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

[Cyrill Gorcunov]

On Wed, Aug 27, 2014 at 04:12:43PM -0700, Hugh Dickins wrote:
> > > 
> > > Weak argument to me.
> 
> Yes.  However rarely it's modified, we don't want any chance of it
> corrupting another flag.
> 
> VM_SOFTDIRTY is special in the sense that it's maintained in a very
> different way from the other VM_flags.  If we had a little alignment
> padding space somewhere in struct vm_area_struct, I think I'd jump at
> Kirill's suggestion to move it out of vm_flags and into a new field:
> that would remove some other special casing, like the vma merge issue.
> 
> But I don't think we have such padding space, and we'd prefer not to
> bloat struct vm_area_struct for it; so maybe it should stay for now.
> Besides, with Peter's patch, we're also talking about the locking on
> modifications to vm_page_prot, aren't we?

I think so.

> > > What about walk through vmas twice: first with down_write() to modify
> > > vm_flags and vm_page_prot, then downgrade_write() and do
> > > walk_page_range() on every vma?
> > 
> > I still it's undeeded,
> 
> Yes, so long as nothing else is doing the same.
> No bug yet, that we can see, but a bug in waiting.

:-)

> 
> > but for sure using write-lock/downgrade won't hurt,
> > so no argues from my side.
> 
> Yes, Kirill's two-stage suggestion seems the best:
> 
> down_write
> quickly scan vmas clearing VM_SOFT_DIRTY and updating vm_page_prot
> downgrade_write (or up_write, down_read?)
> slowly walk page tables write protecting and clearing soft-dirty on ptes
> up_read
> 
> But please don't mistake me for someone who has a good grasp of
> soft-dirty: I don't.

Thanks for sharing opinion, Hugh! (And thanks for second email about
vma-flags) So lets move it to Kirill's way, otherwise indeed one day
it might end up in a bug which for sure will not be easy to catch.
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