From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 77BF53BCD14; Mon, 3 Aug 2026 13:59:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785765589; cv=none; b=lm+hWmeqLyOtUmfEUqfxF0+/LmW70sKyuwuJiKaaYMKAlIujB545DLz+KHAT9bSZRpyyhlu3dxi4q/a389S7qR1kNcADyqoZlF9g57RKCjcTgWYt9f58CoyTPmDsQJgetjPMgSQUewv6S+ev15YCqZW+WK5MTUTI9b3pvLT1mxs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785765589; c=relaxed/simple; bh=BMco7cE6ZUb0x1sY/U4XBBksWpvbpHojwOQxtubK6HA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hl5RUgkce6lCHMieWkU+EChNLtQXBCOAwSRdG09e4fZXXpGaxlNC1KXotjKY+8ClocGsCPAgl7QlPw5NKAklGDC7CxgNCImVIh5naguQcVci7oOl1RMu9VuZ86Wr3/jy3iwdzqRCemdnRrBt4QrHef2cyhgkGdfFie8wDCjk8EI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VdJzXRQm; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="VdJzXRQm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6E4B91F000E9; Mon, 3 Aug 2026 13:59:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785765588; bh=JgJjMGVQwXfN1+lfqi/7mzYbORC4vBXwntqjWOpjj5Y=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=VdJzXRQmCKCQ7JgyKiUD1/ILZ8ALjapAPRXEniDDnzS1EoQJOd2Rh4mnj01mS0kaC SkdwvAXDVmUbSsA21mXsGUsV6UFqBCfzHqr7sKaMJIwGAPUZ/EuQ/qXVQNy9gbWQou k9r4ZeSmv7XSpcXC6vcb89age58OUEyz0Y3jtZ3UkudyTNaZ5gI+4+nRjmeVka/o9q /dN7dYJ78mn13G49euOFXvjaEbz3SgPDfvCdEu2cZqtsnpAXKCLf+lhpTv+wU8hwAu 5uCt9p7JGa3sPVxgYgc/nGl9g631hfgqX2Lq234uZ0mjzzpUHJEqevNTssxpWPfwhD A251zj+Snu4hw== Message-ID: <01f07ea2-8bdd-4450-8a49-548f8b646cfd@kernel.org> Date: Mon, 3 Aug 2026 15:59:35 +0200 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 03/22] mm: introduce MMF_KERNEL flag and set it for init_mm To: Kevin Brodsky , linux-mm@kvack.org Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Pasha Tatashin , Russell King , Catalin Marinas , Will Deacon , Ryan Roberts , linux-arm-kernel@lists.infradead.org, Huacai Chen , loongarch@lists.linux.dev, "James E.J. Bottomley" , Helge Deller , linux-parisc@vger.kernel.org, Madhavan Srinivasan , Michael Ellerman , linuxppc-dev@lists.ozlabs.org, Paul Walmsley , Palmer Dabbelt , Albert Ou , linux-riscv@lists.infradead.org, Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Gerald Schaefer , linux-s390@vger.kernel.org, "David S. Miller" , Andreas Larsson , sparclinux@vger.kernel.org, Richard Weinberger , Anton Ivanov , Johannes Berg , linux-um@lists.infradead.org, Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Ning Sun , x86@kernel.org, tboot-devel@lists.sourceforge.net, Ard Biesheuvel , Ilias Apalodimas , linux-efi@vger.kernel.org, Vishal Moola , Alistair Popple , "Matthew Wilcox (Oracle)" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org References: <20260714-remove_pgtable_cdtor-v1-0-44be8a7685d7@arm.com> <20260714-remove_pgtable_cdtor-v1-3-44be8a7685d7@arm.com> Content-Language: fr-FR From: "Christophe Leroy (CS GROUP)" In-Reply-To: <20260714-remove_pgtable_cdtor-v1-3-44be8a7685d7@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Le 14/07/2026 à 16:03, Kevin Brodsky a écrit : > mm code often needs to know whether some mm represents a kernel or > user address space. This is currently done by comparing the mm > pointer with &init_mm; besides not being particularly elegant, this > ignores the fact that other mm's (e.g. efi_mm) may also represent > parts of the kernel address space. > > Introduce a new mm flag MMF_KERNEL and set it for init_mm. > Subsequent patches will use this flag to replace comparisons with > &init_mm. No functional change is introduced for now. Did you consider performance impact ? This test is usually done in quite critical memory handling functions. init_mm is known at link time. Before your patch 08/22 there is just a comparison of mm (r3) with a constant (loaded in r10): c0014048 : c0014048: 3d 40 c1 09 lis r10,-16119 c001404c: 39 4a 03 98 addi r10,r10,920 c0014050: 7c 03 50 00 cmpw r3,r10 c0014054: 4d 82 00 20 beqlr ... After patch 08/22 we have, it first checks that mm is not 0, then it loads the word located at mm+528 then AND it with 0x1. This load might be costly. c0014048 : c0014048: 2c 03 00 00 cmpwi r3,0 c001404c: 7c 85 23 78 mr r5,r4 c0014050: 41 82 00 10 beq c0014060 c0014054: 81 23 02 10 lwz r9,528(r3) c0014058: 71 29 00 01 andi. r9,r9,1 c001405c: 4c 82 00 20 bnelr ... Christophe > > Assisted-by: Codex:GPT-5.5 > Signed-off-by: Kevin Brodsky > --- > include/linux/mm_types.h | 10 ++++++++++ > mm/init-mm.c | 1 + > 2 files changed, 11 insertions(+) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index a2e0dc5892ff..7838ea3aac00 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -1414,6 +1414,8 @@ struct mm_struct { > char flexible_array[] __aligned(__alignof__(unsigned long)); > }; > > +#define MM_FLAGS_INIT(flags) { .__mm_flags = { BITMAP_FROM_U64(flags) } } > + > static inline bool mm_flags_test(int flag, const struct mm_struct *mm) > { > return test_bit(flag, ACCESS_PRIVATE(&mm->flags, __mm_flags)); > @@ -2001,6 +2003,9 @@ enum { > #define MMF_TOPDOWN 31 /* mm searches top down by default */ > #define MMF_TOPDOWN_MASK BIT(MMF_TOPDOWN) > > +#define MMF_KERNEL 32 /* mm belongs to the kernel */ > +#define MMF_KERNEL_MASK BIT_ULL(MMF_KERNEL) > + > #define MMF_INIT_LEGACY_MASK (MMF_DUMP_FILTER_MASK |\ > MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\ > MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK) > @@ -2008,6 +2013,11 @@ enum { > /* Legacy flags must fit within 32 bits. */ > static_assert((u64)MMF_INIT_LEGACY_MASK <= (u64)UINT_MAX); > > +static inline bool mm_is_kernel(const struct mm_struct *mm) > +{ > + return mm && mm_flags_test(MMF_KERNEL, mm); > +} > + > /* > * Initialise legacy flags according to masks, propagating selected flags on > * fork. Further flag manipulation can be performed by the caller. > diff --git a/mm/init-mm.c b/mm/init-mm.c > index 3e792aad7626..93773269bf87 100644 > --- a/mm/init-mm.c > +++ b/mm/init-mm.c > @@ -34,6 +34,7 @@ struct mm_struct init_mm = { > .pgd = swapper_pg_dir, > .mm_users = ATOMIC_INIT(2), > .mm_count = ATOMIC_INIT(1), > + .flags = MM_FLAGS_INIT(MMF_KERNEL_MASK), > .write_protect_seq = SEQCNT_ZERO(init_mm.write_protect_seq), > MMAP_LOCK_INITIALIZER(init_mm) > .page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock), >