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AJvYcCUu6cDDrAinSnt7GIr4COGosCYfJVufQO1p4+EnRCz/t++nxTifFpY2BXYT0UTE8qoYx5rxW55t5g==@kvack.org X-Gm-Message-State: AOJu0YyDPbgP3kY9VYRKccqsLQQXKAWFTo5Ks8XoxXFF6zYK0YTiQyTk r+D7easAYtgJymbjkTLpBpG4uy6TaK9J3gsxFajhjBCoF0dLPWpzoOC3eYj2p/OfI/w2mbpA+Gp CPnOzL8USGT/pWoBcoen9wB5GOQ== X-Google-Smtp-Source: AGHT+IGz7Acs955+JHt6jWBnGAVlrb2i9ZFcEZvLhMTomk5H/KJdcpKZNG2gPSwMMx7169I77mG09nPrtox3WC7DAA== X-Received: from plsd3.prod.google.com ([2002:a17:902:b703:b0:27d:6f45:42ec]) (user=kaleshsingh job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:3c2c:b0:24e:81d2:cfda with SMTP id d9443c01a7336-290272500c7mr257234325ad.0.1760399769577; Mon, 13 Oct 2025 16:56:09 -0700 (PDT) Date: Mon, 13 Oct 2025 16:51:54 -0700 In-Reply-To: <20251013235259.589015-1-kaleshsingh@google.com> Mime-Version: 1.0 References: <20251013235259.589015-1-kaleshsingh@google.com> X-Mailer: git-send-email 2.51.0.760.g7b8bcc2412-goog Message-ID: <20251013235259.589015-4-kaleshsingh@google.com> Subject: [PATCH v3 3/5] mm: introduce vma_count_remaining() From: Kalesh Singh To: akpm@linux-foundation.org, minchan@kernel.org, lorenzo.stoakes@oracle.com, david@redhat.com, Liam.Howlett@oracle.com, rppt@kernel.org, pfalcato@suse.de Cc: kernel-team@android.com, android-mm@google.com, Kalesh Singh , Alexander Viro , Christian Brauner , Jan Kara , Kees Cook , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Jann Horn , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman , Valentin Schneider , Shuah Khan , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: soahznodwdzgthzmfkfqqchrgibd477b X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: C5D421A000E X-HE-Tag: 1760399770-327516 X-HE-Meta: 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 CsB0tijQ 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 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: The checks against sysctl_max_map_count are open-coded in multiple places. While simple checks are manageable, the logic in places like mremap.c involves arithmetic with magic numbers that can be difficult to reason about. e.g. ... >= sysctl_max_map_count - 3 To improve readability and centralize the logic, introduce a new helper, vma_count_remaining(). This function returns the VMA count headroom available for a given mm. The most common case of checking for a single new VMA can be done with the convenience helper has_vma_count_remaining(): if (!vma_count_remaining(mm)) And the complex checks in mremap.c become clearer by expressing the required capacity directly: if (vma_count_remaining(mm) < 4) While a capacity-based function could be misused (e.g., with an incorrect '<' vs '<=' comparison), the improved readability at the call sites makes such errors less likely than with the previous open-coded arithmetic. As part of this change, sysctl_max_map_count is made static to mm/mmap.c to improve encapsulation. Cc: Andrew Morton Cc: David Hildenbrand Cc: "Liam R. Howlett" Cc: Lorenzo Stoakes Cc: Mike Rapoport Cc: Minchan Kim Cc: Pedro Falcato Signed-off-by: Kalesh Singh --- Changes in v3: - Move vma_count_remaining() out of #if CONFIG_SYSCTL to fix build failure - Use READ_ONCE() for sysclt_max_map_count, per David, Lorenzo - Remove use of ternary op in vma_count_remaining, per Lorenzo - Rebase on mm-new to fix conflicts in vma_internal.h and mm/internal.h include/linux/mm.h | 2 -- mm/internal.h | 3 +++ mm/mmap.c | 24 +++++++++++++++++++++++- mm/mremap.c | 7 ++++--- mm/nommu.c | 2 +- mm/util.c | 1 - mm/vma.c | 10 +++++----- tools/testing/vma/vma_internal.h | 9 +++++++++ 8 files changed, 45 insertions(+), 13 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 5c01c4b59ca6..72ff386ef772 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -203,8 +203,6 @@ static inline void __mm_zero_struct_page(struct page *page) #define MAPCOUNT_ELF_CORE_MARGIN (5) #define DEFAULT_MAX_MAP_COUNT (USHRT_MAX - MAPCOUNT_ELF_CORE_MARGIN) -extern int sysctl_max_map_count; - extern unsigned long sysctl_user_reserve_kbytes; extern unsigned long sysctl_admin_reserve_kbytes; diff --git a/mm/internal.h b/mm/internal.h index a2555be247e5..289aca3bdb6c 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -1682,4 +1682,7 @@ static inline int io_remap_pfn_range_complete(struct vm_area_struct *vma, return remap_pfn_range_complete(vma, addr, pfn, size, prot); } +/* mmap.c */ +int vma_count_remaining(const struct mm_struct *mm); + #endif /* __MM_INTERNAL_H */ diff --git a/mm/mmap.c b/mm/mmap.c index da2cbdc0f87b..d9ea029cd018 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -374,7 +374,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr, return -EOVERFLOW; /* Too many mappings? */ - if (mm->map_count >= sysctl_max_map_count) + if (!vma_count_remaining(mm)) return -ENOMEM; /* @@ -1495,6 +1495,28 @@ struct vm_area_struct *_install_special_mapping( &special_mapping_vmops); } +static int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT; + +/** + * vma_count_remaining - Determine available VMA slots + * @mm: The memory descriptor for the process. + * + * Check how many more VMAs can be created for the given @mm + * before hitting the sysctl_max_map_count limit. + * + * Return: The number of new VMAs the process can accommodate. + */ +int vma_count_remaining(const struct mm_struct *mm) +{ + const int map_count = mm->map_count; + const int max_count = READ_ONCE(sysctl_max_map_count); + + if (map_count >= max_count) + return 0; + + return max_count - map_count; +} + #ifdef CONFIG_SYSCTL #if defined(HAVE_ARCH_PICK_MMAP_LAYOUT) || \ defined(CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT) diff --git a/mm/mremap.c b/mm/mremap.c index 35de0a7b910e..14d35d87e89b 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -1040,7 +1040,7 @@ static unsigned long prep_move_vma(struct vma_remap_struct *vrm) * We'd prefer to avoid failure later on in do_munmap: * which may split one vma into three before unmapping. */ - if (current->mm->map_count >= sysctl_max_map_count - 3) + if (vma_count_remaining(current->mm) < 4) return -ENOMEM; if (vma->vm_ops && vma->vm_ops->may_split) { @@ -1814,9 +1814,10 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm) * split in 3 before unmapping it. * That means 2 more maps (1 for each) to the ones we already hold. * Check whether current map count plus 2 still leads us to 4 maps below - * the threshold, otherwise return -ENOMEM here to be more safe. + * the threshold. In other words, is the current map count + 6 at or + * below the threshold? Otherwise return -ENOMEM here to be more safe. */ - if ((current->mm->map_count + 2) >= sysctl_max_map_count - 3) + if (vma_count_remaining(current->mm) < 6) return -ENOMEM; return 0; diff --git a/mm/nommu.c b/mm/nommu.c index c3a23b082adb..22e55e7c69c4 100644 --- a/mm/nommu.c +++ b/mm/nommu.c @@ -1317,7 +1317,7 @@ static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma, return -ENOMEM; mm = vma->vm_mm; - if (mm->map_count >= sysctl_max_map_count) + if (!vma_count_remaining(mm)) return -ENOMEM; region = kmem_cache_alloc(vm_region_jar, GFP_KERNEL); diff --git a/mm/util.c b/mm/util.c index 088e1f8edcf5..3315e1136c69 100644 --- a/mm/util.c +++ b/mm/util.c @@ -752,7 +752,6 @@ EXPORT_SYMBOL(folio_mc_copy); int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS; static int sysctl_overcommit_ratio __read_mostly = 50; static unsigned long sysctl_overcommit_kbytes __read_mostly; -int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT; unsigned long sysctl_user_reserve_kbytes __read_mostly = 1UL << 17; /* 128MB */ unsigned long sysctl_admin_reserve_kbytes __read_mostly = 1UL << 13; /* 8MB */ diff --git a/mm/vma.c b/mm/vma.c index fba68f13e628..96ba37721002 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -491,8 +491,8 @@ void unmap_region(struct ma_state *mas, struct vm_area_struct *vma, } /* - * __split_vma() bypasses sysctl_max_map_count checking. We use this where it - * has already been checked or doesn't make sense to fail. + * __split_vma() bypasses vma_count_remaining() checks. We use this where + * it has already been checked or doesn't make sense to fail. * VMA Iterator will point to the original VMA. */ static __must_check int @@ -592,7 +592,7 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma, static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma, unsigned long addr, int new_below) { - if (vma->vm_mm->map_count >= sysctl_max_map_count) + if (!vma_count_remaining(vma->vm_mm)) return -ENOMEM; return __split_vma(vmi, vma, addr, new_below); @@ -1345,7 +1345,7 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms, * its limit temporarily, to help free resources as expected. */ if (vms->end < vms->vma->vm_end && - vms->vma->vm_mm->map_count >= sysctl_max_map_count) { + !vma_count_remaining(vms->vma->vm_mm)) { error = -ENOMEM; goto map_count_exceeded; } @@ -2797,7 +2797,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma, if (!may_expand_vm(mm, vm_flags, len >> PAGE_SHIFT)) return -ENOMEM; - if (mm->map_count >= sysctl_max_map_count) + if (!vma_count_remaining(mm)) return -ENOMEM; if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT)) diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h index 3525f5c15e1b..70f11163ab72 100644 --- a/tools/testing/vma/vma_internal.h +++ b/tools/testing/vma/vma_internal.h @@ -1484,4 +1484,13 @@ static inline int do_munmap(struct mm_struct *, unsigned long, size_t, return 0; } +/* Helper to get VMA count capacity */ +static int vma_count_remaining(const struct mm_struct *mm) +{ + const int map_count = mm->map_count; + const int max_count = sysctl_max_map_count; + + return (max_count > map_count) ? (max_count - map_count) : 0; +} + #endif /* __MM_VMA_INTERNAL_H */ -- 2.51.0.760.g7b8bcc2412-goog