On Mon, Sep 15, 2025 at 09:36:34AM -0700, Kalesh Singh wrote: > 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 givine process. > > 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)
Double space after 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 <[email protected]> > Cc: David Hildenbrand <[email protected]> > Cc: "Liam R. Howlett" <[email protected]> > Cc: Lorenzo Stoakes <[email protected]> > Cc: Mike Rapoport <[email protected]> > Cc: Minchan Kim <[email protected]> > Cc: Pedro Falcato <[email protected]> > Signed-off-by: Kalesh Singh <[email protected]> Generally logic looks ok, with various stuff fixed below + in commit msg we should be good to go :) > --- > > Changes in v2: > - Fix documentation comment for vma_count_remaining(), per Mike > - Remove extern in header, per Mike and Pedro > - Move declaration to mm/internal.h, per Mike > - Replace exceeds_max_map_count() with capacity-based vma_count_remaining(), > per Lorenzo. > - Fix tools/testing/vma, per Lorenzo. > > include/linux/mm.h | 2 -- > mm/internal.h | 2 ++ > mm/mmap.c | 21 ++++++++++++++++++++- > 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, 41 insertions(+), 13 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 1ae97a0b8ec7..138bab2988f8 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -192,8 +192,6 @@ 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; > - Nice to get rid of this as a global! :) > 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 45b725c3dc03..39f1c9535ae5 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -1661,4 +1661,6 @@ static inline bool reclaim_pt_is_enabled(unsigned long > start, unsigned long end, > void dup_mm_exe_file(struct mm_struct *mm, struct mm_struct *oldmm); > int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm); > > +int vma_count_remaining(const struct mm_struct *mm); > + > #endif /* __MM_INTERNAL_H */ > diff --git a/mm/mmap.c b/mm/mmap.c > index e5370e7fcd8f..af88ce1fbb5f 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; > > /* > @@ -1504,6 +1504,25 @@ struct vm_area_struct *_install_special_mapping( > int sysctl_legacy_va_layout; > #endif > > +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 = sysctl_max_map_count; David already commented on the READ_ONCE() here, seems wise. > + > + return (max_count > map_count) ? (max_count - map_count) : 0; Not a big deal but would prefer: if (map_count >= map_count) return 0; return max_count - map_count; As the ternary here is a bit less clear, and it puts the 'failure' case first and the 'success' case afterwards. > +} > + As discussed in reply to the kernel bot, you've accidentally placed this in a CONFIG_SYSCTL block, so need to move it :>) > static const struct ctl_table mmap_table[] = { > { > .procname = "max_map_count", > 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; This is much clearer. > > 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; I hate that we do this silly check here, but the time to revisit it is another series... > > return 0; > diff --git a/mm/nommu.c b/mm/nommu.c > index 8b819fafd57b..dd75f2334812 100644 > --- a/mm/nommu.c > +++ b/mm/nommu.c > @@ -1316,7 +1316,7 @@ 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 f814e6a59ab1..b6e83922cafe 100644 > --- a/mm/util.c > +++ b/mm/util.c > @@ -751,7 +751,6 @@ 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 033a388bc4b1..df0e8409f63d 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; > } > @@ -2772,7 +2772,7 @@ 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 3639aa8dd2b0..52cd7ddc73f4 100644 > --- a/tools/testing/vma/vma_internal.h > +++ b/tools/testing/vma/vma_internal.h > @@ -1517,4 +1517,13 @@ static inline vm_flags_t ksm_vma_flags(const struct > mm_struct *, const struct fi > return vm_flags; > } > > +/* 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 */ > -- > 2.51.0.384.g4c02a37b29-goog >
