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hca@linux.ibm.com, gor@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, davem@davemloft.net, andreas@gaisler.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, chris@zankel.net, jcmvbkbc@gmail.com, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, weixugc@google.com, baolin.wang@linux.alibaba.com, rientjes@google.com, shakeel.butt@linux.dev, thuth@redhat.com, broonie@kernel.org, osalvador@suse.de, jfalempe@redhat.com, mpe@ellerman.id.au, nysal@linux.ibm.com, linux-arm-kernel@lists.infradead.org, linux-parisc@vger.kernel.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v5 00/12] mm: establish const-correctness for pointer parameters Message-ID: <16d2d53d-a2d0-4cd8-9323-eb91ba51b061@lucifer.local> References: <20250901123028.3383461-1-max.kellermann@ionos.com> Content-Type: text/plain; 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Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Sep 01, 2025 at 02:30:16PM +0200, Max Kellermann wrote: > For improved const-correctness. > > This patch series systematically adds const qualifiers to pointer > parameters throughout the memory management subsystem, establishing a > foundation for improved const-correctness across the entire Linux > kernel. > > Const-correctness provides multiple benefits: > > 1. Type Safety: The compiler enforces that functions marked as taking > const parameters cannot accidentally modify the data, catching > potential bugs at compile time rather than runtime. > > 2. Compiler Optimizations: When the compiler knows data won't be > modified, it can generate more efficient code through better > register allocation, code motion, and aliasing analysis. > > 3. API Documentation: Const qualifiers serve as self-documenting code, > making it immediately clear to developers which functions are > read-only operations versus those that modify state. > > 4. Maintenance Safety: Future modifications to const-correct code are > less likely to introduce subtle bugs, as the compiler will reject > attempts to modify data that should remain unchanged. I think all of the above is really a lot of noise that doesn't add a huge amount of value. Please in your own words say why you are doing it, and also please mention why you feel it's justified to do: const *const param As mentioned on review of 2/12. I'm pretty well leaning towards - let's just not do the 2nd const at all, unless there's a really good reason to do so. There are also legit cases where you might want to reassign a local variable. Largely, granted, you shouldn't be reassinging params, but it's pretty constraining. > > The memory management subsystem is a fundamental building block of the > kernel. Most higher-level kernel subsystems (filesystems, drivers, > networking) depend on mm interfaces. By establishing > const-correctness at this foundational level: > > 1. Enables Propagation: Higher-level subsystems can adopt > const-correctness in their own interfaces. Without const-correct > mm functions, filesystems cannot mark their own parameters const > when they need to call mm functions. > > 2. Maximum Impact: Changes to core mm APIs benefit the entire kernel, as > these functions are called from virtually every subsystem. > > 3. Prevents Impedance Mismatch: Without const-correctness in mm, other > subsystems must either cast away const (dangerous) or avoid using > const altogether (missing optimization opportunities). > > Each patch focuses on a specific header or subsystem component to ease review > and bisection. All this is unnecessary noise, can you summarise more succinctly. More words than 'const-ify everything' doesn't mean 'several paragraphs of noise'. > > This work was initially posted as a single large patch: > https://lore.kernel.org/lkml/20250827192233.447920-1-max.kellermann@ionos.com/ > > Following feedback from Lorenzo Stoakes and David Hildenbrand, it has been > split into focused, reviewable chunks. The approach was validated with a > smaller patch that received agreement: > https://lore.kernel.org/lkml/20250828130311.772993-1-max.kellermann@ionos.com/ > ^--- All of this should be below the line and is associated with versions not the series as a whole really. > Signed-off-by: Max Kellermann > --- > v1 -> v2: > - made several parameter values const (i.e. the pointer address, not > just the pointed-to memory), as suggested by Andrew Morton and > Yuanchu Xie > - drop existing+obsolete "extern" keywords on lines modified by these > patches (suggested by Vishal Moola) > - add missing parameter names on lines modified by these patches > (suggested by Vishal Moola) > - more "const" pointers (e.g. the task_struct passed to > process_shares_mm()) > - add missing "const" to s390, fixing s390 build failure > - moved the mmap_is_legacy() change in arch/s390/mm/mmap.c from 08/12 > to 06/12 (suggested by Vishal Moola) > > v2 -> v3: > - remove garbage from 06/12 > - changed tags on subject line (suggested by Matthew Wilcox) > > v3 -> v4: > - more verbose commit messages including a listing of function names > (suggested by David Hildenbrand and Lorenzo Stoakes) > > v4 -> v5: > - back to shorter commit messages after an agreement between David > Hildenbrand and Lorenzo Stoakes was found I did ask you to do this in reverse order and add lore links :) I mean these aren't big things but it's REALLY helpful for reviewers. Thanks. > > Max Kellermann (12): > mm: constify shmem related test functions for improved > const-correctness > mm: constify pagemap related test functions for improved > const-correctness > mm: constify zone related test functions for improved > const-correctness > fs: constify mapping related test functions for improved > const-correctness > mm: constify process_shares_mm() for improved const-correctness > mm, s390: constify mapping related test functions for improved > const-correctness > parisc: constify mmap_upper_limit() parameter for improved > const-correctness > mm: constify arch_pick_mmap_layout() for improved const-correctness > mm: constify ptdesc_pmd_pts_count() and folio_get_private() > mm: constify various inline test functions for improved > const-correctness > mm: constify assert/test functions in mm.h > mm: constify highmem related functions for improved const-correctness > > arch/arm/include/asm/highmem.h | 6 +-- > arch/parisc/include/asm/processor.h | 2 +- > arch/parisc/kernel/sys_parisc.c | 2 +- > arch/s390/mm/mmap.c | 7 ++-- > arch/sparc/kernel/sys_sparc_64.c | 3 +- > arch/x86/mm/mmap.c | 7 ++-- > arch/xtensa/include/asm/highmem.h | 2 +- > include/linux/fs.h | 7 ++-- > include/linux/highmem-internal.h | 44 +++++++++++---------- > include/linux/highmem.h | 8 ++-- > include/linux/mm.h | 56 +++++++++++++-------------- > include/linux/mm_inline.h | 26 +++++++------ > include/linux/mm_types.h | 4 +- > include/linux/mmzone.h | 42 ++++++++++---------- > include/linux/pagemap.h | 59 +++++++++++++++-------------- > include/linux/sched/mm.h | 4 +- > include/linux/shmem_fs.h | 4 +- > mm/highmem.c | 10 ++--- > mm/oom_kill.c | 7 ++-- > mm/shmem.c | 6 +-- > mm/util.c | 20 ++++++---- > 21 files changed, 171 insertions(+), 155 deletions(-) > > -- > 2.47.2 >