If madvise(2) advice will result in the underlying vma being split and
the number of areas mapped by the process will exceed
/proc/sys/vm/max_map_count as a result, return ENOMEM instead of EAGAIN.
EAGAIN is returned by madvise(2) when a kernel resource, such as slab,
is temporarily unavailable. It indicates that userspace should retry the
advice in the near future. This is important for advice such as
MADV_DONTNEED which is often used by malloc implementations to free
memory back to the system: we really do want to free memory back when
madvise(2) returns EAGAIN because slab allocations (for vmas, anon_vmas,
or mempolicies) cannot be allocated.
Encountering /proc/sys/vm/max_map_count is not a temporary failure,
however, so return ENOMEM to indicate this is a more serious issue. A
followup patch to the man page will specify this behavior.
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
Documentation/sysctl/vm.txt | 4 ++--
Documentation/vm/ksm.txt | 4 ++++
include/linux/mm.h | 6 ++++--
mm/madvise.c | 51 +++++++++++++++++++++++++++++++++++++--------
mm/mmap.c | 8 +++----
5 files changed, 56 insertions(+), 17 deletions(-)
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -376,8 +376,8 @@ max_map_count:
This file contains the maximum number of memory map areas a process
may have. Memory map areas are used as a side-effect of calling
-malloc, directly by mmap and mprotect, and also when loading shared
-libraries.
+malloc, directly by mmap, mprotect, and madvise, and also when loading
+shared libraries.
While most applications need less than a thousand maps, certain
programs, particularly malloc debuggers, may consume lots of them,
diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
--- a/Documentation/vm/ksm.txt
+++ b/Documentation/vm/ksm.txt
@@ -38,6 +38,10 @@ the range for whenever the KSM daemon is started; even if the range
cannot contain any pages which KSM could actually merge; even if
MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
+If a region of memory must be split into at least one new MADV_MERGEABLE
+or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
+will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).
+
Like other madvise calls, they are intended for use on mapped areas of
the user address space: they will report ENOMEM if the specified range
includes unmapped gaps (though working on the intervening mapped areas),
diff --git a/include/linux/mm.h b/include/linux/mm.h
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1968,8 +1968,10 @@ extern struct vm_area_struct *vma_merge(struct mm_struct *,
unsigned long vm_flags, struct anon_vma *, struct file *, pgoff_t,
struct mempolicy *, struct vm_userfaultfd_ctx);
extern struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *);
-extern int split_vma(struct mm_struct *,
- struct vm_area_struct *, unsigned long addr, int new_below);
+extern int __split_vma(struct mm_struct *, struct vm_area_struct *,
+ unsigned long addr, int new_below);
+extern int split_vma(struct mm_struct *, struct vm_area_struct *,
+ unsigned long addr, int new_below);
extern int insert_vm_struct(struct mm_struct *, struct vm_area_struct *);
extern void __vma_link_rb(struct mm_struct *, struct vm_area_struct *,
struct rb_node **, struct rb_node *);
diff --git a/mm/madvise.c b/mm/madvise.c
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -89,14 +89,28 @@ static long madvise_behavior(struct vm_area_struct *vma,
case MADV_MERGEABLE:
case MADV_UNMERGEABLE:
error = ksm_madvise(vma, start, end, behavior, &new_flags);
- if (error)
+ if (error) {
+ /*
+ * madvise() returns EAGAIN if kernel resources, such as
+ * slab, are temporarily unavailable.
+ */
+ if (error == -ENOMEM)
+ error = -EAGAIN;
goto out;
+ }
break;
case MADV_HUGEPAGE:
case MADV_NOHUGEPAGE:
error = hugepage_madvise(vma, &new_flags, behavior);
- if (error)
+ if (error) {
+ /*
+ * madvise() returns EAGAIN if kernel resources, such as
+ * slab, are temporarily unavailable.
+ */
+ if (error == -ENOMEM)
+ error = -EAGAIN;
goto out;
+ }
break;
}
@@ -117,15 +131,37 @@ static long madvise_behavior(struct vm_area_struct *vma,
*prev = vma;
if (start != vma->vm_start) {
- error = split_vma(mm, vma, start, 1);
- if (error)
+ if (unlikely(mm->map_count >= sysctl_max_map_count)) {
+ error = -ENOMEM;
goto out;
+ }
+ error = __split_vma(mm, vma, start, 1);
+ if (error) {
+ /*
+ * madvise() returns EAGAIN if kernel resources, such as
+ * slab, are temporarily unavailable.
+ */
+ if (error == -ENOMEM)
+ error = -EAGAIN;
+ goto out;
+ }
}
if (end != vma->vm_end) {
- error = split_vma(mm, vma, end, 0);
- if (error)
+ if (unlikely(mm->map_count >= sysctl_max_map_count)) {
+ error = -ENOMEM;
+ goto out;
+ }
+ error = __split_vma(mm, vma, end, 0);
+ if (error) {
+ /*
+ * madvise() returns EAGAIN if kernel resources, such as
+ * slab, are temporarily unavailable.
+ */
+ if (error == -ENOMEM)
+ error = -EAGAIN;
goto out;
+ }
}
success:
@@ -133,10 +169,7 @@ static long madvise_behavior(struct vm_area_struct *vma,
* vm_flags is protected by the mmap_sem held in write mode.
*/
vma->vm_flags = new_flags;
-
out:
- if (error == -ENOMEM)
- error = -EAGAIN;
return error;
}
diff --git a/mm/mmap.c b/mm/mmap.c
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2495,11 +2495,11 @@ detach_vmas_to_be_unmapped(struct mm_struct *mm, struct vm_area_struct *vma,
}
/*
- * __split_vma() bypasses sysctl_max_map_count checking. We use this on the
- * munmap path where it doesn't make sense to fail.
+ * __split_vma() bypasses sysctl_max_map_count checking. We use this where it
+ * has already been checked or doesn't make sense to fail.
*/
-static int __split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long addr, int new_below)
+int __split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long addr, int new_below)
{
struct vm_area_struct *new;
int err;
--
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