Re: [PATCH 3/4] MADV_VOLATILE: Add purged page detection on setting memory non-volatile

[Minchan Kim <minchan@kernel.org>]

On Thu, May 08, 2014 at 02:45:27PM -0700, John Stultz wrote:
> On 05/07/2014 06:51 PM, Minchan Kim wrote:
> > On Tue, Apr 29, 2014 at 02:21:22PM -0700, John Stultz wrote:
> >> +/**
> >> + * mvolatile_check_purged_pte - Checks ptes for purged pages
> >> + * @pmd: pmd to walk
> >> + * @addr: starting address
> >> + * @end: end address
> >> + * @walk: mm_walk ptr (contains ptr to mvolatile_walker)
> >> + *
> >> + * Iterates over the ptes in the pmd checking if they have
> >> + * purged swap entries.
> >> + *
> >> + * Sets the mvolatile_walker.page_was_purged to 1 if any were purged,
> >> + * and clears the purged pte swp entries (since the pages are no
> >> + * longer volatile, we don't want future accesses to SIGBUS).
> >> + */
> >> +static int mvolatile_check_purged_pte(pmd_t *pmd, unsigned long addr,
> >> +					unsigned long end, struct mm_walk *walk)
> >> +{
> >> +	struct mvolatile_walker *vw = walk->private;
> >> +	pte_t *pte;
> >> +	spinlock_t *ptl;
> >> +
> >> +	if (pmd_trans_huge(*pmd))
> >> +		return 0;
> >> +	if (pmd_trans_unstable(pmd))
> >> +		return 0;
> >> +
> >> +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> >> +	for (; addr != end; pte++, addr += PAGE_SIZE) {
> >> +		if (!pte_present(*pte)) {
> >> +			swp_entry_t mvolatile_entry = pte_to_swp_entry(*pte);
> >> +
> >> +			if (unlikely(is_purged_entry(mvolatile_entry))) {
> >> +
> >> +				vw->page_was_purged = 1;
> >> +
> >> +				/* clear the pte swp entry */
> >> +				flush_cache_page(vw->vma, addr, pte_pfn(*pte));
> > Maybe we don't need to flush the cache because there is no mapped page.
> >
> >> +				ptep_clear_flush(vw->vma, addr, pte);
> > Maybe we don't need this, either. We didn't set present bit for purged
> > page but when I look at the internal of ptep_clear_flush, it checks present bit
> > and skip the TLB flush so it's okay for x86 but not sure other architecture.
> > More clear function for our purpose would be pte_clear_not_present_full.
> 
> Ok.. basically I just wanted to zap the psudo-swp entry, so it will be
> zero-filled from here on out.
> 
> 
> > And we are changing page table so at least, we need to handle mmu_notifier to
> > inform that to the client of mmu_notifier.
> 
> So yes, this is one item from my last iteration that I didn't act on
> yet. It wasn't clear to me here that we need to do the mmu_notifier,
> since the page is evicted earlier via try_to_purge_one (and we do notify
> then). But in just removing the psudo-swap entry we need to do a
> notification as well? Is there someplace where the mmu_notifier rules

Hmm, it seems your claim is reasonable so we don't need to call it again, sorry.
But let's double check with KVM people.

> are better documented?
> 
> thanks
> -john
> 
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-- 
Kind regards,
Minchan Kim