Re: mm/ksm.c seems to be doing an unneeded _notify.

[Chris Wright <chrisw@redhat.com>]

* Robin Holt (holt@sgi.com) 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?

It does look extraneous to me as well.

thanks,
-chris

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