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

[Shi, Yang]

On 5/25/2016 5:37 PM, Minchan Kim wrote:
On Tue, May 24, 2016 at 11:58:11AM +0900, Minchan Kim wrote:
On Mon, May 23, 2016 at 10:16:08AM -0700, Yang Shi wrote:
Per the discussion with Joonsoo Kim [1], we need check the return value of
lookup_page_ext() for all call sites since it might return NULL in some cases,
although it is unlikely, i.e. memory hotplug.
Tested with ltp with "page_owner=0".

[1] http://lkml.kernel.org/r/20160519002809.GA10245@js1304-P5Q-DELUXE

Signed-off-by: Yang Shi <yang....@linaro.org>

I didn't read code code in detail to see how page_ext memory space
allocated in boot code and memory hotplug but to me, it's not good
to check NULL whenever we calls lookup_page_ext.

More dangerous thing is now page_ext is used by optionable feature(ie, not
critical for system stability) but if we want to use page_ext as
another important tool for the system in future,
it could be a serious problem.

Can we put some hooks of page_ext into memory-hotplug so guarantee
that page_ext memory space is allocated with memmap space at the
same time? IOW, once every PFN wakers find a page is valid, page_ext
is valid, too so lookup_page_ext never returns NULL on valid page
by design.

I hope we consider this direction, too.

Yang, Could you think about this?

Thanks a lot for the suggestion. Sorry for the late reply, I was busy on 
preparing patches. I do agree this is a direction we should look into, 
but I haven't got time to think about it deeper. I hope Joonsoo could 
chime in too since he is the original author for page extension.


Even, your patch was broken, I think.
It doesn't work with !CONFIG_DEBUG_VM && !CONFIG_PAGE_POISONING because
lookup_page_ext doesn't return NULL in that case.

Actually, I think the #ifdef should be removed if lookup_page_ext() is 
possible to return NULL. It sounds not make sense returning NULL only 
when DEBUG_VM is enabled. It should return NULL no matter what debug 
config is selected. If Joonsoo agrees with me I'm going to come up with 
a patch to fix it.

Regards,
Yang



Thanks.

---
 include/linux/page_idle.h | 43 ++++++++++++++++++++++++++++++++++++-------
 mm/page_alloc.c           |  6 ++++++
 mm/page_owner.c           | 27 +++++++++++++++++++++++++++
 mm/page_poison.c          |  8 +++++++-
 mm/vmstat.c               |  2 ++
 5 files changed, 78 insertions(+), 8 deletions(-)

diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
index bf268fa..8f5d4ad 100644
--- a/include/linux/page_idle.h
+++ b/include/linux/page_idle.h
@@ -46,33 +46,62 @@ extern struct page_ext_operations page_idle_ops;

 static inline bool page_is_young(struct page *page)
 {
-       return test_bit(PAGE_EXT_YOUNG, &lookup_page_ext(page)->flags);
+       struct page_ext *page_ext;
+       page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext)
+               return false;
+
+       return test_bit(PAGE_EXT_YOUNG, &page_ext->flags);
 }

 static inline void set_page_young(struct page *page)
 {
-       set_bit(PAGE_EXT_YOUNG, &lookup_page_ext(page)->flags);
+       struct page_ext *page_ext;
+       page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext)
+               return;
+
+       set_bit(PAGE_EXT_YOUNG, &page_ext->flags);
 }

 static inline bool test_and_clear_page_young(struct page *page)
 {
-       return test_and_clear_bit(PAGE_EXT_YOUNG,
-                                 &lookup_page_ext(page)->flags);
+       struct page_ext *page_ext;
+       page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext)
+               return false;
+
+       return test_and_clear_bit(PAGE_EXT_YOUNG, &page_ext->flags);
 }

 static inline bool page_is_idle(struct page *page)
 {
-       return test_bit(PAGE_EXT_IDLE, &lookup_page_ext(page)->flags);
+       struct page_ext *page_ext;
+       page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext)
+               return false;
+
+       return test_bit(PAGE_EXT_IDLE, &page_ext->flags);
 }

 static inline void set_page_idle(struct page *page)
 {
-       set_bit(PAGE_EXT_IDLE, &lookup_page_ext(page)->flags);
+       struct page_ext *page_ext;
+       page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext)
+               return;
+
+       set_bit(PAGE_EXT_IDLE, &page_ext->flags);
 }

 static inline void clear_page_idle(struct page *page)
 {
-       clear_bit(PAGE_EXT_IDLE, &lookup_page_ext(page)->flags);
+       struct page_ext *page_ext;
+       page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext)
+               return;
+
+       clear_bit(PAGE_EXT_IDLE, &page_ext->flags);
 }
 #endif /* CONFIG_64BIT */

