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 akpm@linux-foundation.org Cc: David Hildenbrand david@redhat.com Cc: "Liam R. Howlett" Liam.Howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Mike Rapoport rppt@kernel.org Cc: Minchan Kim minchan@kernel.org Cc: Pedro Falcato pfalcato@suse.de Signed-off-by: Kalesh Singh kaleshsingh@google.com
---
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 */