Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared

[Cyrill Gorcunov]

On Tue, Aug 26, 2014 at 05:56:12PM +0300, Kirill A. Shutemov wrote:
> > > 
> > > It seems safe in vma-softdirty context. But if somebody else will decide that
> > > it's fine to modify vm_flags without down_write (in their context), we
> > > will get trouble. Sasha will come with weird bug report one day ;)
> > > 
> > > At least vm_flags must be updated atomically to avoid race in middle of
> > > load-modify-store.
> > 
> > Which race you mean here? Two concurrent clear-refs?
> 
> Two concurent clear-refs is fine. But if somebody else will exploit the
> same approch to set/clear other VM_FOO and it will race with clear-refs
> we get trouble: some modifications can be lost.

yup, i see

> Basically, it's safe if only soft-dirty is allowed to modify vm_flags
> without down_write(). But why is soft-dirty so special?

because how we use this bit, i mean in normal workload this bit won't
be used intensively i think so it's not widespread in kernel code

> Should we consider moving protection of some vma fields under per-vma lock
> rather use over-loaded mmap_sem?

Hard to say, if vma-softdirty bit is the reason then I guess no, probably
it worth to estimate how much profit we would have if using per-vma lock.
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