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

[Hugh Dickins]

On Tue, 26 Aug 2014, Cyrill Gorcunov wrote:
> On Mon, Aug 25, 2014 at 09:45:34PM -0700, Hugh Dickins wrote:
> > 
> > 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?

Sorry for making you think you were losing your mind, Cyrill.

I myself have no recollection of any such conversation with you;
but afraid that I might have lost _my_ memory of it - I didn't want
to get too strident about how fragile (though probably not yet buggy)
this down_read-for-updating-VM_SOFTDIRTY-onlyi is, if there had already
been such a discussion, coming to the conclusion that it is okay for now.

I am fairly sure that I have had some such discussion before; but
probably with someone else, probably still about mmap_sem and vm_flags,
but probably some other VM_flag: the surprising realization that it may
be safe but fragile to use just down_read for updating one particular flag.

Hugh
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