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

[Vladimir Davydov]

On Mon, May 04, 2015 at 12:17:22PM +0900, Minchan Kim wrote:
> On Thu, Apr 30, 2015 at 05:50:55PM +0300, Vladimir Davydov wrote:
> > 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
> 
> 
> do_anonymous_page
> page_add_new_anon_rmap
> atomic_set(&page->_mapcount, 0);
> __page_set_anon_rmap
>    anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
>    page->mapping = (struct address_space *) anon_vma;
>    page->index = linear_page_index(vma, address);
> lru_cache_add 
>   __pagevec_lru_add_fn
>   SetPageLRU(page);
> 
> During the procedure, there is no lock to prevent race. Then, at least,
> we need a write memory barrier to guarantee other fields set up before
> SetPageLRU. (Of course, PageLRU should have read-memory barrier to work
> well) But I can't find any barrier, either.
> 
> IOW, any fields you said could be out of order store without any lock or
> memory barrier. You might argue atomic op is a barrier on x86 but it
> doesn't guarantee other arches work like that so we need explict momory
> barrier or lock.
> 
> Let's have a theoretical example.
> 
>         CPU 0                                                                           CPU 1
> 
> do_anonymous_page
>   __page_set_anon_rmap
>   /* out of order happened so SetPageLRU is done ahead */
>   SetPageLRU(page)
>   /* Compilr changed store operation like below */

But it couldn't. Quoting Documentation/atomic_ops.txt:

	Properly aligned pointers, longs, ints, and chars (and unsigned
	equivalents) may be atomically loaded from and stored to in the same
	sense as described for atomic_read() and atomic_set().

__page_set_anon_rmap sets page->mapping using the following expression:

 	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
 	page->mapping = (struct address_space *) anon_vma;

and it can't be split, i.e. if one concurrently reads page->mapping
he/she will see either NULL or (anon_vma+PAGE_MAPPING_ANON), and there
can't be any intermediate result in page->mapping, such as anon_vma or
PAGE_MAPPING_ANON, because one doesn't expect

	atomic_set(&p, a + b);

to behave like

	atomic_set(&p, a);
	atomic_set(&p, atomic_read(&p) + b);

Thanks,
Vladimir

>   page->mapping = (struct address_space *) anon_vma;
>   /* Big stall happens */
>                                                                 /* idletacking judged it as LRU page so pass the page
>                                                                    in page_reference */
>                                                                 page_refernced
>                                                                         page_rmapping return true because
>                                                                         page->mapping has some vaule but not complete
>                                                                         so it calls rmap_walk_file.
>                                                                         it's okay to pass non-completed anon page in rmap_walk_file?
> 
>   page->mapping = (struct address_space *)
>         ((void *)page_mapping + PAGE_MAPPING_ANON);
> 
> It's too theoretical so it might be hard to happen in real practice.
> My point is there is nothing to prevent explict race.
> Even if there is no problem with other lock, it's fragile.
> Do I miss something?
> 
> I think general way to handle PageLRU are ahead isolation or zone->lru_lock.
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