume(). Alternatively it should be possible to pause(), unconditionally resume() after the expensive operations are done and then leave() right away in case of failure. It requires restructuring and might look a bit strange, but can be done if you think it's justified. > > Why would we use enable/disable instead? > >> >> * x86/Xen is currently the only case where explicit handling is >>    required for lazy MMU when context-switching. This is purely an >>    implementation detail and using the generic lazy_mmu_mode_* >>    functions would cause trouble when nesting support is introduced, >>    because the generic functions must be called from the current task. >>    For that reason we still use arch_leave() and arch_enter() there. > > How does this interact with patch #11?  It is a requirement for patch 11, in fact. If we called disable() when switching out a task, then lazy_mmu_state.enabled would (most likely) be false when scheduling it again. By calling the arch_* helpers when context-switching, we ensure lazy_mmu_state remains unchanged. This is consistent with what happens on all other architectures (which don't do anything about lazy_mmu when context-switching). lazy_mmu_state is the lazy MMU status *when the task is scheduled*, and should be preserved on a context-switch. > >> >> Note: x86 calls arch_flush_lazy_mmu_mode() unconditionally in a few >> places, but only defines it if PARAVIRT_XXL is selected, and we are >> removing the fallback in . Add a new fallback >> definition to to keep things building. > > I can see a call in __kernel_map_pages() and > arch_kmap_local_post_map()/arch_kmap_local_post_unmap(). > > I guess that is ... harmless/irrelevant in the context of this series? It should be. arch_flush_lazy_mmu_mode() was only used by x86 before this series; we're adding new calls to it from the generic layer, but existing x86 calls shouldn't be affected. - Kevin