On Wed, Mar 10, 2010 at 10:19:33PM +0200, Izik Eidus wrote:
> On 03/10/2010 09:18 PM, Robin Holt wrote:
> > While reviewing ksm.c, I noticed that ksm.c does:
> >
> > if (pte_write(*ptep)) {
> > pte_t entry;
> >
> > swapped = PageSwapCache(page);
> > flush_cache_page(vma, addr, page_to_pfn(page));
> > /*
> > * Ok this is tricky, when get_user_pages_fast() run it doesnt
> > * take any lock, therefore the check that we are going to make
> > * with the pagecount against the mapcount is racey and
> > * O_DIRECT can happen right after the check.
> > * So we clear the pte and flush the tlb before the check
> > * this assure us that no O_DIRECT can happen after the check
> > * or in the middle of the check.
> > */
> > entry = ptep_clear_flush(vma, addr, ptep);
> > /*
> > * Check that no O_DIRECT or similar I/O is in progress on the
> > * page
> > */
> > if (page_mapcount(page) + 1 + swapped != page_count(page)) {
> > set_pte_at_notify(mm, addr, ptep, entry);
> > goto out_unlock;
> > }
> > entry = pte_wrprotect(entry);
> > set_pte_at_notify(mm, addr, ptep, entry);
> >
> >
> > I would think the error case (where the page has an elevated page_count)
> > should not be using set_pte_at_notify. In that event, you are simply
> > restoring the previous value. Have I missed something or is this an
> > extraneous _notify?
> >
>
> Yes, I think you are right set_pte_at(mm, addr, ptep, entry); would be
> enough here.
>
> I can`t remember or think any reason why I have used the _notify...
>
> Lets just get ACK from Andrea and Hugh that they agree it isn't needed
_notify it's needed, we're downgrading permissions here.
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