Re: page_referenced() and VM_LOCKED

[Hugh Dickins <hugh@veritas.com>]

On Tue, 4 Dec 2007, Ethan Solomita wrote:
> Hugh Dickins wrote:
> > On Fri, 16 Nov 2007, KAMEZAWA Hiroyuki wrote:
> > > On Thu, 15 Nov 2007 20:25:45 -0800
> > > Ethan Solomita <solo@google.com> wrote:
> > >
> > > > page_referenced_file() checks for the vma to be VM_LOCKED|VM_MAYSHARE
> > > > and adds returns 1.
> > 
> > That's a case where it can deduce that the page is present and should
> > be treated as referenced, without even examining the page tables.
> > 
> > > > We don't do the same in page_referenced_anon().
> > 
> > It cannot make that same deduction in the page_referenced_anon() case
> > (different vmas may well contain different COWs of some original page).
> 
> Sorry to come back in with this so late --
> if the vma is VM_MAYSHARE, would there be COWs of the original page?

99.999% correct answer: No, that's why we're testing VM_MAYSHARE,
because we know it has to be that original page which is present
and locked there (it might be readwrite, or it might be readonly,
but it won't be a COWed copy in a VM_MAYSHARE vma; the same would
be true if we tested VM_SHARED, but that would miss readonly cases).

00.001% adjustment: Actually, there's an aberrant case in which
do_wp_page() can put a COW into a VM_MAYSHARE area, supposedly to
suit ptrace().  I overlooked that case when I put the VM_MAYSHARE
test into page_referenced_file(); later when I learnt about it
(and found Linus unwilling to change it), I did an audit of such
oversights, but concluded this test wasn't worth changing.

Hugh

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