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)
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 <a...@linux-foundation.org>
Cc: David Hildenbrand <da...@redhat.com>
Cc: "Liam R. Howlett" <liam.howl...@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoa...@oracle.com>
Cc: Mike Rapoport <r...@kernel.org>
Cc: Minchan Kim <minc...@kernel.org>
Cc: Pedro Falcato <pfalc...@suse.de>
Signed-off-by: Kalesh Singh <kaleshsi...@google.com>
---
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;
-
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;
+
+ return (max_count > map_count) ? (max_count - map_count) : 0;
+}
+
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;
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 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