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f8f3bfc..d27e8b9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -656,6 +656,9 @@ static inline void set_page_guard(struct zone *zone, struct 
page *page,
                return;

        page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext))
+               return;
+
        __set_bit(PAGE_EXT_DEBUG_GUARD, &page_ext->flags);

        INIT_LIST_HEAD(&page->lru);
@@ -673,6 +676,9 @@ static inline void clear_page_guard(struct zone *zone, 
struct page *page,
                return;

        page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext))
+               return;
+
        __clear_bit(PAGE_EXT_DEBUG_GUARD, &page_ext->flags);

        set_page_private(page, 0);
diff --git a/mm/page_owner.c b/mm/page_owner.c
index 792b56d..902e398 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -55,6 +55,8 @@ void __reset_page_owner(struct page *page, unsigned int order)

        for (i = 0; i < (1 << order); i++) {
                page_ext = lookup_page_ext(page + i);
+               if (unlikely(!page_ext))
+                       continue;
                __clear_bit(PAGE_EXT_OWNER, &page_ext->flags);
        }
 }
@@ -62,6 +64,10 @@ void __reset_page_owner(struct page *page, unsigned int 
order)
 void __set_page_owner(struct page *page, unsigned int order, gfp_t gfp_mask)
 {
        struct page_ext *page_ext = lookup_page_ext(page);
+
+       if (unlikely(!page_ext))
+               return;
+
        struct stack_trace trace = {
                .nr_entries = 0,
                .max_entries = ARRAY_SIZE(page_ext->trace_entries),
@@ -82,6 +88,8 @@ void __set_page_owner(struct page *page, unsigned int order, 
gfp_t gfp_mask)
 void __set_page_owner_migrate_reason(struct page *page, int reason)
 {
        struct page_ext *page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext))
+               return;

        page_ext->last_migrate_reason = reason;
 }
@@ -89,6 +97,12 @@ void __set_page_owner_migrate_reason(struct page *page, int 
reason)
 gfp_t __get_page_owner_gfp(struct page *page)
 {
        struct page_ext *page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext))
+               /*
+                * The caller just returns 0 if no valid gfp
+                * So return 0 here too.
+                */
+               return 0;

        return page_ext->gfp_mask;
 }
@@ -97,6 +111,10 @@ void __copy_page_owner(struct page *oldpage, struct page 
*newpage)
 {
        struct page_ext *old_ext = lookup_page_ext(oldpage);
        struct page_ext *new_ext = lookup_page_ext(newpage);
+
+       if (unlikely(!old_ext || !new_ext))
+               return;
+
        int i;

        new_ext->order = old_ext->order;
@@ -186,6 +204,11 @@ err:
 void __dump_page_owner(struct page *page)
 {
        struct page_ext *page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext)) {
+               pr_alert("There is not page extension available.\n");
+               return;
+       }
+
        struct stack_trace trace = {
                .nr_entries = page_ext->nr_entries,
                .entries = &page_ext->trace_entries[0],
@@ -251,6 +274,8 @@ read_page_owner(struct file *file, char __user *buf, size_t 
count, loff_t *ppos)
                }

                page_ext = lookup_page_ext(page);
+               if (unlikely(!page_ext))
+                       continue;

                /*
                 * Some pages could be missed by concurrent allocation or free,
@@ -317,6 +342,8 @@ static void init_pages_in_zone(pg_data_t *pgdat, struct 
zone *zone)
                                continue;

                        page_ext = lookup_page_ext(page);
+                       if (unlikely(!page_ext))
+                               continue;

                        /* Maybe overraping zone */
                        if (test_bit(PAGE_EXT_OWNER, &page_ext->flags))
diff --git a/mm/page_poison.c b/mm/page_poison.c
index 1eae5fa..2e647c6 100644
--- a/mm/page_poison.c
+++ b/mm/page_poison.c
@@ -54,6 +54,9 @@ static inline void set_page_poison(struct page *page)
        struct page_ext *page_ext;

        page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext))
+               return;
+
        __set_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
 }

@@ -62,6 +65,9 @@ static inline void clear_page_poison(struct page *page)
        struct page_ext *page_ext;

        page_ext = lookup_page_ext(page);
+       if (unlikely(!page_ext))
+               return;
+
        __clear_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
 }

@@ -70,7 +76,7 @@ bool page_is_poisoned(struct page *page)
        struct page_ext *page_ext;

        page_ext = lookup_page_ext(page);
-       if (!page_ext)
+       if (unlikely(!page_ext))
                return false;

        return test_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 77e42ef..cb2a67b 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1061,6 +1061,8 @@ static void pagetypeinfo_showmixedcount_print(struct 
seq_file *m,
                                continue;

                        page_ext = lookup_page_ext(page);
+                       if (unlikely(!page_ext))
+                               continue;

                        if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags))
                                continue;
--
2.0.2

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