Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments

Linus Torvalds
2009-01-30T18:15:18Z

On Fri, 30 Jan 2009, Hugh Dickins wrote:
>
> ... what I think you have done is break the vma merging on
> ordinary files: because of that irritating VM_CAN_NONLINEAR
> flag which generic_file_mmap() and some others add in.

Ahh. Yes. VM_CAN_NONLINEAR is a "reverse flag", ie unlike the other flags
it's to some degree about being extra _normal_, not about being odd. Most
special flags tend to disable the VM from doing some clever thing, this
one enables it.

> To break the merging won't cause anyone much trouble,
> but is a slight regression we should fix.

Yeah. Just masking it off when comparing is probably the simplest option.
Make it a separate #define just for readability. There might be other
flags like this in the future.

> I'd have been very upset not to find something ;)

Yay for you ;)

And yes, this is the kind of thing that probably does mean that we're
better off with the no-semantic-changes patches in -stable.

Linus

---
mm/mmap.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index d3fa10a..c581df1 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -658,6 +658,9 @@ again: remove_next = 1 + (end > next->vm_end);
validate_mm(mm);
}

+/* Flags that can be inherited from an existing mapping when merging */
+#define VM_MERGEABLE_FLAGS (VM_CAN_NONLINEAR)
+
/*
* If the vma has a ->close operation then the driver probably needs to release
* per-vma resources, so we don't attempt to merge those.
@@ -665,7 +668,7 @@ again: remove_next = 1 + (end > next->vm_end);
static inline int is_mergeable_vma(struct vm_area_struct *vma,
struct file *file, unsigned long vm_flags)
{
- if (vma->vm_flags != vm_flags)
+ if ((vma->vm_flags ^ vm_flags) & ~VM_MERGEABLE_FLAGS)
return 0;
if (vma->vm_file != file)
return 0;