2021-05-27 00:12:41

by Waiman Long

Subject: [PATCH-next] mm/memcontrol.c: Fix potential uninitialized variable warning

If the -Wno-maybe-uninitialized gcc option is not specified, compilation
of memcontrol.c may generate the following warnings:

mm/memcontrol.c: In function ‘refill_obj_stock’:
./arch/x86/include/asm/irqflags.h:127:17: warning: ‘flags’ may be used uninitialized in this function [-Wmaybe-uninitialized]
return !(flags & X86_EFLAGS_IF);
~~~~~~~^~~~~~~~~~~~~~~~
mm/memcontrol.c:3216:16: note: ‘flags’ was declared here
unsigned long flags;
^~~~~
In file included from mm/memcontrol.c:29:
mm/memcontrol.c: In function ‘uncharge_page’:
./include/linux/memcontrol.h:797:2: warning: ‘objcg’ may be used uninitialized in this function [-Wmaybe-uninitialized]
percpu_ref_put(&objcg->refcnt);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix that by properly initializing *pflags in get_obj_stock() and
introducing a use_objcg bool variable in uncharge_page() to avoid
potentially accessing the struct page data twice.

Signed-off-by: Waiman Long <[email protected]>
---
mm/memcontrol.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cb864f87b01d..b9a6db6a7d4f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2108,6 +2108,7 @@ static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
struct memcg_stock_pcp *stock;

if (likely(in_task())) {
+ *pflags = 0UL;
preempt_disable();
stock = this_cpu_ptr(&memcg_stock);
return &stock->task_obj;
@@ -6840,6 +6841,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
unsigned long nr_pages;
struct mem_cgroup *memcg;
struct obj_cgroup *objcg;
+ bool use_objcg = PageMemcgKmem(page);

VM_BUG_ON_PAGE(PageLRU(page), page);

@@ -6848,7 +6850,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
* page memcg or objcg at this point, we have fully
* exclusive access to the page.
*/
- if (PageMemcgKmem(page)) {
+ if (use_objcg) {
objcg = __page_objcg(page);
/*
* This get matches the put at the end of the function and
@@ -6876,7 +6878,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)

nr_pages = compound_nr(page);

- if (PageMemcgKmem(page)) {
+ if (use_objcg) {
ug->nr_memory += nr_pages;
ug->nr_kmem += nr_pages;

--
2.18.1

2021-05-27 00:14:39

by Andrew Morton

Subject: Re: [PATCH-next] mm/memcontrol.c: Fix potential uninitialized variable warning

On Wed, 26 May 2021 15:36:02 -0400 Waiman Long <[email protected]> wrote:

> If the -Wno-maybe-uninitialized gcc option is not specified, compilation
> of memcontrol.c may generate the following warnings:
>
> mm/memcontrol.c: In function ‘refill_obj_stock’:
> ./arch/x86/include/asm/irqflags.h:127:17: warning: ‘flags’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> return !(flags & X86_EFLAGS_IF);
> ~~~~~~~^~~~~~~~~~~~~~~~
> mm/memcontrol.c:3216:16: note: ‘flags’ was declared here
> unsigned long flags;
> ^~~~~
> In file included from mm/memcontrol.c:29:
> mm/memcontrol.c: In function ‘uncharge_page’:
> ./include/linux/memcontrol.h:797:2: warning: ‘objcg’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> percpu_ref_put(&objcg->refcnt);
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Fix that by properly initializing *pflags in get_obj_stock() and
> introducing a use_objcg bool variable in uncharge_page() to avoid
> potentially accessing the struct page data twice.
>

Thanks. I'll queue this as a fix against your "mm/memcg: optimize user
context object stock access".

2021-05-27 01:00:38

by Waiman Long

Subject: Re: [PATCH-next] mm/memcontrol.c: Fix potential uninitialized variable warning

On 5/26/21 4:43 PM, Andrew Morton wrote:
> On Wed, 26 May 2021 15:36:02 -0400 Waiman Long <[email protected]> wrote:
>
>> If the -Wno-maybe-uninitialized gcc option is not specified, compilation
>> of memcontrol.c may generate the following warnings:
>>
>> mm/memcontrol.c: In function ‘refill_obj_stock’:
>> ./arch/x86/include/asm/irqflags.h:127:17: warning: ‘flags’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>> return !(flags & X86_EFLAGS_IF);
>> ~~~~~~~^~~~~~~~~~~~~~~~
>> mm/memcontrol.c:3216:16: note: ‘flags’ was declared here
>> unsigned long flags;
>> ^~~~~
>> In file included from mm/memcontrol.c:29:
>> mm/memcontrol.c: In function ‘uncharge_page’:
>> ./include/linux/memcontrol.h:797:2: warning: ‘objcg’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>> percpu_ref_put(&objcg->refcnt);
>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Fix that by properly initializing *pflags in get_obj_stock() and
>> introducing a use_objcg bool variable in uncharge_page() to avoid
>> potentially accessing the struct page data twice.
>>
> Thanks. I'll queue this as a fix against your "mm/memcg: optimize user
> context object stock access".
>
Thanks for that.

Cheers,
Longman