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

[Cyrill Gorcunov]

On Mon, Aug 25, 2014 at 09:45:34PM -0700, Hugh Dickins wrote:
> > +static int clear_refs(struct mm_struct *mm, enum clear_refs_types type,
> > +                      int write)
> > +{
...
> > +
> > +	if (write)
> > +		down_write(&mm->mmap_sem);
> > +	else
> > +		down_read(&mm->mmap_sem);
> > +
> > +	if (type == CLEAR_REFS_SOFT_DIRTY)
> > +		mmu_notifier_invalidate_range_start(mm, 0, -1);
> > +
> > +	for (vma = mm->mmap; vma; vma = vma->vm_next) {
> > +		cp.vma = vma;
> > +		if (is_vm_hugetlb_page(vma))
> > +			continue;
...
> > +		if (type == CLEAR_REFS_ANON && vma->vm_file)
> > +			continue;
> > +		if (type == CLEAR_REFS_MAPPED && !vma->vm_file)
> > +			continue;
> > +		if (type == CLEAR_REFS_SOFT_DIRTY &&
> > +		    (vma->vm_flags & VM_SOFTDIRTY)) {
> > +			if (!write) {
> > +				r = -EAGAIN;
> > +				break;
> 
> Hmm.  For a long time I thought you were fixing another important bug
> with down_write, since we "always" use down_write to modify vm_flags.
> 
> But now I'm realizing that if this is the _only_ place which modifies
> vm_flags with down_read, then it's "probably" safe.  I've a vague
> feeling that this was discussed before - is that so, Cyrill?

Well, as far as I remember we were not talking before about vm_flags
and read-lock in this function, maybe it was on some unrelated lkml thread
without me CC'ed? Until I miss something obvious using read-lock here
for vm_flags modification should be safe, since the only thing which is
important (in context of vma-softdirty) is the vma's presence. Hugh,
mind to refresh my memory, how long ago the discussion took place?
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