Re: [PATCH] mm: check the return value of lookup_page_ext for all call sites

[Minchan Kim]

On Wed, Jun 01, 2016 at 01:40:48PM -0700, Shi, Yang wrote:
> On 5/29/2016 11:11 PM, Minchan Kim wrote:
> >On Fri, May 27, 2016 at 11:16:41AM -0700, Shi, Yang wrote:
> >
> ><snip>
> >
> >>>
> >>>If we goes this way, how to guarantee this race?
> >>
> >>Thanks for pointing out this. It sounds reasonable. However, this
> >>should be only possible to happen on 32 bit since just 32 bit
> >>version page_is_idle() calls lookup_page_ext(), it doesn't do it on
> >>64 bit.
> >>
> >>And, such race condition should exist regardless of whether DEBUG_VM
> >>is enabled or not, right?
> >>
> >>rcu might be good enough to protect it.
> >>
> >>A quick fix may look like:
> >>
> >>diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
> >>index 8f5d4ad..bf0cd6a 100644
> >>--- a/include/linux/page_idle.h
> >>+++ b/include/linux/page_idle.h
> >>@@ -77,8 +77,12 @@ static inline bool
> >>test_and_clear_page_young(struct page *page)
> >> static inline bool page_is_idle(struct page *page)
> >> {
> >>        struct page_ext *page_ext;
> >>+
> >>+       rcu_read_lock();
> >>        page_ext = lookup_page_ext(page);
> >>+       rcu_read_unlock();
> >>+
> >>	if (unlikely(!page_ext))
> >>                return false;
> >>
> >>diff --git a/mm/page_ext.c b/mm/page_ext.c
> >>index 56b160f..94927c9 100644
> >>--- a/mm/page_ext.c
> >>+++ b/mm/page_ext.c
> >>@@ -183,7 +183,6 @@ struct page_ext *lookup_page_ext(struct page *page)
> >> {
> >>        unsigned long pfn = page_to_pfn(page);
> >>        struct mem_section *section = __pfn_to_section(pfn);
> >>-#if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PAGE_POISONING)
> >>        /*
> >>         * The sanity checks the page allocator does upon freeing a
> >>         * page can reach here before the page_ext arrays are
> >>@@ -195,7 +194,7 @@ struct page_ext *lookup_page_ext(struct page *page)
> >>         */
> >>        if (!section->page_ext)
> >>                return NULL;
> >>-#endif
> >>+
> >>        return section->page_ext + pfn;
> >> }
> >>
> >>@@ -279,7 +278,8 @@ static void __free_page_ext(unsigned long pfn)
> >>                return;
> >>        base = ms->page_ext + pfn;
> >>        free_page_ext(base);
> >>-       ms->page_ext = NULL;
> >>+       rcu_assign_pointer(ms->page_ext, NULL);
> >>+       synchronize_rcu();
> >
> >How does it fix the problem?
> >I cannot understand your point.
> 
> Assigning NULL pointer to page_Ext will be blocked until
> rcu_read_lock critical section is done, so the lookup and writing
> operations will be serialized. And, rcu_read_lock disables preempt
> too.

I meant your rcu_read_lock in page_idle should cover test_bit op.
One more thing, you should use rcu_dereference.

As well, please cover memory onlining case I mentioned in another
thread as well as memory offlining.

Anyway, to me, every caller of page_ext should prepare lookup_page_ext
can return NULL anytime and they should use rcu_read_[un]lock, which
is not good. :(