Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables V2 (resend)

[Mel Gorman <mgorman@suse.de>]

On Tue, Jul 24, 2012 at 12:23:58PM -0700, Hugh Dickins wrote:
> On Tue, 24 Jul 2012, Mel Gorman wrote:
> > On Mon, Jul 23, 2012 at 06:08:05PM -0700, Hugh Dickins wrote:
> > > 
> > > So, after a bout of anxiety, I think my &= ~VM_MAYSHARE remains good.
> > > 
> > 
> > I agree with you. When I was thinking about the potential problems, I was
> > thinking of them in the general context of the core VM and what we normally
> > take into account.
> > 
> > I confess that I really find this working-by-coincidence very icky and am
> > uncomfortable with it but your patch is the only patch that contains the
> > mess to hugetlbfs. I fixed exit_mmap() for my version but only by changing
> > the core to introduce exit_vmas() to take mmap_sem for write if a hugetlb
> > VMA is found so I also affected the core.
> 
> "icky" is not quite the word I'd use, but yes, it feels like you only
> have to dislodge a stone somewhere at the other end of the kernel,
> and the whole lot would come tumbling down.
> 
> If I could think of a suitable VM_BUG_ON to insert next to the ~VM_MAYSHARE,
> I would: to warn us when assumptions change.  If we were prepared to waste
> another vm_flag on it (and just because there's now a type which lets them
> expand does not mean we can be profligate with them), then you can imagine
> a VM_GOINGAWAY flag set in unmap_region() and exit_mmap(), and we key off
> that instead; or something of that kind.
> 

A new VM flag would be overkill for this right now.

> But I'm afraid I see that as TODO-list material: the one-liner is pretty
> good for stable backporting, and I felt smiled-upon when it turned out to
> be workable (and not even needing a change in arch/x86/mm, that really
> surprised me).  It seems ungrateful not to seize the simple fix it offers,
> which I found much easier to understand than the alternatives.
> 

That's fair enough.

> > 
> > So, lets go with your patch but with all this documented! I stuck a
> > changelog and an additional comment onto your patch and this is the end
> > result.
> 
> Okay, thanks.  (I think you've copied rather more of my previous mail
> into the commit description than it deserves, but it looks like you
> like more words where I like less!)
> 

I did copy more than was necessary, I'll fix it.

> > 
> > Do you want to pick this up and send it to Andrew or will I?
> 
> Oh, please change your Reviewed-by to Signed-off-by: almost all of the
> work and description comes from you and Michal; then please, you send it
> in to Andrew - sorry, I really need to turn my attention to other things.
> 

That's fine, I'll pick it. Thanks for working on this.

-- 
Mel Gorman
SUSE Labs

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