Re: [PATCH v3 3/3] proc: add kpageidle file

[Vladimir Davydov <vdavydov@parallels.com>]

On Thu, Apr 30, 2015 at 05:25:31PM +0900, Minchan Kim wrote:
> On Wed, Apr 29, 2015 at 12:12:48PM +0300, Vladimir Davydov wrote:
> > On Wed, Apr 29, 2015 at 01:35:36PM +0900, Minchan Kim wrote:
> > > On Tue, Apr 28, 2015 at 03:24:42PM +0300, Vladimir Davydov wrote:
> > > > +#ifdef CONFIG_IDLE_PAGE_TRACKING
> > > > +static struct page *kpageidle_get_page(unsigned long pfn)
> > > > +{
> > > > +	struct page *page;
> > > > +
> > > > +	if (!pfn_valid(pfn))
> > > > +		return NULL;
> > > > +	page = pfn_to_page(pfn);
> > > > +	/*
> > > > +	 * We are only interested in user memory pages, i.e. pages that are
> > > > +	 * allocated and on an LRU list.
> > > > +	 */
> > > > +	if (!page || page_count(page) == 0 || !PageLRU(page))
> > > > +		return NULL;
> > > > +	if (!get_page_unless_zero(page))
> > > > +		return NULL;
> > > > +	if (unlikely(!PageLRU(page))) {
> > > 
> > > What lock protect the check PageLRU?
> > > If it is racing ClearPageLRU, what happens?
> > 
> > If we hold a reference to a page and see that it's on an LRU list, it
> > will surely remain a user memory page at least until we release the
> > reference to it, so it must be safe to play with idle/young flags. If we
> 
> The problem is that you pass the page in rmap reverse logic(ie, page_referenced)
> once you judge it's LRU page so if it is false-positive, what happens?
> A question is SetPageLRU, PageLRU, ClearPageLRU keeps memory ordering?
> IOW, all of fields from struct page rmap can acccess should be set up completely
> before LRU checking. Otherwise, something will be broken.

So, basically you are concerned about the case when we encounter a
freshly allocated page, which has PG_lru bit set and it's going to
become anonymous, but it is still in the process of rmap initialization,
i.e. its ->mapping or ->mapcount may still be uninitialized, right?

AFAICS, page_referenced should handle such pages fine. Look, it only
needs ->index, ->mapping, and ->mapcount.

If ->mapping is unset, than it is NULL and rmap_walk_anon_lock ->
page_lock_anon_vma_read will return NULL so that rmap_walk will be a
no-op.

If ->index is not initialized, than at worst we will go to
anon_vma_interval_tree_foreach over a wrong interval, in which case we
will see that the page is actually not mapped in page_referenced_one ->
page_check_address and again do nothing.

If ->mapcount is not initialized it is -1, and page_lock_anon_vma_read
will return NULL, just as it does in case ->mapping = NULL.

For file pages, we always take PG_locked before checking ->mapping, so
it must be valid.

Thanks,
Vladimir