Re: [PATCH 00/16] Sanitize usage of ->flags and ->mapping for tail pages

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Mar 24, 2015 at 04:42:48PM -0700, Hugh Dickins wrote:
> On Mon, 23 Mar 2015, Kirill A. Shutemov wrote:
> > Yes, it works until some sound driver decide it wants to use
> > page->mappging.
> 
> (a) Why would it want to use page->mapping?

No idea.

> (b) What's the problem if it wants to use page->mapping?

It would need to be initalized for all subpages to get core mm see correct
value. And this doesn't match with current ->mapping users of __GFP_COMP
page (THP and hugetlb) which initialize ->mapping only for head pages.

> (c) Or perhaps some __GFP_COMP driver does already use page->mapping?

I haven't found any.

> > It's just pure luck that it happened to work in this particular case.
> 
> We were lucky that it fitted together without needing extra code, yes.
> But this didn't happen by accident, it was known and considered.

I don't agree it was considered well enough.

> > You only need to pay the expense if you hit tail page which is very rare
> > in current kernel. I think we can pay this cost for correctness.
> 
> But it's correct as is.

See above.

> > 
> > We will shave some cost of compound_head() if/when my refcounting patchset
> > get merged: no need of barrier anymore.
> 
> And if these changes are necessary for that, sure, go ahead:
> but as part of that work.

I believe the patchset has value by its own. And having it merged makes my
life easier. But up to Andrew.

-- 
 Kirill A. Shutemov

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