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

[John Stultz]

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
are better documented?

thanks
-john

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