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Message-ID: <827c844f-7106-4f62-a108-1f83544aa56e@lucifer.local>
Date: Thu, 4 Sep 2025 17:02:52 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Kalesh Singh <kaleshsingh@...gle.com>
Cc: akpm@...ux-foundation.org, minchan@...nel.org, kernel-team@...roid.com,
android-mm@...gle.com, David Hildenbrand <david@...hat.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: centralize and fix max map count limit checking
On Wed, Sep 03, 2025 at 04:24:35PM -0700, Kalesh Singh wrote:
> The check against the max map count (sysctl_max_map_count) was
> open-coded in several places. This led to inconsistent enforcement
> and subtle bugs where the limit could be exceeded.
>
> For example, some paths would check map_count > sysctl_max_map_count
> before allocating a new VMA and incrementing the count, allowing the
> process to reach sysctl_max_map_count + 1:
>
> int do_brk_flags(...)
> {
> if (mm->map_count > sysctl_max_map_count)
> return -ENOMEM;
>
> /* We can get here with mm->map_count == sysctl_max_map_count */
>
> vma = vm_area_alloc(mm);
> ...
> mm->map_count++ /* We've now exceeded the threshold. */
> }
>
> To fix this and unify the logic, introduce a new function,
> exceeds_max_map_count(), to consolidate the check. All open-coded
> checks are replaced with calls to this new function, ensuring the
> limit is applied uniformly and correctly.
>
> To improve encapsulation, sysctl_max_map_count is now static to
> mm/mmap.c. The new helper also adds a rate-limited warning to make
> debugging applications that exhaust their VMA limit easier.
>
Yeah this is nice thanks!
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> ---
> include/linux/mm.h | 11 ++++++++++-
> mm/mmap.c | 15 ++++++++++++++-
> mm/mremap.c | 7 ++++---
> mm/nommu.c | 2 +-
> mm/util.c | 1 -
> mm/vma.c | 6 +++---
> 6 files changed, 32 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1ae97a0b8ec7..d4e64e6a9814 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -192,7 +192,16 @@ 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;
> +/**
> + * exceeds_max_map_count - check if a VMA operation would exceed max_map_count
> + * @mm: The memory descriptor for the process.
> + * @new_vmas: The number of new VMAs the operation will create.
> + *
> + * Returns true if the operation would cause the number of VMAs to exceed
> + * the sysctl_max_map_count limit, false otherwise. A rate-limited warning
> + * is logged if the limit is exceeded.
> + */
> +extern bool exceeds_max_map_count(struct mm_struct *mm, unsigned int new_vmas);
No new externs please (as Pedro also pointed out! :)
>
> extern unsigned long sysctl_user_reserve_kbytes;
> extern unsigned long sysctl_admin_reserve_kbytes;
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 7306253cc3b5..693a0105e6a5 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 (exceeds_max_map_count(mm, 0))
Yeah this last 'mystery meat' parameter isn't amazing though to be honest, it's
kinda hard to read/understand.
E.g.:
int map_count_capacity(const 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;
}
Then this would become;
if (!map_count_capacity(mm)) {
// blah.
}
> return -ENOMEM;
>
> /*
> @@ -1504,6 +1504,19 @@ 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;
> +
> +bool exceeds_max_map_count(struct mm_struct *mm, unsigned int new_vmas)
> +{
> + if (unlikely(mm->map_count + new_vmas > sysctl_max_map_count)) {
> + pr_warn_ratelimited("%s (%d): Map count limit %u exceeded\n",
> + current->comm, current->pid,
> + sysctl_max_map_count);
Yeah not a fan of this, we aren't warning elsewhere, it's actually perfectly
normal for somebody to exceed this, this isn't at the level of a warning.
> + return true;
> + }
> + return false;
> +}
> +
> static const struct ctl_table mmap_table[] = {
> {
> .procname = "max_map_count",
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e618a706aff5..793fad58302c 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 (exceeds_max_map_count(current->mm, 4))
> return -ENOMEM;
In my version this would be:
if (map_count_capacity(current->mm) < 4)
return -ENOMEM;
And etc. etc. I won't do them all :)
I do think this is clearer...
>
> if (vma->vm_ops && vma->vm_ops->may_split) {
> @@ -1811,9 +1811,10 @@ 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 (exceeds_max_map_count(current->mm, 6))
> return -ENOMEM;
>
> return 0;
> diff --git a/mm/nommu.c b/mm/nommu.c
> index 8b819fafd57b..0533e1e3b266 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 (exceeds_max_map_count(mm, 1))
> 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 3b12c7579831..f804c8ac8fbb 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -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 (exceeds_max_map_count(vma->vm_mm, 1))
> 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) {
> + exceeds_max_map_count(vms->vma->vm_mm, 1)) {
> 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 (exceeds_max_map_count(mm, 1))
> return -ENOMEM;
>
> if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT))
> --
> 2.51.0.338.gd7d06c2dae-goog
>
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