On 03/11/2010 08:23 AM, Hugh Dickins wrote:
On Wed, 10 Mar 2010, Andrea Arcangeli wrote:
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.
Robin is not questioning that it's needed in the success case;
but in the case where we back out because the counts don't match,
and just put back the original entry, he's suggesting that then
the _notify isn't needed.
Yes exactly, and at that 'counts don`t match' path -
there is no need to call to _notify.
(I'm guessing that Robin is not making a significant improvement to KSM,
but rather trying to clarify his understanding of set_pte_at_notify.)
Yea, it won`t run unless at very rare cases
Hugh
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