Messages in this thread | | | Subject | Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments | From | Lee Schermerhorn <> | Date | Fri, 30 Jan 2009 14:53:10 -0500 |
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On Fri, 2009-01-30 at 10:14 -0800, Linus Torvalds wrote: > > 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;
I tried this patch atop 29-rc3 + your patch from yesterday with my simple test program at http://free.linux/hp.com/~lts/Tests/mmap_lock.c.
The test program shows the /proc/<pid>/maps before and after the mmap and attempted merge. It's not merging:
7fd20a668000-7fd20a66a000 rw-p 7fd20a668000 00:00 0 7fd20a68a000-7fd20a68b000 r--s 00000000 68:23 6608852 /tmp/tmpfVx1SFL (deleted) 7fd20a68b000-7fd20a68c000 r--s 00001000 68:23 6608852 /tmp/tmpfVx1SFL (deleted) 7fd20a68c000-7fd20a690000 rw-p 7fd20a68c000 00:00 0
Ad hoc instrumentation shows that it's the VM_ACCOUNT flag that is different between the existing file segment and the one attempting the merge:
is_mergeable_vma: !mergable: vma flags: 0x80020f9:0x1020f9 | |-VM_ACCOUNT +-----------VM_CAN_NONLINEAR
So happens, I'm mapping with MAP_SHARED, so the VM_ACCOUNT flag gets cleared later in mmap_region(). Comments say that this is for checking memory availability during shmem_file_setup(). Maybe we can move the temporary setting of VM_ACCOUNT until just before the call to shmem_zero_setup()?
Lee
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